component si contents else « computer tomography batteries in series connectioi dry battery charger experiments with batteries Circuits for: audio, music. B sound genera' audio transfer equalizer blow that synthesizer' heat sink monitor hi-fi headphone amplifier loudspeaker protection melodic sawtooth microphone pre amplifier with mi MOSFET power amplifier sound level indicator swell pedal wah-wah oox for guitars cars b bicycles 8 78 8 65 8-32 8 25 8 83 8 96 8 40 8 76 8 87 computers & microprocessors current loop for modem direct reading digitizer faultfinding probe lor *iPs floppy centring unit floppy disk dnve hexadecimal Keyboard mams interface measuring wth the BBC micro morse training with the Junior Computer Ql RAM extension RS232 interface serial ime dnvet and receiver simple video inverter for ZX81 Sync inverter for QL keyboard for Apple ii clock design ideas combining digital circuils designing a low noise amplifier fast opto coup er fast opto-isoiator I power supply sequencing for opamj • thrifty LEO indicator time lapse unit generators B oscillators noise generator programmable baud rate gener rectangular pulse generator two-frequency oscillator hobby Ef games 'absorption type metal detector digital | 0 ystick interface 8 72 8 48 8 52 . 9-00 electronic dog infra-red light barrier lumbo displays 8 47 8 82 8 67 8 86 8 75 8 44 8-54 8 54 8 56 RF & electronic VHF/LJHF aerial switch NAVTEX receiver RTTY calibration indicator RTTY/CW filter send receive ident simple field strengh indicator simple video inverter for ZX81 spot frequency receiver sync separator video amplifier for B/W television sets video buffer/ repeater video dislnbution amplifier video selector VI F converter test b measurement faultfinding probe for pf*S GHz prescaler measunng with the BBC micro meter amplifier metering selector noise generator opamp tester RTTY calibrator indicator simplified word comparator ,/nie\a\ detector 8-35 miniature running lights 8-23 model aircraft monitor 8-56 "on the an" indicator 8 97 set pomtei 8 95 time-lapse unit 8-37 home 6 garden automatic sliding door ^ burglar deterrent “ ‘ CH boiler control ” flashing light with twilight switch lour position touch dimmer ” hotel switch f 50 1 infra red light barrier 1 LED direction indicator mams voltage monitor !'!? S wiring locator l pipe detector ®-?8 smofe and gas detector temperature regulator with ze power supplies, batter chargers. Er ancillaries 12 V NiCd battery charger active rectifier without diodes battery charging indicator battery fitness centre DC /DC converter direct voltage doubler economical power supply lead acod battery charger interface power supply with primary regulation voltage monitor negative supply converter simple zero crossing detector variable 3 A power supply 8 30 8 66 8 39 8-20 8 22 8 82 8 43 8 84 8 73 8 75 8 20 . 8 72 8 34 8 46 8-36 8 79 8 66 8 51 8-77 8 93 lia Aug/Sept 1985 O UNLIMITED Electronic and Computer Components UNLIMITED is the only word that can correctly describe the range of components available at our new showroom! rr^stshcwroomin^ntsand^ The ' ary ilVQomputer v. p^mniC&LOny _ PCBs and Components for ELEKTOR Projects are also available with us. UNLIMITED Electronic & Computer Components Neelam Manzil, 1st Floor, 350, Lamington Road, Bombay 400 007 Phone: C/o. 384634 HM 203 HM 204 Scientific announces two new additions to its trusted and time-proven range of HM Series Oscilloscopes: HM 203 and HM 204 Dual Trace 20 MHz Oscilloscopes. These sleek new low-line portables incorporate the latest in international product design and features: a 140 mm rectangular flat-face CRT with illuminated internal graticule, 8 x 10 cm display, high 2 mV sensitivity and 20 MHz bandwidth, HF triggering up to 50 MHz. single touch component tester, sweep delay, and many more features never before available in this price range. So remember HM 203 and HM 204, when you order an oscilloscope next. You will be glad you did. the affordable portables ! Manufactured by: SCIENTIFIC MES-TECHNIK PVT. LTD. Formerly known as: SCIENTIFIC INSTRUMENTS (INDORE) PVT. LTD. B 14, Pologround. Industrial Estate. Indore 452 003 Telephone: 31777. 31778; Cable: SCOPE. Telex: 0735-267 Customer Services at : I Bombay. Bangalore. Calcutta. Delhi. Hyderabad. Madras. THE AUTO LOGIC MONITOR LOGICLIP LC-16 16 Channel Logic Monitor imported PULSECHO SYSTEMS ■ Bombay -400 019. we stock: elektof Magazines c IZUMIYA 1C INC. P.C.B. Drafting Aids TELEDYNE RELAYS Device offers ELECTRONIC EDUCATIONAL & T€STING D€VIC€S IlCil KOIMTAKT raMHUMBE Raytheon] Semiconductors Cleaners Semiconductors FAIRCHILD Semiconductors Texas Instruments Semiconductors 1/D7IT Trimming jrJM.7 Potentiometers Bread Boards 9GS Semiconductors Transistors Metal Film Resistors Zener Diodes SPECIAL EX STOCK OFFER CA 307E CA 324E CA 339E CA 555E CA741E CA 723E CA 747E CA 3065E CA 3068 CA 3086 2N 3055 Bulk importers and users in India may Kindly forward their specific enquiries © detiae Elaatrcniss Pte. lid. 101 Kitchener Road *02-04, Singapore Electrical, Electronics and Hardware Centre, Singapore 0820. Telex : DEVICE RS 33250 for enquiries CALL: 298 6455 (4 lines) HENSON VIDOC CO., (PTE) LTD. TESTICA T-3 ONLY .400001 THE ONLY MULTIMETER WITH PROMPT SERVICE AFTER SALES ACCURATE! ROBUST! ECONOMICAL! AVAILABLE AT ALL COMPONENT SHOPS MANUFACTURERS : ELECTRICAL INSTRUMENT LABORATORIES, 339/68. RAJESH BUILDING. LAMINGTON ROAD. BOMBAY-400 007. PHONE-36 07 49. ^pnnect'i emits Chhotani Building, 52-B, 1st Floor, Proctor Road, Bombay 400 007 Phone: 360709 What's the best possible way to please everyone? By offering a variety of products sure to meet every customer's needs. That's what we at C0NNECTR0NICS have done. The connectors shown above are just a few of the high quality products we offer. Variety and quality - that's what makes CONNECTRONICS customers keep coming back. Widest Choice in Connectors! Technical Perfection In Desoldering Art Simple thumbloading Central loading shaft. Two auxiliary guide bars for well defined plunger movement. Positive shaft latch with push button release. Anodised knurled barrel for no-slip handling. Low-mass plunger and shaft assembly. Low Static construction. Safe for CMOS LSI. Powerful suction. Ideal for PTH disoldering. Non stick Teflon nozzle. Spare nozzles available. Industrial Radio House Gandhi Bhavan, First Floor, Chunam Lane, Off. Dr. Dadasaheb Bhadkamkar Marg, Bombay 400 007, Phones: 382104. India's largest selling desoldering system, for quick, easy and efficient desoldering. Measurement of INDUCTANCE, CAPACITANCE, RESISTANCE are greatly simplified by VLCR 7. No balancing, no adjustments. Connect the component to the terminals. VLCR 7 gives you directly the digital reading of value and its loss factor simultaneously. FOUR TERMINAL measurement eliminates inherent errors due to lead resistance. GUARD TERMINAL provided eliminates errors due to lead capacitance. VLCR 7 is the only instrument in India covering the widest ranges of 0.1 pf /uH /m ohm (ie. 0.0001 ohm) to 20,000 uf/200H/20 M ohm. J J V Here are 13 reasons why every small scale industry should take the new Fire Insurance Policy Lightning Riots Strikes Malicious damage Explosions Floods Storms Cyclones Landslides Earthquakes Volcanic eruptions Aerial damage Here is one weighty reason why even the tiniest industry can afford it: Premiums considerably reduced: you pay as little as One rupee for a Thousand insured. < 2 > GENERAL INSURANCE CORPORATION OF INDIA National Insurance ] Company Limited. \ The New India i Assurance Company Limited. K The Oriental I Insurance ' Company Limited. United India Insurance Company Limited. ■ mains power supply with " orimarv regulation The unusual circuit shown in figure 1 has an unusual efficiency: according to SGS, this amounts to no less than 37 per cent at an output voltage of 3 V and output current of 2 A. With tra- ditional secondary regulation, an ef- ficiency of about 8 per cent would have been normal. The output voltage can be varied over the range 1.2... 25V, and the output current can be 1.5 A at any of these voltages, provided IC 2 is mounted on a suitable heat sink. Another advantage of primary regu lation is that the power supply is pro- tected against variations in the mains supply. This aspect is normally ignored with secondary regulation, as it is assumed that primary fluctuations have no effect on the secondary regu lation. The present circuit is, therefore, of particular importance for use where the mains supply is subject to large variations. The regulation functions so that the voltage drop across voltage regulator 1C, is held constant. This voltage drop is transferred by current source T, into a current through the LED in the opto-coupler. When the voltage drop diminishes, the current through the LED is smaller. The transistor in the opto coupler gets less drive, and the voltage at pin 3 of 1C, drops. Voltage regulator 1C, contains a com- plete circuit for phase gating control with silicon-controlled rectifier Tri, . The gating angle of this triac depends on the comparison between the direct voltage at pin 3 and an on-chip generated sawtooth signal, the fre- quency of which is determined by capacitor C, (= 100 n). In our example, the triac switches the mains voltage earlier so that buffer capacitor C, receives more energy. Noise caused by the phase gating cir- cuit must be prevented of entering the mains supply by a mains noise filter as shown. SGS application battery fitness centre Tomezzoli This circuit is designed primarily for maintaining lead-acid batteries that are often not used for long periods in good working order. It charges the battery, after which the battery discharges slowly through its internal resistance and the present circuit. When the state of charge reaches a predetermined level, the charger is switched on again, the battery charges, and so the cycle repeats itself. The circuit is based on Schmitt trigger T,/T 2 . Zener diode D 7 determines the state of charge at which the charger is switched off. Resistor /? 2 provides the required hysteresis. With the mains disconnected and no battery con- nected to the battery terminals, check with voltages (from a regulated power supply) of 13.6 V and 12.5 V applied across the battery terminals that the relay switches off and on respectively. The "on" threshold may be corrected by, for instance, connecting a 1N4148 (cathode to + line!) in series with D 7 . The "off" threshold is corrected by altering the value of /? 2 , for example, by replacing this component with a 100 Q preset. It is, of course, possible to replace the mains transformer and bridge rectifier by a battery charger (see, for instance, Eiektor, July 1984, p. 7-39), in which case the rest of the circuit can be fit- ted inside the charger. 8.20 eiektor indie Aup/Sept It is not possible to connect a full discharged battery to the circuit, because the relay would not be energized. Such a battery should first be charged to above 10 V, but it is also possible to fit a switch in parallel with the relay contact and switch on the mains with that. It is possible, of course, to maintain two 12 V batteries in condition by doubling the secondary voltage of the mains transformer, the zener voltage of D 7 , the hysteresis, the rated coil voltage, and connecting the batteries in series across the terminals. Fuse F, is necessary to provide pro- tection against short circuits. The transformer primary circuit may also be protected by .. fuse (like F, a delayed action type) rated at 1 A. The circuit does not need a smoothing ried out by the battery, capacitor because that function is car- video buffer/repeater This universal video amplifier is intended as a buffer/repeater in a long coaxial cable to keep the signal at a reasonable level. Its gain is about 6 dB. The circuit is built from readily available components: some tran- sistors and a few others. The circuit consists of a two-stage amplifier, T, and T 2 , and an emitter follower that functions as impedance converter. The bandwidth at —3 dB is not less than 20 MHz. Current con- sumption at a suplly voltage of 12 V amounts to about 20 mA. The power supply needs to be regulated to pre- vent lines and other noise on the The buffer/repeater is very suitable for being combined with the video selec- tor featured elsewhere in this issue. The present circuit, with R, omitted. is then used as a buffer for the output of the inverter. Its input impedance is then around 4 kQ. serial line driver and receiver This circuit owes its existence to the -< need for data communication over relatively long distances (up to 100 metres), inexpensively, reliably, and suitable for speeds up to 2400 bauds. At the distances considered, the main expense is normally the cable, so here a readily available 60 Q coaxial cable is used. Because of its relative immunity to noise, current drive is employed. In the line driver — figure 1 — transis- tor T„ diode D 4 , and resistors. /? 3 and /? 4 form a current source that can be fed direct from a non-regulated supply of 8. . .10 V. The transistor should be mounted on a heat sink. The current level of 40 mA ensures an adequate input signal to the line receiver. Tran- sistor T 2 is a current switch that short-circuits the current source and the cable to earth of the input to the driver is logic high: only when that input is logic low, is the current of 40 mA fed into the cable. Diodes D 2 and D 3 protect the driver against noise emanating from the cable, while capacitor C, decouples the supply line. The line receiver is based on a type LM 311 comparator. Matching of the input is effected by a wire link at a rel- evant tap of resistive divider 3 R b -R 6 -R,IR e (in our case: 60 Q). Resistors /? 9 and /?,„, and diode D 5 protect the LM 311 against noise emanating from the cable. The sensi- tivity of the receiver is set with P,. Resistor R u provides some hysteresis. Pull-up resistor /?, 6 ensures that 1C, provides at its pin 7 a TTL output signal that is in phase with the input signal to the line driver. The circuit is best calibrated with the 85443-3 aid of an oscilloscope once it has been installed in its final position. The level P,. The setting of P, is optimum the input voltage (wave form B in of input to the receiver is then com- when the voltage at its wiper (wave figure 3). pared with the voltage at the wiper of form A in figure 3) is exactly opposing DC/DC converter W. Jitschin In circuits where two signal paths must be electrically isolated, use is often made of an opto-coupler. Unfor- tunately, these devices require two power supplies: one for the sender, and the other for the receiver. In industrial and professional undertak- ings this requirement is met by a pro- prietary DC/ DC converter. As these are by and large very expensive, they are not of very much interest to the average hobbyist. However, the do-it- yourself converter presented here is much less expensive and, moreover, easy to build. The circuit diagram in figure 1 shows that the converter consists of an oscil- lator, IC„ and a driver, IC 2 , on the 12v primary of the isolating transformer, primary side, and of a rectifier, Q f The voltage induced in the secondary D, . . . D 4 , and buffer capacitor, C 3 , at w winding is rectified and smoothed by the secondary. ^ C 3 . The stated value of that capacitor In our prototype, operating from a u ' IC 2 is more than adequate for the rela- 12 V battery at the maximum 74 per ^ tively high secondary frequency of cent efficiency, we measured a sec- y 200 kHz. ondary output voltage of 10.64 V, and ® The isolating transformer is a DIY a secondary output current of 9 mA item: it is wound on a pot core of (the corresponding primary current 22 mm dia. and 13 mm high with amounted to 10.8 mA). The second- course, 0 per cent! In other words: the 0.35 mm dia. enamelled copper wire ary current should not exceed 10 mA, circuit works optimally at a secondary - 80 turns for the primary and 80 because the secondary output voltage load current of 9 mA. turns for the secondary. The specific then drops below 10 V and the effi- Oscillator 1C, operates at a frequency inductance, A L , of the core should be ciency deteriorates. That applies also of around 100 kHz. Its two output 400 nH. The core should not have an to low-load conditions: when the signals are each amplified in three air gap. Insulating foil should be secondary is open-circuit, the output parallel-connected buffers contained placed between the two windings to voltage is 14 V, but the efficiency is, of in IC 2 , and then applied to the ensure an isolating voltage of 4 kV. 8.22 elektor India Aug/Sep! 1985 SEE miniature running lights J P Truong voltages on pins 12 and 13 determine the voltage range swept by the LEDs. The reference voltage for D, 6 is pro- vided via pin 5 of 1C, and amounts to about % of the supply voltage to the 555. The reference voltage for D, is determined by the potentional at the junction of < = pin 12 of IC 2 ), which with values shown amounts to about 3 V. Current consumption 30 mA. so that battery possible with two PP3s 12 V regulator. around supply is only series and a electronic dog To produce a faithful reproduction of the voice of man's best friend, we have borrowed several ideas from our music synthesizer. When push button switch S 2 is pressed, the frequency of voltage-controlled oscillator (VCO) A,-A 2 changes in about an eigth of a second from almost 0 Hz to a preset- table value of 100. . .1000 Hz. That signal is passed through band-pass filter A 5 -A 6 , the centre frequency of which corresponds with the highest VCO frequency. Voltage-controlled amplifier (VCA) T, ensures that the single pulse generated by the VCO when S 2 is open cannot be heard. Gates N, and N 2 form a monostable relaxation oscillator. When S 2 is closed, a short pulse appears at the output of N 2 that charges capacitor C 2 . Because of fl 3 , the pulse shape will be as shown in figure 1. This pulse controls the output frequency of the VCO as also shown in figure 1. Poten- tiometer P, determines the highest frequency: its setting depends on whether you want the sound. of a yap- ping poodle or the deep bark of an alsatian. The function of C 4 is similar to that of C 2 : it shapes the pulse applied to the VCA. This transistor behaves as an °i£Trnf^' ; C: Woo electronic potentiometer, i.e., it operates as a voltage-controlled resistor. Adjusting potentiometer P 2 influences the manner in which the tone decays after the switch has been released. Instantaneous dying of the tone would sound just as unreal as its microphone amplifier with mute switch Microphones, unfortunately, produce only a small signal and they, therefore, require a special pre-amplifier to boost their output. Because small signals are involved, the signal-to-noise ratio of the pre-amplifier is a very important parameter. In this article, we present two circuits for a pre-amplifier suitable for virtually all occasions: a symmetrical and an asymmetrical version. We have incor- porated a mute switch, which speakers can use when they want to clear their throat. As there is an number of low-noise operational amplifiers available nowadays, the cost of these pre-amplifiers is rela- tively low. The asymmetrical version is shown in figure 1. Switching between high and low impedance matching is possible with switch S 2 . Opamp A, is arranged as an AC amplifier with a gain of around 27 dB. This stage may also be used as a DC amplifier: /? 3 and C, are then omitted, and the value of R 2 is lowered to 22 k. Capacitor C 2 limits the bandwidth of the amplifier to ensure stable operation. Irrespective of whether A, functions the pre-amplifier with symmetrical inpi eleklor india Aufl/Sept 1985 Figure 2. Circuit of ,8.25 Ri,R 4 = 330 Q metal film R 2 ,R 3 « 22 k metal (ilm R5.R7 = 6k8 R 6 = 1k6 Rs.RtO = 1k2 R9.Rtt.R12 - 5k6 R 13 = 12 k R,4 = 1 M Rl5 = 150 k R 16 .R 17 = 120 k Rib = 10 k Rig = 270 k as a DC or an AC amplifier, the DC component in its output is blocked by C 3 . The amplified AC signal is applied to muting stage T,. This field-effect transistor (FED normally conducts and the output of A, is then further amplified in A 2 by about 5. Finally, the signal is taken to the output ter- minal via high-pass filter ft, 3 -C s . The load must be greater than 10 kQ. Capacitors: Ct = 22 p C 2 - 1p5 MKT* C 3 = 47 n C4,C 5 = 220 n Ce - '00 n Semiconductors: Di = AA119 Tt = BF256C T 2 = BC547B ICt,IC 2 = LM833; NE5532: TL072 When mute switch S, is pressed, the FET receives a negative voltage at its gate and is switched off. Capacitor C s determines the speed with which muting occurs within certain limits. Capacitors C u C 3 , and C 6 may be electrolytic types: measure the DC level at both terminals to determine which way they should be connected! The symmetrical version of the pre- Miscellaneous: St = spring-loaded push to make switch S 2 = miniature SPST switch PCB 85450-1 *MKT = metal-plated plastic polythereftalate For Components, Sources See Page 9-38 amplifier is shown in figure 2. The only difference between this and that in figure 1 is that the input stage now consists of A,, A 2 , and A 3 to obtain symmetry. Opamps A, and A 2 provide a total gain of about 20 dB. Opamp A 3 functions as a differential amplifier to ensure that common-mode noise and interference is effectively sup- pressed. ndia Aug/ Sept 1985 8.26 elek.ot I 3 3 burglar deterrent Most burglar deterrent systems are based on the same principle: once the presence of an unwanted or sus- picious individual has been detected (by electronic or other means), some action ensues which makes it clear to passers-by or neighbours that something is amiss. It is often overlooked that the unwanted visitor first had to ascertain that there is nobody at home. The majority of burglars who operate by daylight just ring the bell. Once they have repeatedly rung without anyone answering the door, they go about their nefarious ways. Once inside, they may well set off a conventional alarm, but by then it is already too late. The circuit proposed here was designed to prevent the intruder getting that far. When the bell is rung, a number of monostables is actuated, which, after a suitable delay, switches on a cassette player that generates an awesome sound. This can vary from the barking of a large dog to the roar of a lion, depending on the premises. Sometimes a simple "sorry, no can- vassers" may be adequate. The circuit consists basically of two monostables. The delay between the ringing of the bell and the cassette player being switched on is preset with P, between 0.22 and 2.4 seconds. The time the cassette player operates is set with P 2 between 47 s and 8 m 37 s. The cassette player is switched on via the relay contacts. The circuit is powered by the bell transformer. In the circuit it is as- sumed that this is a 6 V type, and the relay is, therefore, also a 6 V type (which here draws a current of 50 mAI. siren In spite of its modest configuration, the circuit shown here is capable of generating quite a sound. This is made possible by the n-channel MOSFET, T,, which drives the loud- speaker. Such a MOSFET can be driven direct by CMOS logic circuits, and the type chosen here has an output ^drain- source) resistance of only three ohms. Moreover, its drain current can be as high as 1.7 A, while the maximum drain-source voltage is 40 V. These parameters are independent of the polarity of the applied voltage, since the device has internal diode pro- tection. Since the MOSFET is virtually indestructible, it is perfectly bll right to load it with just a loudspeaker. The circuit can be controlled simply from a computer, and is operated by making the ENABLE input logic high (which can also be done with a simple 9... 12 V switch instead of a computer). When the input at pin 5 of gate N 2 is high, the pulses from Schmitt trigger N, cause N 2 to oscillate. The output of N 2 is applied to the MOSFET via buffer N 3 . The frequency of N 2 can be adjusted with P,. As to applications, this siren is particularly suitable for use in alarm installations. EBB metal pipe detector Water and gas pipes, as well as elec- trical conduit, embedded in walls are not easy to trace, although this is essential when work is to be carried out to the wall. This handy little unit will be a godsend at such times. The principle of the detector is based on the property of metals of absorbing magnetic energy when they are brought into a magnetic field. Transistor T, in figure 1 is a simple LC oscillator, of which the sensor, L,, forms a part. The oscillator frequency is around 15 kHz. When energy is withdrawn from the magnetic field around L f by a metal object, the alter- nating voltage across the LC circuit will diminish. By rectifying that voltage in 1C,, and applying the resul- tant direct voltage to a differential amplifier, IC 2 , which compares it with a voltage preset with P 3 , an on /off indication is obtained. When L , is brought in the vicinity of metal, D 4 goes out. The sensitivity of the detec- tor is set with P, and P 3 . The unit is powered by a 9 V battery IPP3). To calibrate the detector, adjust P, for maximum resistance and connect an oscilloscope to the collector of T,. Adjust the peak value of the oscillator signal with P 2 so that the oscillator just does not stop working. This is checked by adjusting P 3 so that the LED just lights. If then a coin is held near the ferrite rod, the LED should go out, indicating that the oscillator has ceased working. At the start of the search, use the smallest peak value of the oscillator signal (P, at maximum resistance). IC2 = CA 3130, CA3140 combined with the lowest trigger level (wiper of P 3 to earth). After the location of the pipes has been ascer- tained roughly, the peak value of the oscillator signal and the trigger level can be increased until the required accuracy is obtained. CH boiler control If you still alter your central heating -Ja system's boiler thermostat according to the season (many people nowadays leave it at the same — fairly high — setting throughout the year), this may cause the boiler to be switched on and off too frequently when the weather is unseasonly cold (see figure la). This problem may be resolved by the pres- ent circuit which prevents the boiler being switched on for some time, t a , after the switch-off temperature, T 2 , has been reached. After t a has lapsed, the boiler temperature, T, should have dropped well below the switch-on temperature, T } (see figure 1b). The circuit in figure 2 is an extension of the central heating monitor (, Eiektor , August/September 1984, p. 8-24) . The make contacts of a relay 1b quent change of logic level at the out- put selected by S, causes the reset of the bistable via N 4 . This is the end of The set input of the bistable is con- nected to the collector of T 3 in the central heating monitor via N, and R 3 -C 2 and R 2 -C y . That transistor drives the LED that indicates the inter- rupted heat request of the boiler ther- mostat. Delay time t a can be set within wide limits with P, and S,. A period of 10 minutes is probably a good starting point. It is then impossible for the boiler to be switched on and off more than six times per hour. Based on the number of times the relevant LEDs in the central heating monitor light, you can alter the delay time with P, and S,. Briefly connect the junction of R 3 -C 2 to earth: this causes the bistable to be set; D 3 goes out and are inserted into the 24 V boiler cir- bistable is set, T, conducts and delay the delay period starts. Adjust P, so cuit. The state of bistable N 2 -N 3 time f d commences. At the same that D3 lights again after 5, 10, or 20 determines whether transistor T, is time, the reset of counter IC 2 is minutes. It is also possible to use on or off, i.e., whether the 24 V circuit cancelled. After some time, IC 2 has periods of 4, 8, or 16, or 6, 12, or 24 is open or closed. As soon as the reached maximum count: the conse- minutes. S B i temperature sensor The LM35 is a temperature sensor Celsius. This means that if the tern- 19.8 °C, the output voltage is 0.198 V. which provides an output voltage that perature is 0 °C, the output voltage is This is an important advantage over is directly proportional to the tempera- 0 V. The output voltage increases by other temperature sensors that are ture being measured in degrees 10 mV for every degree Celsius, i.e., at calibrated in kelvin. Using such sen- elektor indie Aug/Sept 1985 8.29 sors to measure in degrees Celsius requires a very stable reference voltage that must be deducted from the reading. Another advantage of the LM35 is its very low current consumption of less than 60 t>A. This means a long battery life and small internal power dissi- pation, so that errors caused by internal heat are minimal: 0.1 °C with a battery voltage of 4 V. The sensor can be connected direct to an analogue or digital multimeter, or, more interestingly, to a computer which can then process and store the information. A suitable interface for this purpose is described in direct reading digitizer elsewhere in this B /|\ ©* ~ The accuracy of the LM35/LM35C is typically 0.4 °C at 25 °C. To keep the self-heat minimal, the load should be not smaller than 5 kQ. If a long screened cable is used between the sensor and indicator, an RC network (10 Q in series with 1 pF) should be connected between the output of the sensor and earth to pre- vent any oscillations. 0 fl 2 12-volt NiCd battery charger tmAl 13- If you attempt to charge a 12 V NiCd 1 battery from a 12 V lead-acid car bat- tery, you will soon find that that is not really possible: the charging voltage should be somewhat higher than the nominal battery voltage. A 12 V bat- tery should be charged from a source of about 14 V. The present circuit is, therefore, a voltage doubler based on the well- known 555 1C. The 1C oscillates, which means that output 3 is con- nected alternately with earth and the +12 V supply voltage. When pin 3 is logic low, C 3 is charged via D 2 and D 3 to almost 12 V. When pin 3 is logic high, the voltage at the junction of C 3 and D 3 becomes almost 24 V, because the negative ter- minal of C 3 is at +12 V and the 2 capacitor itself is charged to about 12 V. Diode D 3 is then reverse biased, but D„ conducts, so that C 4 is charged to just over 20 V, which is ample for our purposes. The 78L05 in the IC 2 position func- tions as a current source, which tends to keep its output voltage, U 0 . appearing across R 3 , at 5 V. The out- put current, /„, is therefore easily calculated from /„ = U 0 / R 3 — 5/680 = 7.4 mA. The 78L05 itself also draws current: the central terminal (normally earthed) delivers about 3 mA. The total load current is, therefore, of the order of 10 mA, which is a good value for con- tinuously charging NiCd batteries. The LED has been incorporated to indicate that charging current flows. The characteristic of the charging cur- rent versus battery voltage in figure 2 shows that the circuit is not perfect: a 12 V battery will be charged with a current of only about 5 mA. There are several causes for this: ■ the output voltage of the circuit tends to drop with increasing current; ■ the voltage drop across the 78L05 is about 5 V to which must be added the 2.5 V the 1C needs to operate correctly; ■ there is a voltage drop of about 1.5 V across the LED. None the less, a 12 V NiCd battery with a rated capacity of 500 mAh can be charged continuously with a cur- rent of 5 mA, which is 1 per cent of its capacity. 8.30 ktc* India Aug/Sept EDI bicycle lights and alarm A bicycle or tricycle should, as everyone knows, be fitted with front and rear lights. The noteworthy aspect of the lights circuit described here is that it also provides a visible alarm, which is primarily intended for invalid road users. When such handi- capped people are in need of assist- ance during the day, this is quickly spotted by passers by. At night, this is, unfortunately, r.vi so, whence the The usual dynamo or battery is replaced by a 6 V rechargeable lead- acid battery, which ensures that the bicycle lights are operational even when the bicycle is not moving. When the rider is in need of assistance, the alarm can be switched on: in addition to the normal lights, a small display with the word "HELP" will then flash. Such a signal for help is not easily overlooked! The circuit is based on an astable multivibrator, which does not operate when alarm switch S 2 is open. Pro- vided S, is closed, the front and rear lights are on, however. When the alarm switch, S 2 , is closed, the multivibrator operates, which causes the normal lights and the HELP lights to flash alternately. The circuit is powered by a 6 V 1.8 Ah lead-acid battery which, when prop- erly charged, is sufficient to keep the lights on for about three hours. The circuit can be fitted in a small, preferably water-proof, case. Lamps La 4 ...La 6 light the letters "HELP" that have been cut out in the lid. The BC141 should be fitted onto a small heat sink. Because of the need of regularly charging the battery, the case should be fitted to the vehicle in a manner which allows easy removal and attach- ment. A circuit for a suitable charger is given in figure 1b. This provides a constant charging voltage of 6.9 V (preset with P,), while the charging current is limited to about 650 mA. This enables the battery to be fully charged in around 3 hours. The charg- ing voltage should be set carefully, otherwise the battery will not be charged correctly. LM317 B1=B80C1500 I IDS melodic sawtooth H Millian Even in this era of programmable, 2 polyphonic synthesizers, interest in simple, monophonic keyboard instruments remains. Many FORM- ANT owners are still proud of their, probably first, home-built synthesizer and are still on the look-out for new circuits for the generation of exotic sounds. For all those, here is an easy- to-build circuit that can convert a sawtooth signal at its input into an output of double the frequency and half the peak value of the input signal (figure 1). Comparator 1C, transforms the sawtooth signal into a rectangular 1 /Wl/1 /VWI /H/M/I/VM signal (see figure 2). Adder IC 2 com- bines the original input signal and the rectangular signal. An additional LFO (low frequency oscillator) connected as shown pro- vides pulse-width modulation of the rectangular signal, which has a greatly beneficial effect on the output signal. When switch S, is set to position b, it is possible to inject a rectangular signal whose frequency is indepen- dent of the sawtooth frequency, which greatly increases the number of melodic variations, as anyone acquainted with synthesizers knows. Power requirements can be met direct by the FORMANT or any other +15 V symmetrical supply. Current con- sumption is not higher than 10 mA. IDS infra-red light barrier 8.32 The transmitter, shown in figure 1, consists of an astable multivibrator (AMVI, IC 3 . The output of the AMV, pin 3, consists of a pulse stream with a duty factor of about 30 per cent. The output is connected to a constant-current source, T 2 . This source provides infra-red transmit diodes D ? and D 8 with a current of just over 20 mA, which pulsates in rhythm with the output signal of the AMV. The infra-red light is, therefore, transmitted in rhythm with the pulse stream also. The receiver, shown in figure 2, is based on an SL486 demodulator, 1C,. The output of the demodulator, pin 11, also consists of a 10 kHz pulse train with a duty factor of around 30 per cent. This pulse stream is applied to integrator /? 2 -C, 2 . The logic level at the input of N, remains low as long as D 4 receives the pulsating infra-red light. Because of this, monostable N 2 is disabled, and oscillator N 4 , which drives a piezoelectric buzzer, is switched off. Relay Re, is, however, energized via N 4 and transistor T,. When the pulse stream between D 7 -D 8 and D 4 is broken, the logic level at pin 11 of 1C, goes high, so that the output of N, becomes logic 0, which triggers monostable N 2 via D,. Oscil- lator N 4 is then switched on and actuates the buzzer. At the same time, N 3 ensures that T, is switched off, so that the relay returns to its quiescent state. When the monostable pulse decays, which with the stated values of /? 4 and C, 0 is after about 5 seconds, oscillator N 4 stops and the alarm tone ceases. Diode D 3 ensures that the relay remains in its rest state, however, by transferring the high voltage level of the collector of T, to the input of N 3 whose consequent low logic out- put continues to hold the transistor off. The equipment switched by the relay contacts, therefore, does not only indicate when the light barrier has been interrupted, but also when the supply voltage has failed. The relay is re-energized when reset switch S, is operated. If D 3 and S, are omitted, the relay is re-energized when the monostable pulse has decayed. Current consumption of the transmit- ter is about 50 mA; that of the receiver around 10 mA. The printed circuit board shown in fig- ure 3 is intended to be cut into three along the dashed lines, although it may not be necessary in some situ- ations to cut the relay section from the receiver section. If the latter two are separated, they should on completion be interconnected by a suitable cable. 1 Aug/Sept i8.33 negative supply converter It is sometimes required in certain cir- cuits that are powered from just one battery to derive a negative supply voltage from the positive battery potential. As the loading of such negative lines is normally pretty minimal, it is possible to use a TL 497 A 1C to provide the negative voltage. This saves a transformer, rec- tifier, and a smoothing capacitor. The TL 497A is a switch-mode 1C from Texas Instruments, that may be used as an upwards/downwards trans- former, but also as a negative supply converter. Inductor L makes it all possible, because when the on-chip transistor is switched off, a fairly large back- e.m.f. is generated across L, which causes a negative potential at the emitter of the transistor. The diode then conducts, and capacitor C f charges. The output voltage, U 0 , is determined by U 0 = \ — U b tJt 0 N where U b is the supply voltage; t, is the time the transistor is switched on; t 0 is the time the transistor is switched off. Period t, is determined by the value of C T . The output voltage is devided across /?1 and R2 and applied to the inverting input of an on-chip comparator, whose + input is a 1.2 V reference voltage. When the actual value of U 0 lies below the wanted value, the com- parator toggles and switches on the oscillator, which in turn drives the transistor. The TL 497A also contains a current limiting circuit which ensures that the coil cannot be saturated and that that transistor is not affected by voltage spikes. Coil L may be any fixed inductor with a value of 100. . .500 pH. The output voltage is calculated from U a = — IN+1.2IV wheie N is the numerical value of R2 in kilohms. The output current should not exceed 50 mA. Texas Instruments Application E D E water-diviner driver stage for the alarm buzzer. The sensor consists of a waste piece of wiring board, about 40 x 20 mm. Connect all odd and all even tracks together with wire links, that is, 1 to 3 to 5, and 2 to 4 to 6. Tin the tracks to protect them against corrosion. When the board is dry, the resistance between the two sets of tracks is high, but when it is wet, the resistance drops sharply. The sensor is in series with resistor R 2 and the two together, therefore, form a humidity-dependent voltage divider, which resets the R-S bistable when input 1 of N 2 goes low. Oscillator N 3 is then switched on, and driver N a energizes the buzzer. The bistable is set automatically on power up via the series combination /?, and C,. The circuit can also be used as a lie- detector. The sensor is then replaced by two lengths of wire of which the ends have been stripped. The bare wires are then placed in the hands of the person being interrogated. If the lies (which causes his hands to become damp) the buzzer will sound. The sensitivity of the circuit is deter- mined by the value of R 2 : some experimenting may be necessary here. The oscillator (and, therefore, the buzzer) is disabled by closing switch S,. metal detector In contrast to the other metal detector in this issue, the present one works on the principle that the frequency of an LC oscillator changes when the inductance is altered. Any metal object brought near the inductor will modify the inductance. The degree by which the frequency changes depends on the nature of the metal and on the frequency. If the fre- quency is very high, a metal object will act as a shorted turn, which lowers the inductance, so that the fre- quency increases. If the frequency is low enough for eddy-current losses to be ignored, it is possible to distinguish ferrous from non-ferrous metals. The inductance required for an oscil- lator frequency of not greater than 200 Hz would be pretty difficult to make, and the oscillator in the present circuit, therefore, works at about 300 kHz. The inductance then needed is quite easy to make and consists of a single turn of coaxial cable as shown in the accompanying diagram. The circuit consists of oscillator T,. frequency-to-voltage converter 1C,, and BiMOS operational amplifier IC 2 . With a detector coil diameter of c. 440 mm, the values of capacitors C, and C 2 ensure an oscillator frequency of around 300 kHz. Smaller diameter coils need more turns. The level of the oscillator signal should be at least 500 mV,,, to be able to drive the 4046B satisfactorily. At that level, the phase comparator ensures that the internal phase-locked loop always locks. The source follower output at pin 10 is fed to a CA3130 where it is amplified substantially. The centre frequency of the phase- locked loop, and, therefore, the zero of the centre-zero microammeter, is set with P,; fine adjustment with P 2 may be necessary if the sensitivity of the opamp is high. That sensitivity is set with P 3 which is connected in the negative feedback loop to the invert- ing input. There is also positive feed- back via the microammeter and R l0 to the non-inverting input. If, therefore, a meter with a different resistance is used, it may be necessary to alter the values of R a , /7 10 , and /?„ accordingly. Note that in treasure hunts the size of the objects sought should have some relation to the diameter of the detec- tor coil: looking for coins with a 440 mm (17.5 in) diameter coil is a fruitless task! ,8.35 S i D simplified word comparator Primarily intended as a trigger source for an oscilloscope in the testing of digital circuits, the comparator is a derivative of the word recognizer and delayed trigger published in the July/August 1981 issue of Eiektor When an 8-bit binary word is recognized during a comparison with a pre-determined value, the present circuit issues a short trigger pulse. In contrast to the original circuit, the present one has no provision for either a delayed trigger pulse or an external trigger input. None the less, the com- parator remains an almost indispens- able aid in the testing of digital circuits. The unit is based on two four-bit com- parators, 1C, and IC 2 . The reference level for them can be set separately with switches S,...S 4 and S B . . .S 8 respectively. With these switches set as drawn, inputs A and B are intercon- nected: this is the don't care position. With a switch set to its centre pos- ition, a high reference level is ob- tained, while when it is set to the extreme right position, a low reference level is obtained. When all A and B inputs agree, the A = B output of IC 2 goes logic high, taken of the transit delay which drawn by the LS241, and 10 mA by Gates N, . . . N 4 suppress short amounts to 24 ns per comparator. In the LS85. This enables the current spurious pulses that arise during the some tests this may lead to an unac- consumption of multiple comparators stabilization of the comparator inputs, ceptable delay if several comparators to be calculated quite easily. The size of the binary word can be are used. Note that each additional 1C must be increased by cascading two or more The current consumption is about separately decoupled by a 100 n comparators. Account must then be 60 mA per comparator: 32 mA is capacitor. ® i S RTTY/CW filter An appreciable part of short-wave radio traffic takes place via morse and radio teletype transmission. To ensure optimum reception of these types of transmission, a practical bandwidth of about 300 Hz is required in the receiver. Such a bandwidth allows for some drift of both transmitter and receiver, and also for the frequency shift of RTTY signals. As commercial- ly available filters meeting these requirements are still rather expensive, it pays to build your own: a suitable one is shown in the accompanying diagram. The crystals used are inexpensive types, commonly found in computer systems. Inductor L y is made by winding 2 times 20 turns enamelled copper wire of 0.3 mm diameter onto a T50/2 RF 8.36 eiektor India Aug/Sepl >985 hours are possible between .exposures. As the signal at the wiper of S 2 is a square wave, which is — by definition - logic 1 for half the time, it is essen- tial that it is shaped in a monostable. The duration of the consequent pulses is determined by P 3 . Their width should, of course, not exceed the period of the clock oscillator. Many film cameras are provided with a miniature socket via which they can be operated for single frame exposures and film transport. Con- tacts X and Y of relay Re, should be connected to this socket via a suitable cable. If you have any trouble with this, or are not sure of the socket con- nections, it is best to seek advice from your local photographic dealers. S § 2 opamp tester All types of operational amplifier can tester has, of course, a self test facility Opamp A 3 functions as the summing be functionally checked with the tes- so that the error-free operation of it stage whose output is fed to two tran- ter proposed here. can be readily ascertained. sistors that drive LEDs. The principle of the tester is quite Opamps A, and A 2 form a triangular The specimens are connected as simple: a triangular voltage is applied pulse generator. Opamp A, operates inverters in either positions to the inverting ( — ) input of the as an integrator: capacitor C, is Ap, . . .Ap 4 or Ap 5 . In the design it specimen. This voltage is, of course, charged, and as soon as the voltage was assumed that the most frequently inverted. If then the inverted and the across it reaches the threshold value encountered opamps are contained in original triangular voltage are added, of Schmitt trigger A 2 , resistor /? 4 is a 14-pin DIL housing (as, for instance, the result should be zero. Any devi- connected to earth, and C , the TL 084 used for A, . . ,7\ 3 I, or in ations from this are taken as a mal- discharges until the voltage across it an 8-pin DIL package (such as the function which is indicated by one of reaches the second threshold of A 2 , LM 355 or LM 3871. For different two light-emitting diodes (LEDs). The when the process repeats itself. packages, the specimen connections elektor indie Aug/Sept 1985 8.37 in figure 1 should be modified ac- cordingly. When a specimen is defect, the out- put of A 3 consists of a triangular voltage superimposed on the (DC) offset. This is sufficient to bias the drive transistors and one or both LEDs flash in rhythm with the triangular voltage. The frequency of that signal is about 10 Hz, and this can be altered to some extent by changing the value of /? 4 and/or C,. It is clear that the voltage at the out- put of A 3 must be greater than ±0.6 V, otherwise the bias for the transistors is too small. Preset P, should therefore be adjusted so that the LEDs just do not light when an opamp that is known to work cor- rectly is inserted in the relevant specimen position. The self test function is easily check- ed: when P 2 is turned from one extreme of its travel to the other, first one LED, then both, and finally the other LED should light. In positions 1. . .4 of switch S„ the four opamps contained in, say, a TL 084 can be tested sequentially: in position 5, the single opamp con- tained in, say, an LM 355; and pos- ition 6 is the self test setting. g S H crystal tester Many electronics hobbyists have crystals lying about, but don't know whether these are still working all right. The crystal tester described here will quickly show whether a crystal can be used or should be discarded. Transistor T, and the crystal under test form an oscillator. Capacitors C, and C 2 form a voltage divider in the oscillator circuit. If the crystal is in good order, the oscillator will work. Its output voltage is then rectified and smoothed by D, and C a respectively. The resulting direct voltage at the base of T 2 is sufficient to switch this The circuit is suitable for use with 100 kHz and 30 MHz. Current con- transistor on, so that the LED lights, crystals of a frequency between sumption is about 50 mA. 8.38 elektof india Aug/Sept 1985 metering selector When just one meter is used to measure the voltage of three different sources, it is, of course, possible to use a three position rotary switch to select any one of the sources. How- ever, care must be taken here, because the switch must break before make, otherwise two sources are intercon- nected and this is normally highly undesirable. Any electronic equivalent of the rotary switch must, of course, also break before make. Unfortunately, tran- sistors have the property of switching on much faster than switching off. For example, a well-driven BC 547 takes a couple of »js to switch off, but far less than that to switch on. The present circuit circumnavigates these potential troubles by using the output level as criterion, whereby a 4028 serves as the referee. The 4028 is a one-of-ten active high decoder which drives one of three transistors, T, . . ,T 3 . Let us assume that T, is on: its collector voltage is low, and so is input A of the 4028. The other two collectors are high, and so are inputs B and C of the decoder. The 4028 therefore sees binary code 6 (110) at its input and this causes pin 6 to go logic high, so that T, is driven hard. When in this condition another key, for instance, S 2 , is pressed, a wrong code, i.e., 4 (100), ensues. Output 4 of the 4028 is, however, not connected, T, switches off, but T 2 is not yet driven. Only after T, has actually switched off, and its collector goes high, does 5 (101) arise at the input of the 4028: T 2 will then be driven. In practice, the voltage at the collector may be used to control a CMOS switch that arranges the change over of the meter or the sound channel. It is also possible to replace the collector resistor by a suitable relay, but this would, of course, introduce even longer delay times (of the order of milliseconds). In that case, the feed- back to the input must be effected by a separate contact of the relay, but there is then, of course, absolute cer- tainty that switching is correct! Another variant is including a resist- ance in each feedback loop and shunting each switch contact by a capacitor. This RC network will ensure a reasonable delay during the change Current consumption of the 4028 is small (CMOS!), while that of the tran- sistors depends on the value of the collector resistors. With values as shown, it amounts to 18 mA for a supply voltage of 10 V. S K a battery charging indicator Sealed 6 V or 12 V lead-acid batteries, under normal charging conditions, are charged at a constant voltage of 2.3 V per cell. The charging current reduces during the charging: when it reaches a value of 10 mA, the battery is deemed fully charged. To check this, you do not need an expensive ammeter. The present circuit uses an LED (light- emitting diode) to indicate when the battery is fully charged. The green indicator LED is connected in the collector circuit of a p-n-p tran- sistor. As soon as the transistor con- ducts, the LED lights. This happens when the voltage drop across resistor R i reaches the forward bias threshold of the base emitter junction (about 0.6-V). When this resistor has a value of 56 Q, a charging current of around 10 mA will cause this drop. To ensure that the charging current can exceed 10 mA, /?, is shunted by diode D, which limits the voltage drop across the resistor to about 0.7 V. The maxi- mum charging current depends on the diode used and lies between 1 and 3 A. The LED does not light when the charging current is less than about 10 mA, i.e., when the battery is fully charged, when the battery is con- nected with wrong polarity, or when the output is short-circuited. The red LED will light when the battery is con- nected with reversa polarity. The indicator should be connected between the charger and the battery. It may either be built into the charger housing, or be constructed in a small case that can conveniently become part of the charging cable. ,8.39 twin bell-push It is often desirable for a single door- bell to be operated by two bell- pushes, for instance, one at the front door and the other at the back-door. The additional bell-push, S 2 , in series with the break contact of relay Re,, is connected in parallel with the original bell-push, S,. When S 2 is pressed, the bell voltage is rectified by D, and smoothed by C v After a time t = R^R 2 C 2 , the direct voltage across C 2 has risen to a level where T, switches on. Relay Re, is then energized and its contact breaks the circuit of S 2 , so that the bell stops ringing. After a short time, C, and C 2 are discharged, the relay returns to its quiescent state, and the bell rings again. In this way, S, will cause the bell to ring continuously, while S 2 makes it ring in short bursts, so that it is immediately clear which bell-push is operated. sound-level indicator This novel indicator is ideally suitable for use in a discotheque. It consists of eight equi-distant columns of eight LEDs arranged in a starlike pattern, so that corresponding LEDs in the eight columns form concentric circles, as shown in figure 1b. The higher the sound level, the more circles light, giv- ing the impression of a star of con- stantly varying brightness. As can be seen in figure 1b, the eight LEDs in any one of the eight circles are connected in series. Each of these series chains is driven by a transistor: T|...T 8 in figure la. Dropping resistors are not required: the positive supply voltage provides just over 1.8 V per LED, which is a perfect value for red LEDs to show up nicely. Transistors T, . . ,T 8 are driven by dif- ferential amplifiers A,...A 8 , which compare the audio-dependent direct voltage across C 2 , which is buffered by A, 2 , with the potential determined by D„ and /?„ . . ./?, 8 . If the result of the comparison is positive, the associ- ated driver transistor is switched on, and the appropriate circle of LEDs lights. The LED in the centre, D 4 , is driven by T 9 , and only lights when the sound level is very low. The direct voltage across C 2 results from full-wave rectification in A, 0 and A, , of the input signal after this has 8.40 eleklor in X V 4^xs ; \ u£C£ \ -« V \ \ \“k^0^aH — J5V / / 3 « **. been amplified in A 9 . The input sensi- tivity is about 600 mV for saturation, i.e., to light all sixty-four LEDs; it can be increased by lowering the value of Ft 2 . The speed with which variations in sound intensity are indicated depends on the value of C 2 : if this is 10 pF, the light pattern changes slowly, whereas when the capacitor is omitted, it reacts instantly to different sound levels. The indicator is constructed on two printed circuit boards (figures 2 and 3). The LED board in figure 3 has not been provided with a component layout because of aesthetic consider- ations. The layout is, however, given on the PCB in figure 4 for those who want to use it all the same. The two boards can be fitted together with the use of spacers: appropriate holes have been provided for this in a manner which ensures that the 11 terminals for interconnections on the boards are opposite one another. An interesting optical effect arises when a sheet of red perspex is mounted in front of the LED board. Refraction in this material causes the LEDs to show up as sources of dif- fused, rather than pinpointed, light. The current consumption of 800 mA at saturation may be reduced by lowering the supply voltage to, say, 12 V, but this will, of course, reduce the brightness of the display. Ri = 270 k Ri\ Rh = 10 k R 3 = 100 k R<- - R8.R19. . .R27 = 15 k Rg = 22 k Rio = 1k8 R11.R12 = 27 k R 13 = 18 k R 15 = 8k2 R16 = 6k8 R17 = 2k2 Ri8 “ 1 k R 28 - 820 0 Pi - preset potentiometer, 250 k Capacitors: C, = 560 n Ci* = 0 ... 10 p/16 V C 3 = 47 p/16 V C 4 . .C 6 = 100 n Semiconductors: Ti...T 8 = BC550C Tg = BC560C D 1 .D 1 = 1N4148 D 3 - zener diode 5V6/400 mW D4. . .D 68 = LED red IC 1 .IC 1 = LM324 IC 3 = TL084 For Components Sources See Page 9-38 1 Aug/Sepl 1985 8.41 temperature regulator with zero crossing switch This temperature regulator can be built without special ICs and may be used with powers up to 3.5 kVA. The circuit is based on a two-point regulator with a thermistor as the tem- perature sensor. As the load current is switched only during zero crossing of the mains, no additional interference suppression is necessary. The series combination R,C, serves to lower the mains voltage to a level suitable as supply voltage for trigger T,. As /?, is small compared with the reactance of C„ the current leads the voltage by nearly 90°. If the ambient temperature is higher than a given value, determined by potentiometer P„ the resistance of /? Ih is low enough to cause T, to con- duct. Silicon controlled rectifier Th, is supplied with gate current and switches on during the negative half When the temperature drops below the negative half cycle, Th 3 switches cycle of the mains, because the cur- the value determined by P„ transistor on. rent through /?,C, leads the voltage. T, and thyristor Th, remain off, so During the positive half cycle, C 2 is When Th, is on, thyristors Th 2 and that Th 2 conducts. As the voltage charged via R, and D 2 , and so pro- Th 3 remain in the blocked state, so across zener diode D, leads the mains vides the gate current to switch on that no current flows through heating voltage, Th 2 switches on when the Th 3 at the onset of the negative half element /? L . remains crosses zero. At the onset of cycle. E @ D economical power supply The power supply described here uses a silicon-controlled rectifier (SCR) that, depending on the load current, selects taps on the secondary of the mains transformer. The output voltage of around 9 V is eminently suitable as input voltage for a 5 V regulator, which consequently works with the absolutely minimum power dissi- pation. With low to medium load currents, the SCR is in the blocking state. Recti- fication of the secondary transformer voltage then takes place in D,, D 2 , D 6 , and D 6 only. The load current flows during the positive half cycle via D,, load, and D 6 ; during the negative half wave it flows through D 2 , load, and D 6 . The tapped secondary voltage amounts to 8 V in either case, while a 2 V section remains unused. With increasing load current, the out- put voltage drops until no current and D 6 are reverse biased. suppressors are therefore necessary, flows any more through the zener As the voltage across the zener diode To build this supply, you need a mains diode. Transistor T ; switches off is always lowest during the zero cross- transformer with a 12 V secondary which removes the short circuit from ing of the secondary voltage, the SCR that has taps at 2 V steps: the gate of the SCR, which then con- always switches on at or near that in- 2-4-6-8-10-12 V. For load currents up to ducts. As soon as that happens, the stant. This prevents high current 1.5 A, a 2 A transformer will suffice; full secondary transformer voltage is pulses and other noise often associ- an output current of up to 2 A requires rectified by D, . . . D 4 , while diodes D 5 ated with SCR switching: no further a 3 A transformer. ESS 8.43 measuring with the BBC micro The BBC micro, one of the best value- for-money computers on the market, can be used for a variety of appli- cations thanks to the various inter- faces provided as standard. The four analogue inputs, each with a resol- ution of 10 bits, make it particularly suitable for measuring all kinds of pro- cesses. There is unfortunately one drawback: the rather poor reference voltage associated with the analogue inputs. That voltage is obtained from three normal diodes connected in series. The alternative described here has been in use in our BBC micro for some LM 336Z Diodes D 6 . . -D s in the diagram pro- vide a reference voltage of 1.8 V, which is fine for use with a joystick interface, but will not do where absol- ute values are to be measured. The three diodes are, therefore, replaced by one zener diode, a 2.5 V type LM336Z. This diode deviates no more than 1.8 mV over the temperature range of 0. . .70 °C; its long-term stability is better than 20 p.p.m. at 25 °C. Its internal resistance is 0.4 Q, which makes it ideal for our purpose. Moreover, it is easily fitted into the micro without the need for any altera- tions other than the removal of D 6 ...D 8 . The micro remains, of course, fully compatible with existing software. Cut off the adjust terminal from the LM336Z, and unsolder D 6 . . .D s from the computer. Solder the anode and cathode of the zener to the cathode connection of D 6 and the anode con- nection of D 8 respectively. A good- quality small soldering iron is indispensable here! NAVTEX receiver NAVTEX, the international maritime service that provides navigational and meteorological information via RTTY (radio teletype) on 518 kHz, makes use of FECTOR. This is a system in which the information is transmitted twice, with a particular interval between the first character and the repeat. FECTOR is decoded automati- cally by a microprocessor that is coupled to the ship's medium wave receiver. It is, of course, not desirable that the decoder is taking up the medium wave receiver continuously. On the other hand, navigational officers, and many amateur radio listeners, do not want to miss one iota of NAVTEX infor- mation. Obviously, a second receiver is the answer, and this can, of course, be coupled to the decoder night and day. Since only one frequency, 518 kHz, and one type of trans- mission, FSK (frequency shift keying), needs to be received, the circuit can be kept quite simple. The circuit is based on a type TCA440. The AGC (automatic gain control) provided by this 1C is not used because the IF amplifier, due to its internal symmetry, is already an excellent limiter for FSK signals. The internal oscillator is not used either: it is replaced by a crystal oscil- lator, T,, operating on 5185 kHz, that is followed by a decade scaler, IC 2 . The exact frequency of the crystal depends on the requirements of the decoder; trimmer C 3 enables it to be varied by a few kHz, i.e. , a few hun- dreds of Hertz at the output. Thanks to the TCA440, the remainder of the receiver is fairly simple without the need of special components. Standard chokes can be used in the 8.44 elektor India Aug/Sept £^3 4 ».I‘ v Calibration is very simple: adjust input Current consumption is not greater trimmers C, and C 2 for maximum out- than 10 mA. The supply voltage may ||i meter amplifier A meter amplifier is intended for use between a sensor or other measuring device, such as a probe, and the indi- cator. It is characterized by a high input impedance, typically 1 MQ, and a differential input. A differential input ensures that the output signal cannot be affected by hum or noise on the meter leads. The input signals are buffered by dif- ferential amplifiers A, and A 2 . The 22 pF capacitors in the C, and C 2 positions obviate any tendency to oscillate. The output of opamp A 3 is a function of the difference between the two input signals. Opamp A 4 serves to compensate for any offset and also to set the amplification at exactly 1. The bandwidth of the circuit as shown is not less than 100 kHz, and the phase shift is 0°. As already mentioned, the amplifier may be used with any sensor, for instance, in computer control of the central heating, or to monitor the ambient temperature in rooms. It can also be used with a multimeter or oscilloscope. The peak-to-peak level of the input signal should not exceed about 80 per cent of the supply voltage. Current consumption is not greater than 25 mA at a supply voltage of ±18 V. Calibrate the unit by adjusting P 2 under no-signal conditions for zero tt output, and setting the amplification to exactly 1 with P,. If you aim at perfection, use 1 per cent resistors. ktor indis Aug/Sept 1985 8.45 RTTY calibration indicator S § 3 To calibrate an RTTY (radio teletype) decoder correctly in accordance with the marks and spaces, an oscilloscope is needed. The mark and space signals are applied to the X and Y inputs of the instrument respectively, when, on correct calibration, the screen of the oscilloscope displays the well-known RTTY cross. If an oscilloscope is not available, the circuit shown here can be used. It consists of two amplifiers with high- impedance input, T, and T 8 , that are followed by driver stages T 2 . . ,T 4 and T 5 . . .T 7 . The driver stages control three LEDs, D, . . . D 3 direct. Diode D, (red) is the mark indicator, D 2 (green) is the space indicator, and D 3 (amber) indicates whether the decoder has been calibrated symmetrically. Preset potentiometers P, and P 2 determine the amplification of the field-effect transistors. Proper setting of these components enables the indi- cator to be matched with the filter outputs of any RTTY decoder. After the indicator has been coupled to the RTTY decoder, that unit can be calibrated as follows: ■ tune the short-wave receiver to the marks; the BFO knob must be adjusted until the red and amber LEDs both flash brightly; ■ the RTTY decoder is then adjusted to the correct frequency deviation, indicated by the flashing of the green LED. If the amber LED lights continu- ously, the decoder has been calibrated correctly. Otherwise, the above pro- cedure should be repeated carefully. thrifty LED indicator It is often necessary that the current consumption of an essential status indicator is minimal. In the circuit shown, dependent on the level of the supply voltage, a number of LEDs drawing a current of only 10. . .15 mA may be switched on or off as desired. Moreover, the entire indicator may be switched off if none of the LEDs lights. The circuit is based on switched cur- rent source T,. The base current of this transistor is set at c. 15 mA with R,. The value of this resistor is calculated from R, = (4x10 6 /(6/ b — 0.7)) Q where U b is the supply voltage in Transistor T 2 conducts when the input to inverter N, is logic 0: when this becomes a logic 1, the current source and, consequently, the indi- cator are switched off. If the input to one of the buffers N 2 . . .N 4 is a logic 1, the associated LED is switched on. More LED-FET combinations may be added to the circuit as long as the supply voltage permits this. Also, the dissipation of T, has to be kept within certain limits. A BC557B can be used for T, over the supply voltage range of 5. . ,18 V. The circuit is intended for CMOS ICs; Note that the buffers must be pow- if devices of other logic families are ered from the same supply as the cur- used, remember to take account of rent source, the different logic threshold levels. o T4 N3 sea faultfinding probe for juPs Anyone who has ever tried to faultfind in a microprocessor system with a test probe will have experienced the uselessness of it. This is because the signals at the address, data, and con- trol buses are constantly - and rap- idly - changing. This means that it is not just the signal level that is import- ant, but also the instant the signals are present. For faultfinding properly, you need a logic analyser, which is capable of indicating several signals simultaneously. If you have no logic analyser, the probe presented here may provide the solution. Strictly speaking, this is nothing more than a bistable multivibrator (FF,|. Data are simply read direct and cause D, to light or stay out, depending on the state of FF, . The bistable only reads at the instant its clock input (pin 3) switches from low to high. The clock signal is thus the key for all measurements carried out with the probe and that means it must be chosen with some care for every test. Suppose you have to check whether a certain portion of memory is all right. The CE signal in the memory is then connected to the QUAL input of the probe. Switch S 4 must be closed, because CE is active low. The probe can then only read data during a"CE of the RAM under test. The CLK input of the probe is connected to the RD signal of the memory. Reading must then be carried out during the trailing, i.e., the positive-going, edge. Switch S, must, therefore, be closed. Resistors: R, = 10 k R 2 = 390 Q Semiconductors: D, = LED, red T, = BC557B 1C, = 74LS00 or HCITI ICj = 74LS74 or HCIT) Miscellaneous: S, . . . S 4 = 4-way DIL switch Reading is effected by, for instance, a PEEK command in BASIC. Diode D, will then light in accordance with the signal emanating from the RAM dur- ing this process. Be careful that this BASIC is not used by the RAM section being tested, because then there will be more than one read process and the probe will only retain the last of these. There is no easy solution in that case, but often it will be possible with the aid of a monitor to make the microcomputer execute only one command in machine language. To keep the probe small, DIL (dual in line) switches are used in the S,. . ,S 4 positions. Note that only S, or S 2 and S 3 or S 4 should be closed simul- taneously at any one time. LS type ICs may be used, but as these put a relatively high load on the circuit during tests, HCT types are better. These are fully compatible with the LS types but have high impedance inputs. HC types should only be used where systems are already executed entirely in CMOS; the supply voltage can then be higher than 5 V. Current consumption of the circuit is small: 10 mA for the LED and 5 mA for the ICs (if these are TTL). 9-38 " “ ■ programmable baud-rate , „ generator Only some computers, e.g., the Sam- son 65, enable you to reprogram the ACIA (asynchronous communications interface adapter), or whatever your serial interface may be, if you want to connect a printer and a modem to your computer. With most other micros, you have to use an additional circuit like the one proposed here. The circuit is based on a presettable, synchronous down counter, a CMOS 1C type 40103. Another CMOS 1C, type 4060B, serves as a crystal con- trolled Clock generator. The crystal fre- quency, f x , is 2.4576 MHz, while the clock, f c , is 153.6 kHz. The output frequency, f 0 , of the generator is determined from f 0 = (153.6/ (A/ + 1)1 kHz where N is the decimal equivalent of the number that is input to the J0. . . J7 terminals of the 40103 (see table). The number /Vis provided by the com- puter and from there written into, and stored by, the 74LS374. The table gives various baud rates (also for RTTV — radio teletype) and the corre- sponding decimal and hexadecimal numbers. The address decoder in the circuit diagram is arranged for a Z80 com- puter, as can readily be seen from the control signals, but this is purely taken as an example. The signal from the data bus is applied to t he 74LS3 74 at the leading edge of the STROBE pulse at the output of the decoder. The articles address decoding and memory timing in the February and March 1984 issues of Elektor respect- ively contain all the necessary infor- mation for the design of an address decoder for any type of computer. The address decoder in the diagram is shown for the decoding of the hexa- 1 S N 4 256 OUT 240, N T1TZ electronic VHF/UHF aerial 111 switch There are many situations where it is useful, or downright essential, to be able to switch between two VHF/UHF aerials at the aerial mast without introducing losses in the signal paths. The switch proposed here does all this over the usual coaxial down lead. The switch and its small associated power supply are fitted near the rel- evant receiver. The power supply, con- sisting of a small mains transformer, a rectifier diode, and a three-pin voltage regulator, provides a direct voltage of 5 V, the polarity of which can be reversed by DPCO (double-pole change-over) switch S,. The poles of the switch are connected to the co- axial cable via decoupling network L r C,. Resistor /?, serves as a current limiter for p-i-n diodes D, and D 2 . Whichever of these diodes conducts depends on the polarity of the voltage across the coaxial cable. The signal from the aerial connected to the con- ducting diode is passed to the input of tuner or receiver, while the other signal is blocked. A p-i-n diode is a semiconductor diode that contains a region of i-type semiconductor between the p-type and n-type regions. They are Invariably used as switching diodes. Their most important property is a very low self-capacitance, while at high frequencies they are virtually purely resistive (see Elektor, July 1983, p. 7-26). Choke L 3 is made from four turns enamelled copper wire of 0.3 mm dia. around a ferrite bead. If the aerials have no 75 Q termination, this may be provided by L , and L 2 which convert the 300 Q balanced aerial impedance to the asymmetrical 75 Q required by the receiver input. These inductors are made by winding 7 turns of two-core flat cable on a T50-2, T50-3, or T50-6 toroid as shown in figure 2. If the switch is mounted in the open, it should be well protected from the elements: potting in araldite is best. audio transfer equalizer Limiting the bandwidth of an audio system to 20 kHz affects the behaviour of the system in the pass band. The steeper the filter characteristic, the greater the phase shift in the pass band. That phase shift stands in non-linear relation to the frequency, and this causes a frequency-dependent delay of the signals (increasing with frequency from about 4. . .6 kHz). This effect is audible. The CD (compact disc) player is an example of a system in which the bandwidth has been so limited. Particularly the Sony CD player and its clones suffer from a frequency- dependent transfer time. The Philips (and Philips-derived) system does not suffer from this effect. The effect can be negated by introducing a delay in the transfer time of the frequencies below 4. . .6 kHz, which equalizes the delay over vir- tually the entire audio range. In other words, transfer of all audio fre- quencies is carried out at the same speed as it should. Such a delay is realized by phase shifter A 2 (left-hand channel) and A 4 (right-hand channel) in the ac- companying figure. The maximum delay for the lowest frequencies is 2/? 5 C 5 = 2 R 6 C 6 = 36 M s. The circuit is connected between the output of the CD player and the AUX or CD input of the main amplifier. 15V(+> 15V& A1 ,A2 = IC1 ■ NE5532N A3.A4 = IC2 - NE5532N rlHT ,8.49 It is often required to switch an elec- tric light or apparatus from various positions in a building. A typical example of this is the hotel switch, which makes it possible to control lights from a number of positions. With some electronics and electric wiring, the number of switching pos- itions may be extended ad infinitum. The actual switching is effected by a relay that is controlled by an R-S bistable, N 2 /N 4 , via transistors T, and T 2 . The state of the bistable is of import to the position of logic switches N, and N 3 . A trigger pulse at the junction of R, and C, is only applied to that input of the bistable which causes the bistable to toggle. In other words, a train of trigger pulses, 0;1;0;1;0. . ., with a minimum interval between pulses of a few seconds, results in a series of logic level changes which causes the relay to be actuated and de-energized alternately. The trigger pulses arise when one of the push buttons, S, . . . S„, is pressed briefly. The push buttons are all con- nected in parallel, so that they can be interlinked by a two-wire system. It would be possible to fit an LED at every switch position, but this would entail an additional wire. Such LEDs would, of course, also be in parallel, so that it is advisable to use similar The value of resistor R m is calculated /7 )0 = HU - 2)// 0 n) Q where U is the supply voltage in volts; / D is the current through each LED in A; and n is the number of LEDs. B 0 @ four position touch dimmer Any electric light may be adjusted with this dimmer to very low, low, medium and maximum, which in most cases will be sufficient. After all, it is all very well to be able to control an electric light over the whole range of its brightness, but how often is that facility really used? Moreover, in every- day use, position control has practical advantages: setting, for instance, takes a second or two. The circuit is based on an LS7237 and some discrete components. The dim- mer may also be used as an electronic on/off switch, in which case the mode select pin (7) must be connected to earth (pin II. Such a switch does not produce sparks and consequent noise in nearby electronic equipment. 8.50 elektor India Aug/Sepl 1985 Another possibility is leaving pin 2 open, whereby a three position dim- mer ensues: low, medium, and maximum. The LS7237 has all the necessary facilities to drive silicon-controlled rec- tifier (SCR) Tri,. Resistor R 2 and capacitor C 4 filter a 50 Hz signal from the mains that serves to synchronize the on-chip phase locked loop. Network /?„ C 2 . and D 2 provide the supply for the LS7237, while filter 1-,/C, prevents excessive noise from reaching the mains supply. Different types of triac may be used, as long as these can provide the required current, and are suitable for operating voltages of not less than 400 V. For safety's sake, no deviations from the stated voltage ratings of the various components should be tolerated. The two 4M7 resistors pro- vide ample safety for the user: under no circumstances should these be replaced by a single 10 MQ resistor! The complete circuit is small enough to be accommodated in the pattress or plaster box of a light switch. simple video inverter for ZX81 The inverter must be connected before the TV modulator in the ZX81. Switch S, enables bypassing of the inverter when inversion of the picture is not required. The composite video signal is inverted by gate N,. Gates N 2 and N 3 separate the sync signal from the input: the sync signal is then available at the output of N 3 at a level of 5 V op . The inverted video signal and amplified sync signal are then added again, resulting in an inverted video signal with the sync signal in the correct position and at the right level. Preset P, serves to adjust the contrast. The circuit can be constructed on a piece of veroboard so small that it can easily be added in the ZX81 case. The power supply can be taken from 1C, in the ZX81: +5 V at pin 40 and earth (0 V) at pin 34. :8.51 The excellent properties of counter/divider 1C type 4059 have, so far, not been given the prominence in Elektor they deserve. One of these properties is the provision of divide ratios anywhere between 3 and 15 999 depending on the logic level at inputs J, . . . J le and the setting of switches The 4059 is clocked by a relaxation oscillator, N,-N 2 , which could have been a crystal-controlled type instead of the one shown in figure 1. The dual-D bistable type 4013 at the output is essential because the width of the pulses at pin 23 of the 4059 is comparable to the clock frequency. The bistable ensures that the pulses emanating from pin 23 are reshaped into rectangular form. The Q output of the bistable is, of course, half the fre- quency of the wave train at pin 23 of the divider. N1, N2 = 54 IC1 =4093 FF1 = V> IC3 = 4013 Inputs J, . . . J, 6 of IC 2 are divided into groups of four. The binary information at these inputs is called T A ...T 0 . Inputs J, . . . J 4 are further sub-divided into D, and D 2 . In total there are, therefore, five data inputs, of which the smallest, D 2 , is only one bit wide. Furthermore, the 4059 has three mode control inputs, K a . . .K c , whose com- position results in a factor K as shown in table 1. If K=10, input D, becomes four bits wide: J 4 then forms part of D,l The divisor, n, is then calculated n =10 3 T o +10*T C +10 T b + D, In all other cases, n = K(10 3 D 2 +10*10 +10T C +T B ) + D, The 4059 can be programmed by com- puter, by hand, or with the aid of an up-or-down counter. The generator can be used in elec- trophonics, in measuring techniques, as timer, and even as a digital phase- locked loop frequency synthesizer in FM tuners. The circuit operates from a 4. . .15 V supply and current consumption is absorption-type metal detector The action of the detector, which indicates the presence of ferrous as well as non-ferrous metals, depends on the absorption of magnetic energy. An inductor, which forms part of a tuned oscillator circuit, radiates a magnetic field. When a metal object is introduced into this field, enough magnetic energy is absorbed to cause the oscillator to stop working. The Colpitt's oscillator in figure 1 operates at a frequency of around 70 kHz. Inductor 4, also serves as the sensor. Because of the high value of the emitter resistor, ft,, the oscillator only just operates. This is desirable, otherwise any losses in the tuned cir- cuit would easily be replenished by the transistor. The oscillator output is rec- tified by D, and D 2 , and the resulting direct voltage is applied to the invert- ing input of Schmitt trigger 1C,. If that voltage drops below the level at pin 3 (preset by P,), the output becomes logic high, and the relay is energized. The detector is best constructed on the printed circuit board shown in fig- ure 2 (this is, unfortunately, not available ready made). Inductor 4, is not intended to be fitted on the board. This is a standard non-screened choke of 100 mH. If the oscillator does not readily start at any setting of P„ the value of ft, must be reduced. If, on the other hand, the oscillator does not stop working when a metal object is held near 4,, the value of /?, must be increased. The stated value of /?, has been found right when 4, is a Toko type. Starting with the wiper of P, to earth, adjust the preset so that the relay just does not operate. If a lower sensitivity is required, advance the wiper slightly Miscellaneous: L, = choke. 100 mH, non-screened IToko) Re, = relay, 12 V, coil resistance not smaller than 240 Q Current consumption is determined primarily by whether the relay is energized or not; in any case it is not greater than 50 mA. ;8.53 “ morse training with the j ° g Junior Computer J Sgonina Here is yet another small program to be added to the large amount of soft- ware already available for the Junior Computer. It is intended to teach pro- spective short wave listeners to read morse code. The program can be used even with the basic version of the JC. The only additional hardware is the amplifier stage shown in the accom- panying figure. The input to this is taken from port line PB5. The number and speed of the morse characters can be predetermined. After the program has started, the JC will generate 1 to 6 morse characters, which the trainee should decode and write down. The letters corresponding to the generated characters appear on the display after a short delay, so that the trainee can check his decoding with the actual text. During this phase, the computer is on stand by until an arbitrary key, other than ST and RST, is pressed. The hex dump given is sufficient to write the program into the JC. Once that has been done, you can prepare the start, but the program needs the following information before it can ■ in address 0010 write data 00. . .05; ■ in address 0011 write data 01 ... 55 (max); ■ in address 0014 write data from table 1 for the first character to be generated minus 1; ■ in address 0015 write data from table 1 for the last character to be generated. Now, the program can be run; it starts in address 0020 when key GO is pressed. Programming example: the JC is to generate morse characters for the letters B to G. Before the start, the folowing data should be written: ■ in address 0010 - data 05 ■ in address 0011 - data 55 ■ in address 0014 data 02 ■ in address 0015 - data 07 0 1 2 3 4 5 6 0200: A9 FF 8D 83 1A 8D 81 0210: 85 05 18 A5 11 65 11 0220: 02 A8 B9 CB 02 95 00 0230: 06 21 B0 07 AS 11 85 0240: 8D 02 C6 20 D0 EA A5 0250: D0 F8 CA 10 C9 20 6F 0260: 20 AC ID F0 F0 20 6F 0270: 48 A9 FF 8D 81 1A 8D 3280: CE 7C 02 10 F6 A9 05 0290: 83 1A EE 82 1A D0 FB 02A0: 40 D0 FC C6 13 D0 F8 02B0: ED 85 E8 A2 04 B5 E8 02C0: C5 14 30 E6 85 30 68 02D0: 0E 42 09 7A 72 0A 47 02E0: 41 01 36 11 64 79 24 02F0: 84 A4 83 01 24 C3 04 0300: 43 03 81 23 14 63 94 0310: E5 F5 FD 789ABCDEF 1A 85 01 85 02 85 03 85 04 65 11 85 12 A5 10 AA 20 A8 B9 EF 02 85 21 29 07 85 20 13 4C 3F 02 A5 12 85 13 20 12 85 13 C6 40 D0 FC C6 13 02 20 AC ID F 0 F8 20 6F 02 02 C6 40 D0 F9 4C 00 02 8A 83 1A A2 08 A5 04 20 E3 ID 8D 7C 02 68 AA 60 A9 FF 8D C6 13 D0 F7 A5 11 85 13 C6 60 8A 48 38 A5 E9 65 EC 65 95 E9 CA 10 F9 C5 15 B0 EA AA A5 30 60 08 03 27 21 06 48 2B 23 0C 18 2F 52 07 63 30 19 12 02 78 00 10 40 42 02 74 A3 44 C2 82 E3 64 D4 B4 C4 7D 3D ID 0D 05 85 C5 As soon as these data have been writ- ten, the program starts when key GO is pressed. The hex data for the letters of the alphabet and numbers 0 ... 9 are given in table 1. The most important addresses are given in table 2. E 0 u QL RAM extension Sinclair's QL has as standard a 128 K RAM, which sounds like a lot in com- parison with most 64 K machines. Unfortunately, the software writers, in the knowledge that there is more than enough memory, have been rather wasteful in their work, so that at the end of the day, there is not all that much more in the QL than in the 64 K machines. So, you need more memory. . . The accompanying circuit is an appli- cation of the TMS4500A as RAM extension for the 68008. This chip can drive a maximum of 128 K dynamic RAM and provides virtually everything: mul tiplex in g o f the address lines, RAS, CAS, and REFRESH. The memory ICs are 64 K x 1 (128 or 256 refresh are both permitted) and have a speed of better than 150 ns. Since the QL uses a clock frequency of 7.5 MHz rather than the normal 8 MHz. such a RAM can run without wait cycles. An 8 MHz CPU that regularly has to carry out a wait cycle is appreciably slower than a 7.5 MHz type! The 68000 family is provided with a data acknowledge input. As with other processors, the CPU places addresses and data onto the bus and indicates the validity with an address strobe and data strobe respectively. It continue s to do so until the memory sends a DTACK signal. The present extension generates this signal with the aid of the LS156. Normally, this acknowledgment is given almost immediately, but it may happen that the 4500 is in the middle of a refresh. In that case, the CPU has to wait, which is arranged via the ready output (pin 2). To prevent the QL waiting forever when an addre ss is rea d that has no memory, the DTACK is generated internally: this must, however, be disabled for addresses where the RAM extension is located, and for- tunately this can be done easily via DSMC. By making this logic high as quickly as possible, the internal DTACK is cancelled. If you cannot get the 2N2905 transis- tor. you may use a BS250, in which case resistor /?, can be omitted and R 2 should be replaced by a wire link. The circuit as shown is for the 128 K version. It is also possible to omit the eight RAMs connected to RAS1 and make a 64 K extension. Input A of the LS138 must then be connected to A 16 and pin 11 instead of pin 13 must be used as CS . There is no 5 V supply available on the connector, but there is a 9 V line. This can be reduced to 5 V by a standard 7805. The current drawn depends on the types of RAM and will be 200... 300 mA. It is important to decouple the supply lines properly: each RAM 1C and the 4500 require a 100 n capacitor! E 3 0 model aircraft monitor Older, i.e. , not using a computer, radio controlled model aircraft are highly vulnerable to breaks in radio com- munication, which can lead to a crash or the model landing out of reach, or both. Owing to the allocated radio fre- quency range being usurped by pirates, its is essential for every model flyer to make sure that the channel to be used is free. Even if it is, it is advisable to continue monitoring it. In combination with a short-wave receiver, the circuit presented here enables monitoring the 27 MHz radio control band. The aerial signal is filtered (26. . .41 MHz) and applied to the input of differential amplifier T,-T 2 . Since the current source of this stage consists of an oscillator, the amplifier functions as a mixer. The crystal oscillator can operate with almost any crystal between 2 and 32 MHz. The output circuit, £ 4 -C 5 -C 6 , is tuned to about 27.2 MHz. This fre- quency is inversely proportional to the values of the coil and capacitors. The values of the crystals are based on a 40-channel set-up. In switch pos- ition A, the circuit functions as an aerial amplifier; in position B, chan- nels 38. . .49 are converted to 8. . .19; in position C, channels 50... 53 are converted to 20. . .23; and in position D, channels 61 . . .79 are converted to 21 . . .39. coupled need not be suitable for FM on FM by detuning the monitor a few The receiver into which the monitor is reception: an AM receiver can work kHz. E 2 E RS232 interface This circuit is intended as an interface between the Elektor modem ( E/ektor , November 1984) and a computer. The software for each individual computer must, of course, be written separately. Since the writing of a terminal program can only be carried out in machine language, the interface can be kept quite simple. Signals at TTL level are sufficient to operate the modem and LS05 buffers are therefore used. Complete address decoding of the 6551 is ensured by IC 2 and IC 3 so that only four locations in the memory are required, and these should be available on virtually any computer. The fourteen common address bits are selected with S,...S 14 : a closed switch represents an address bit or 0. The input buffers are standard RS232 line receivers so that they can cope with any voltage 8.56 elektor indie Aug/Sep: 1985 levels that may be present on an RS232. The interface is also suitable for con- necting a serial printer to a computer, provided it can operate from TTL levels, which normally is the case. The accompanying tables show some of the possibilities of the 6551 and are intended as an aid in the writing of the terminal program. VLF converter Strictly speaking, the VLF (very low frequency) band stretches from 3 kHz to 30 kHz, and the LF (low frequency) band, often called the long-wave- band, from 30 kHz to 300 kHz. The converter described here covers the frequency range 10... 150 kHz and falls, therefore, half-way between being a VLF and an LF converter. Frequencies between 10 kHz and 150 kHz are converted to 4.01 . . . 4.15 MHz which can be fed to any short-wave receiver capable of accepting those frequencies. The con- verter is connected to the aerial input of the receiver via coaxial cable. Many converters suffer from break through of the mixer/oscillator fre- quency in the output signal, which is normally caused by the mixer being asymmetrical. Because of that, the present converter uses the well- known S042P frequency changer, the symmetry of which can be set accurately with a 1 k preset poten- tiometer connected between pins 10 and 12. To prevent reception of image fre- quencies, the aerial signal is first applied to an LC band-pass filter, before it is fed to the frequency changer. The output of the frequency changer (pin 2) is applied to an LC circuit that is tuned to the frequency range 4.01 . . .4.15 MHz. This circuit, con- sisting of a 100 pH inductor in parallel with a 100 n capacitor and a 60 p trim- mer, effectively suppresses any spurious signals produced in the fre- quency changer. The 60 p trimmer is used to tune in to the desired transmitter in the 10... 150 kHz range (loudest recep- tion)). The symmetry of the frequency changer is set by tuning the short- wave receiver to the frequency of the quartz oscillator, i.e., 4.00 MHz, and then adjusting the 1 k preset for minimum output from the converter, that is, minimum deflection of the S meter, or other field strength indi- cator, on the receiver. During this cali- bration, the input of the frequency changer, point A in the diagram, should be short-circuited to earth. All inductors are standard RF chokes. The value of the output inductor, 12 pH, is not critical. The aerial should be as long a wire as possible. 5! S D power supply sequencing for opamps Most designers know that many prob- lems may arise between the paper design and the practical realization of that design. We are, of course, no exception, and one incident that we experienced recently illustrates a prob lem that is of interest to pass on. Measurements were being carried out on a circuit that contained some type NE 5532 opamps which were powered from a +12 V symmetrical supply. When the circuit was switched on, it did not function correctly. Measuring the supply lines revealed that the positive supply was —0.6 V instead of + 12 V. When the +12 V line only was switched off and immediately on again, the malfunction disappeared. Switching off the mains and immedi- ately on again made the defect reap- pear. Using new opamps made no difference. After some research in relevant literature, it appeared that on switching symmetrical power supplies temporary polarity reversal may occur. Because of the complex internal structure of integrated circuits, it may happen that this polarity reversal causes parasitic components on the chip to be actuated which places the 1C in a stable but malfunctioning state. The book we consulted. Intuitive 1C Opamps, suggests that the mal- function we experienced was prob- ably caused by a parasitic thyristor being triggered owing to the negative supply not rising fast enough. The remedy proposed was to connect two diodes across the supply lines as shown in the accompanying figure: these diodes effectively prevent po- larity reversal. This simple remedy certainly cured the malfunction in our circuit and is prob ably the simplest protection circuit in this issue. Literature: intuitive iC Opamps by Thomas M Frederiksen National Semiconductor Corporation S3? automatic switch off If you are one of the many who fre- quently forget to switch off their digital multimeter, this circuit is for When this little circuit, which is in- tended to be incorporated in the multimeter, is switched on, capacitor C, is connected to the +9 V line via D, . Since (7, is discharged, the gate of T 3 is also at +9 V which causes T 3 and T 2 to conduct. The meter is then switched on. Capacitor C, slowly charges via R v After about 2 or 3 minutes, the poten- tial at the gate of T 3 becomes too low to keep the FET in conduction. Tran- sistor T 2 then also switches off, and the battery is disconnected from the multimeter. Transistor T, ensures that when the multimeter is switched off manually, capacitor C , is discharged. When the multimeter on/off switch, S,, is opened, a base current will flow to the negative terminal of C, via R, and R 2 . Transistor T, then conducts and discharges C v The circuit is thus immediately ready for use again. Without T,, there would have to be a delay of a few minutes before the cir- cuit could be switched on again. The circuit is best built on a small piece of vero board and then fitted between the on /off switch and the meter itself. A final tip: T 2 could be replaced by a Darlington, such as a BC516, in which case a 1 MQ resistor would have to be inserted in the connection to the drain of T 3 . This arrangement would have the advantage that the BC516 is more easily obtainable than the BS250, but the disadvantage of causing a slightly larger voltage drop across the circuit: 0.8 V as compared with less than 0.1 V when a BS250 is used. The cur- rent in both cases is 10 mA. mains wiring locator The accompanying circuit shows a simple means of locating current- carrying conductors. The detector coil is a telephone pick-up with suction pad. The magnetic field of a current- carrying conductor induces a very small voltage in L , that is amplified in opamps A, and A 2 . Capacitors C 2 . . -C 5 have a value which ensures maximum amplification in A, and A 2 of signals around 50 Hz. Diode D, will light during positive halve-waves of the mains current. :tor indla Aufl/Sep ,8.59 S 1 2 direct reading digitizer The computer to which this digitizer is voltmeter. The input range of the 1C The 3-digit information at the output coupled reads a 3-digit number that is stretches from -99 mV to 999 mV: of the 3162 is multiplexed. The data a direct representation of the the resolving power is, therefore, 1098 can, for instance, be written into the measured voltage in millivolts. units. In other words, this converter micro via seven PIA (peripheral inter- The analogue-to-digital converter is offers a resolving power that is better face adapter) input lines. That means, an RCA type CA3162, which was than that of a standard 10-bit device however, that some machine designed for use in a 3-digit digital for the price of an 8-bit device. language is required to be loaded into 5 V 8.60 elektor India Aug/Sapl 1985 the RAM every time the converter is to be used. The present circuit uses hardware to obviate this difficulty. The 3-digit information, which is emit- ted every 20 ms, is automatically loaded into three 4-bit buffers, IC 8 , IC 9 , and V2 1C i 0 , whose outputs are connected direct to the data bus. Each of these buffers has its own address. Writing the converted value into the computer has become simply a matter of reading the three memory locations, which can be carried out by PEEKS in BASIC. The address decoder consists of IC 3 . . ,IC 6 and 1C,. The present circuit occupies a block of eight addresses of which only the first four are used. When the first address is read, monostable IC„ is started, which causes 1C, to commence the conversion process. When the monostable returns to its stable state, 1C, goes to the HOLD mode, and the measured voltage can be read. An interval of not less than 50 ms is required between the start of the con- version process and the reading of the The eight successive memory locations required for the digitizer may be placed anywhere in the memory range by means of the open inputs of gates N 6 . . . N, 6 . If any of these inputs is connected to +5 V, the relevant address line becomes logic 1; if the input is linked to 0 V, the address line goes logic low. Assuming that the decoding has been set to address $E300, the first address is read with a PEEK, which starts the conversion. Wait for 50 ms. Write the data from address SE301, which is the least significant bit ( LS B ) , i.e., the extreme right-hand digit of the 3-digit number. Then write SE302 and finally $E303. At each of these transfers, an AND action must by car- ried out with 00001111 (binary) or 15 (decimal), because only the four lowest data bits are of import. If the converted voltage during the further processing of the three written digits is negative, this is indicated by the data at address $E303, which is 10. Overflow is also easily recognized: if the value read from address $E301 is 11, the voltage is greater than 999 mV; if the value is 10, there is a negative overflow. The small BASIC program given here is an example of a possible conversion routine for the Junior computer. The circuit as shown can be used with a 6502 pP; if it is required to be used with a Z80, RD must be connec ted to th e R/W l ine via an inverter, and IORQ or MREQ is put onto the 02 line via an inve rter. Th e choice between IORQ and MREO depends on whether the digitizer is located in the I/O or the memory range respectively. Take care during the construction to keep the connections marked with an asterisk (0 V and +5 V to 1C,) as short as possible. These lines go together to C 3 and C t , from where the connection is made to the 0 V and + 5 V lines of the digital part of the cir- cuit. Keeping these lines short prevents possible interaction between the analogue and digital parts of the circuit. Four inputs of IC, 0 are not used in the present circuit, and they can, therefore, serve as four additional digital inputs. During the reading of address SE303 (in the example), the highest four data bits indicate the state of these four inputs. 10 A= 14* 14*3*3)! 14*2: REM ADDRESS *E300 20 B=PEEK(A) : REM START COWERS ION 30 FOR T=1 TO 15: NEXT: REM DELAY 40 X=PEEK(A* 1) AND 15 50 Y=PEEK(A*2) AND 15 40 Z=PEEK(A*3) AND 15 70 S=1 80 IF Z=10 THEN Z=0: S=-S: REM SIGN IS NEGATIV IF Z=10 90 AD=SX< lOOSfZ* 109EY+X) 100 IF X=ll THEN PRINT ■ POS.OVERFLON 'j CHR$<13);: GOTO 130 110 IF X=10 THEN PRINT 1 (CG.tWERFLON CHR$(13);: GOTO 130 120 PRINT' l£ , sAD;’ wU ' ; CHR$( 13) ; 130 GOTO 10 a 3 a discomixer This mixer is a typical example of the way modern components can, and do, simplify the realization of good quality audio circuits. In the given configuration it is eminently suitable for use as a discomixer, but the number of input channels can easily be enlarged. As can be seen in figure 1, in its basic form the mixer has four input chan- nels. These could, for instance, serve as inputs for a microphone, stereo pick-up, and cassette player or tape recorder. The power supply has been kept as simple as possible; if it proves difficult to obtain the XR4195 regulator 1C, it may be replaced by a combination of a 78L15 and 79L15. The transformer is preferably of the PCB type to keep the mixer as compact as possible. The values of C, and /?, are depen- dent on the type of microphone used. If this is a high-impedance type, the 1 values should be 470 nF and 22 kQ respectively, whereas with low- impedance types, 10 pF and 680 Q are required. Unfortunately, miniature bipolar elec- trolytic capacitors (C,, C y , C 9 , and C 9 -l are not yet available everywhere, although they are almost indispens- able in applications such as described here. Standard electrolytics may be used with maximum reverse voltages of 1 V, but their use introduces distor- tion and premature ageing (because of the reverse polarity). Provision has been made on the printed circuit board for up to four channels. Two or more PCBs may be connected together; the output and supply sections may then be cut off as required. Current consumption is about 10 mA per channel. 2 79L15 78L15 8 8 79L15 78L15 Table 1. loudspeaker protection There are many ways of protecting loudspeakers against the switch-on 'plop': many of these rely on a clamp circuit across the power amplifier input to hold this at 0 V for a few seconds after switch-on. Others, like the one suggested here, depend on a relay to switch off the loudspeakerls). Terminals A and B of the circuit in figure 2 are connected to one of the sensing circuits in figures la. . If, of which the pros and cons will be dis- cussed shortly. Whichever of these circuits is used, A is shorted to B immediately the power is switched on. This cuts off transistor T1 instantly, which causes capacitor Cl to charge. After a few seconds, the voltage across Cl causes zener D2 to break down. Transistor T2 and T3 then con- duct; the relay is energized, and the loudspeakers are connected in circuit. When the power is switched off, T1 conducts and this causes Cl to dis- charge very rapidly. The voltage across Cl quickly drops below the breakdown level of D2; transistors T2 and T3 are cut off, and the relay returns to its quiescent state, which disconnects the loudspeakers. Input circuit la relies on a light- dependent resistor (LDR) fitted close to the mains on indicator lamp. When the lamp lights, the resistance of the LDR drops sharply, so that terminal A is virtually shorted to B. The input in 1b relies on a reed relay connected to the secondary winding of the mains transformer. As soon as the mains is switched on, the relay contacts close. The third possibility, shown in 1c, is that the mains on/off switch has a third contact that connects A to B when the mains is switched on. A further option is illustrated in Id, where a transistor is connected to the secondary of the mains transformer via a diode and resistor. The transistor conducts when the mains is switched The inputs in 1e and If also provide power for the protection circuit. That in 1e has a bridge rectifier connected across the secondary winding of the mains transformer. When the mains is switched on, the BC 547 conducts and shorts A to B. Finally, the circuit in If is connected direct to the mains. Here again, as soon as the mains is switched on, the BC 547 conducts and terminal A is shorted to B. Whichever of the input circuits is used depends on circumstances and/or individual preferences. If one of cir- cuits la... Id is used, a separate power supply is required for the pro- tection circuit. As suggested, the out- put voltage, 4/ v , of this should be 40. . . 60 V d.c. For lower values of (J v , the rating of D2 must be reduced accordingly. Resistance R w depends on the relay used, and is calculated from /?„ = l<6/„ - U, - 2.5)//,] C where U, and /, are the operating voltage (in volts) and current (in amperes) of the relay used respect- The relay contacts must be able to carry a large current: 10 A is not un- usual in many amplifiers. The rating of /?„ is 16//,] W. If the 'plop' is still heard, increase the value of R3 as required — in reason- ably small steps. ■ India Aug/Sept 1986 8.65 s a s fast opto-coupler The opto-coupler in the normal com- mon emitter circuit at the output of a phototransistor is invariably too slow for use in data communication. Its great advantage remains, of course, the excellent isolation between transmitter and receiver. To retain the advantage, the phototransistor has been integrated into a cascode circuit, as shown in figure 1. The photograph illustrates data transfer in a conventional circuit (top) and in the cascode circuit - the fast opto-coupler — (bottom) at a fre- quency of about 30 kHz. The cascode circuit's faster operation is due to the transistor's internal Miller capacitance being of no consequence as the collector voltage remains con- stant. The result is a faster transistor. The base of T 2 is biased at about 1.5 V by voltage divider /?,//? 2 . Capacitor C, ensures that, even with rapid fluctuations in current, this voltage remains stable. If you consider T 2 as an emitter follower, it is clear that the collector of T, is always pro- vided with a constant (direct) voltage, and this causes the Miller (base-to- collector) capacitance to be inactive. A disadvantage of the fast opto- coupler is that its output signal does not go down to 0 V but at best to 1 V. TTL devices like this just as little as they do a supply voltage of 12 V. \f\f\T Take care during experimenting not to exceed the maximum LED current (in the TIL 111) of 100 mA (this is the reason for dropping resistor /? v ). The value of /?, is calculated from /?. =l(t/„— 1.5)// l60 IQ where U„, is in volts and / LED in amperes. E 1 0 simple field strength indicator A practically proven small circuit that is very popular with many model fliers, as it enables them to verify that their remote control transmitter is actually transmitting. Any doubt as to whether a fault lies in the receiver or transmit- ter is also quickly resolved. The only active element in the circuit is a transistor that is used as a con- trolled resistance in one of the arms of a metering bridge. The base of the transistor is connected to the wire or rod aerial. The increasing HF voltage out of equilibrium. A current then transistor. The meter should be zeroed at the base of the aerial drives the flows through ff 2 , the mA meter, and with P, before the transmitter is transistor so that the bridge is brought the collector-emitter junction of the switched on. E 3 0 active rectifier without diodes The active rectifier proposed here is based on the property of an oper- ational amplifier that its output cannot become negative if its power supply is asymmetrical. We have used an RCA type CA3130 opamp which is eminently suitable, because it can cope with input voltages down to 0 V, and has a CMOS output stage that 8.66 alekloi India can also work down to 0 V. With a supply voltage of 15 V, the maximum input level is about 1.2 V lms . The frequency range, for not more than 1 dB change in output, extends from DC to just over 25 kHz. Negative half cycles at the input of the opamp are inverted and amplified by a factor /? 2 //7, . Positive half cycles are also inverted, but, as stated, the out- put of the opamp cannot become negative, and it therefore remains at 0 V. The positive half cycles are also applied to the output of the opamp via a resistive divider, /?,-/? 2 -/? 3 -P 2 . The result of all this is that only positive half cycles are present at the output, just as if full-wave rectification had i Aug/Sept taken place. If the asymmetry of the supply is set correctly with P 2 , the peak values of the inverted negative half cycles and the positive half cycles Preset P, should be adjusted to give zero output when the input of the opamp is connected to earth. The rectifier has a low-impedance input (source impedance should be not greater than 100 Q) and a high- impedance output (load impedance should be not less than 1 MQ). If these requirements as to source and load impedance cannot be met, the values of /?, and/or /? 3 should be modified: /?, + source impedance should be about 2k2, while the parallel combination of R- s and the load must be around 10 kQ. floppy disk drive This is a much simplified version of -j the circuit published in the May 1984 issue of E/ektor, but it is, unfortu- nately, not usable with all disk drive motors. First, a recap of the operation. The drive motors are switched on when one of the drives is accessed by a DISK SELECT signal. There is a delay of a few index pulses before access proper to give the motor speed time to stabilize. A few seconds after all the drives have been deselected, the motor is switched off. This arrange- ment reduces operation of the drive mechanisms, the heads, and the disks to a minimum, which ensures a longer life of these devices. In contras t to the earlier published article, the READY output of the drive mechanism is used, wherein lies the reason that the older circuit cannot be as compact and simple as the present 2 one: it has to take into consideration that not all drive mechanisms have this output. However, as far as we can find out, most drive mechanisms do have it, but there must be some, of course, that do not. Figure 1, which is part of the circuit of the floppy controller board (E/ektor (UK), November 1982), shows the new wiring of port A 7 . The x at plug PL 2 represents pin 3 of the type FD-55x drive mechanism, and pin 6 of the BASF 6106. As this latter input corresponds to Disk Select 4, not more than three BASF 6106 drives can be connected to the present circuit. It is a wise precaution to break the connection between pin 10 of gate Njs and pin 6 of PL 2 , but it is not strictly necessary. As long as you do not select drive 4 (with the Ohio DOS, drive D), nothing can go wrong. One connection that must be broken is that between pin 16 of PL 2 and earth. Instead, pin 16 must be con- nected to pin 8 of IC 2 as shown in If you are really a dab hand at soldering, you may be able to make the changes, with the appropriate lengths of wire, on the relevant printed circuit board. Most of you will, how- ever, find it much easier to use a 15x20 mm piece of veroboard, which after completion can be glued or screwed on short spacers underneath C, 6 on the floppy controller board. ■ India Aug/Sept 1985 8.67 E 1 D audio tester A simple millivoltmeter and an equally simple sine wave generator are ideal instruments for checking and testing audio equipment. The audio tester combines the two, as shown in fig- ure 1, where A, and A 2 form the millivoltmeter circuit, while the sine wave generator is built from A 3 and As the audio tester is supplied (asym- metrically) from a 9 V battery, this supply must be halved for the oper- ational amplifiers. This is essentially done by zener diode D,. This zener is biased by /? 6 , and the reference voltage is taken from the junction of diodes D 8 -D 9 via resistor /?,. The ref- erence voltage is, therefore, about Fi 5.3 V. The constant voltage drop across the two diodes is applied across preset P 3 which serves to negate the offset voltage of A 2 (enabling the millivoltmeter to be cali- brated to zero). The input signal is applied across high pass filter C,/^i to the non-inverting input of A,. For all practical purposes, * ©- Semiconductors: D,...D 6 .D 8 .D 9 = 1N4148 D, = zener diode 4V7/0.4 W T,.T 2 = BC547B 1C, TL084 Miscellaneous: Ml = moving coil meter, 50 jjA (see text! PP3 (9 VI battery with dual miniature clip SPST on/off switch (optional - see text) For Components Sources See Page 9-38 this sets the input impedance at 1 MQ. Note that the circuit is fully driven with an input signal of 50 mV, ros . Higher inputs necessitate a voltage divider at the input or a reduction in gain in A, by dropping the value of R 3 . When this resistor is reduced to 6k8, for instance, the gain of A, is 2, and the input sensitivity is 275 mV lm5 . Full-scale deflection of the meter is set by P,. Opamp A, together with diodes D 3 ...D 6 , functions as an active full-wave rectifier. The meter is connected in of the diagonals of the diode bridge. To ensure that even small AC voltages can be measured, the potentials at both inputs of A 2 must be absolutely equal. Because of this, a small offset voltage is applied to the non-inverting input via The sine wave generator is essentially a Wien bridge oscillator, A 2 , whose frequency determing components are P 2 , C 3 . and C t . To ensure stable oper- ation, an active feedback loop takes part of the output signal of buffer amplifier A 4 , rectifies this ID,, D 2 ) and applies the consequent DC voltage to the inverting input of A 3 via buffer stages T, and T 2 . The output voltage of the generator section is 2 V p0 . The audio tester is best constructed on the printed circuit board of fig- ure 2. The meter can be almost any type from 50 pA to 1 mA. Note, how- ever, that the value of P, in figure 1 applies to a 50 pA instrument; for a different f.s.d., this value can be changed inversely proportional. For instance, if the instrument is a 500 p A type, P, should be 2k5. The millivoltmeter is calibrated by tap- ping the reference voltage with a divider of 820 Q in series with 100 kQ: the voltage at their junction will be 45 mV. Apply that voltage to the non- inverting ( + ) input of A1, and adjust P, till the meter reads "45". The current consumption of the audio tester is some 10 mA, and it is therefore advisable to incorporate an on /off switch. The frequency range extends from 150 Hz to 20 kHz. simple zero crossing detector Zero crossing detectors are often con- tained in rather complex circuits, or they are part of an integrated circuit, the rest of which is not required. Basically, such a detector is required to give a pulse every time the mains voltage passes through the zero potential. The detector proposed here is very simple indeed: the mains voltage is transformed down, rectified in D,, and smoothed by C , to give a direct voltage of 17 V. Part of the mains voltage is taken from across R 2 and used to drive transistors T, . . ,T 3 . Dur- ing positive half cycles T, conducts and T 2 and T 3 are off, whereas negative half cycles switch on T 2 and T 3 , while T, is off. When the momen- tary voltage across R 2 lies between + 0.6 V and —0.6 V, none of the tran- sistors conducts, so that the output voltage is high. In this way, a short positive pulse is produced every time the mains voltage passes through zero potential. Since operation is direct from the mains, there is no phase shift caused by the usual isolating trans- former. (5>-J 1 Where the direct voltage output of the circuit is used for supplying external circuits, attention should be paid to the current required by those circuits and the rating of the transformer. It may also be necessary to increase the value of C,. Finally, remember that the circuit and, therefore, any external units are con- nected direct to the mains! ,8.69 designing a low noise amplifier To design a low noise amplifier, it does not suffice to choose a low noise opamp, because the components associated with the opamp, particularly resistors, are themselves sources of noise. The noise in a resistor, which is caused by random movement of electrons, increases by the square root of the increase in resistance. Figure 1 shows a very convenient characteristic for determining opti- mum values of input resistance. The y-axis gives the square of the sum total of noise voltage produced in a circuit (in nV over the bandwidth con- sidered), while the *-axis gives the value of the source resistance. 2a For instance, a noisy opamp like the 741, which produces some 70 nV of noise over its bandwidth, can cope with an input impedance of some 200 k (higher values would cause the input impedance to generate more noise than the opamp!). On the other hand, the less noisy TCA 520, which generates about 30 nV of noise over its bandwidth, should have an input impedance not greater than about 50 k. It is not always convenient to use such relatively low values of resistance. For example, the audio amplifier in figure 2a is intended to operate down to 0.3 Hz; because of that, the time constant, t=RC, must be fairly long. The input (=source) impedance of the opamp is determined primarily by /?,. Lower values of this resistor would require a higher value of C, and this is not acceptable on cost grounds. The solution to this problem is shown in figure 2b, where both the DC and AC amplification are the same as in la, but because /?, is 10 times as small, its noise voltage is reduced by V 10. Sources Figure 1: intuitive 1C opamps (T M Frederiksen — National Semiconductor) Figure 2: technical note 068 (Philips) I It is sometimes useful, or even necessary, to use the same screen for more than one video source. Some simple video selectors used for this purpose suffer rather badly from crosstalk. The present circuit does not have this drawback: the unused chan- nel(s) is shorted out with a switch. When CH(annel) 1 is switched in, electronic switches ES, and ES 2 are closed and ES 4 is open. The other channel(s) is effectively choked because ES 5 and ES 6 are both open and any residual crosstalk is shorted to earth by ES 8 . Each channel uses its own 1C so that there is no risk of cross channel interference via the chips. As the switches have a certain impedance in the on state, there will be some losses when the output is ter- minated into 75 Q. It is, therefore, best to buffer the output; for instance, with the video buffer/repeater described elsewhere in this issue. The input of the video selector must be terminated into 75 Q. The — 3 dB 8.70 elektoi mdia Aug/ Sent 1985 bandwidth is about 8 MHz. Current because the electronic switches then consumption amounts to 1. . .2 mA have the lowest impedance in the on depending on the supply voltage. A state, high supply voltage is preferable. I 1 current loop for modem A modem, such as the direct-coupled modem featured in the November 1984 issue of Elektor, opens a whole new world to the computer user by making possible communication be- tween two computers anywhere in the world (provided, of course, they can be coupled to a telephone line). Ironi- cally, although the distance between the computers may be very large, that between computer and modem is strictly limited. This is because the RS 232 input is voltage driven and is, therefore, very susceptible to noise. This is not a new problem: it existed many years ago when, for instance, two telex machines had to be inter- connected. The solution then found, and still in use today, is the current loop. Such a current loop can also be used when the distance between the modem and the computer is relatively large: up to 1 km. A current loop so used converts RS 232 compatible voltages into RS 232 compatible currents. The stan- dard in the RS 232 protocol is a cur- rent loop of 20 mA. In view of the arrangement of the cir- cuit it is possible for the current loop to be used as a voltage driven input and output. In the receiver, the opto- isolator converts the input current into an output voltage via T,. The output voltage is ±12 V. As the current loop is closed via V* and V , mind the polarity. If you want to use an input voltage instead of a current, apply the input between V and earth. The input voltage to the transmitter may vary from TTL level to +12 V. Its output signal is available as either a voltage or a current: the former between V* and earth and the latter between V * and V . Current consumption in the quiescent state is zero; with full load, it amounts to 20 mA. The maximum bit rate at which the circuit operates reliably is 1200 baud, but this can be increased by the use of a faster opto-isolator. SgE sync inverter for the QL N1 = % IC1 ■ 74HC00 For some unknown reason, the Sinclair QL land perhaps some other personal computers) provides positive, instead of the usual negative, field synchronizing pulses to the monitor. Inverting these pulses with a suitably fast NAND gate or inverter is, of course, no problem. What is a prob- lem is where to power this gate from: a special supply would be nonsense. However, in the circuit proposed here, the gate is supplied from the sync signal itself. A monitor with TTL input for the sync signal draws only a very small current at logic 1, so that the additional load presented to the input pulse by the diode and electrolytic capacitor is inconsequential. Instead of the HC-MOS gate shown, it is also possible to use a buffered CMOS gate, for instance, a type HEF4011B. Standard CMOS devices, such as the 4011, cause a very small delay, which in practice does not mat- ter, and certainly not with a field sync signal. Note that it is important, as always with CMOS devices, to con- nect unused pins to earth (pin 7) or to U\y (pin 14). elefctor india Aug/Sept 1985 8.71 mains voltage monitor It is often desirable to know at a glance whether the mains voltage is at the low side; for instance, when you are about to work on a computer program. The danger is, of course, that when it is already low, further loads may cause the mains to drop below an acceptable level. The supply for the present circuit is taken direct from the mains, which exists across R, and P,. The 15 V stabilized voltage produced by R 2 , C„ C 2 , D,, and D 2 provides two reference voltages. These voltages are com- pared in A, and A 2 with a fixed pro- portion of the mains. If the mains is below 210 V, D 7 lights, and when it is higher than 250 V, D 8 lights. When neither D, nor D 8 lights, T, switches on and causes D 4 to light, indicating that the mains voltage is within acceptable limits. The mains voltage limits are set with P, with the aid of a multimeter and a variac; where perfectionism is not required. the preset may be set to roughly the centre of its travel. Remember that this circuit is not isolated from the mains and it must, therefore, be housed in a man-made fibre case. 1 i S noise generator Noise is normally defined as unwanted electrical signals spread over a rela- tively flat, wide frequency spectrum. In most equipment, great care is taken to reduce the amount of noise to a minimum, resulting in a low noise factor. None the less, noise is useful for measuring the behaviour of a circuit under varying input conditions. A noise generator is used, for instance, for measurements on coaxial cables, microwave links, and RTTY (radio teletype) and CW (continuous wave = radio telegraphy) decoders. The present circuit may also be used to imitate the sound of wind, mos- quitoes, bees, and other buzzing The circuit consists of a relaxation oscillator, IC„ which is provided with positive and negative feedback via P,-/? 2 and PyP 2 -R 3 -C, respectively. Zener diode D, functions as noise source. The amplification of the noise is determined by the setting of P 3 (coarse) and P 2 (fine). The setting of P, determines the noise bandwidth: a small effective value results in a nar- row band, and increasing values cause wider bands. Due to the negative feedback, the opamp also functions as a low-pass filter: a small feedback factor results in a low roll-off frequency. The pass band of the opamp also depends .on the value of C 2 . a value of 47 n causes a noise similar to the buzzing of a mosquito or bee. Diodes D 2 and D 3 serve as input limiters. The output level of the generator can be adjusted with P 4 . Current consumption is not greater than 10 mA at 12 V. 8.72 altktor in brake lights monitor R Kambach The circuit described below monitors your car's brake lights, and indicates by a light-emitting diode whether they both function correctly. In that sense, it can save you money by preventing your being fined for driving with defective brake lights, and it also leads to increased road safety. The monitor depends inevitably on the voltage drop across the supply lines to the two lamps. For the circuit to work correctly, that drop needs to be greater than 0.6 V. If this is not so, the drop must be increased by adding a 5 A diode in series with each lamp. Transistor T, and T 2 in figure 1 form a Schmitt trigger, which reacts to the voltage drop across the supply lines to the two brake lights. This reaction manifests itself in D, lighting via T 3 . If one of the brake lights is faulty, the switch-on current drawn by the other lamp will cause D, to light briefly when the brake pedal is pressed. If both brake lights are defective, D, will not light at all. All three possible states of the brake lights are thus indicated. The hysteresis of the trigger, and, therefore, the sensitivity of the circuit, can be adjusted within narrow limits with P,. The preset is best adjusted with one lamp out of action in a man- ner which makes D, light briefly as described above. If you find it disturbing that D, lights every time you brake, the operation can be reversed by replacing the 8C557B in the T 3 position by a BC547B (n-p-n). The collector of T 3 is then connected to the positive supply line, and the emitter to ft 6 . On the printed circuit board this means that the flat edge of T 3 must be turned the other way. A second base connection has also been provided on the PCB. Note, however, that this configuration lead-acid battery charger Although in electronics more NiCd than PbH 2 S0 4 batteries are used lor so we're told), there is still a healthy demand for good chargers- for the lead-acid types. The present one enables 6- or 12-volt types to be charged rapidly; switches itself off automatically; and is protected against thermal overload, short cir- cuits, and polarity reversal of the battery. If you are not fully acquainted with modern sealed lead-acid batteries, here are some of its more important properties. It may be used in any pos- ition, even upside down. The charging voltage should be 2.3 V per cell (2.45 V for fast charging): i.e., 6.9 V for a 6 V battery and 13.8 V for 12 V types. The charging current need not be limited to 0.1. . .1 C ( = capacity in Ah — the actual figure depends on the manufacturer). The battery is charged when the charging current has dropped to 1 per cent of the capacity. Some manufacturers state that it is preferable that their batteries Table 2. 1N4001 1N4001 1N5401 1N5401 R 2 provide current limiting, but R 2 is only necessary if a charging current above 0.5 A is required or to enable the output current more precisely. The current is limited to 10.451/?,+ /? 2 )//?,/? 2 ] A; its actual value is indicated by M,. The L200 may be mounted on a small heat sink, but this is not strictly necessary since the device has internal thermal protection. Normally, the battery charger works from the mains, but it can also operate from a 12 V (carl battery. All possible situations, some of which are highly undesirable, are enumerated in table 1. The one exception is that when the battery is really flat, the table does not apply. The battery must then be seen to be connected correctly to the charging terminals. Also, the LED indication will then initially not work. Resistors: Ri - 1 2 R 2 - see text R 3 = 820 0 R.i 560 Q (see text) R 6 470 0 P, = 500 O preset (see textl C| P 1000 p/25 V (see textl C 2 = 330 n C 3 = 1 p/16 V Miscellaneous: Mt = moving coil meter, 500 mA t.s.d 12 V. 600 mA (see textl Si - DPST mains on/off switch F, = fuse, 100 mA. delayed action heat sink for ICi (optional - see textl D,...D 4 ,D7,08 = 1N4001 (see table 2) D 5 ,D 9 = 1N4148 D 6 = LED ICi = L200 For Components Sources See Page 9-38 are charged in a horizontal position. Never charge these batteries with a NiCd battery charger!! The circuit of the suggested charger is based on a type L200 voltage regu- lator which ensures a constant charg- ing voltage. The actual level of the charging voltage is set with P, in the absence of a battery. Resistors /?, and which is permissible under all cir- cumstances). If the charger is required for 12 V bat- teries, the mains transformer must have a secondary voltage of at least 18 V, and capacitor C, must become a 35 V type. Furthermore, resistor /? 4 should be increased to 1k8 and preset P, to 1 kQ. EBB mains interface This circuit is of use, for instance, when a computer is required to moni- tor a mains-operated equipment. Opto-isolator TIL111 ensures complete isolation between the mains and the computer. With the mains on, during every positive half-wave a current of about 1 mA flows through the LED in the opto-isolator. The associated transis- tor then conducts and its collector current of about 100 pA is sufficient to drive T,. Remember, however, that this is a pulsating current: capacitor C, ensures that T, conducts continu- ously as long as the mains is on. If a 50 Hz square wave is required at the collector of T,, C, should, of course, be omitted. EBB smoke and gas detector This circuit is intended for use as a preventive device. We all know about accidents that occur through the accumulation of gas or of people over- come by smoke. The preventive character manifests itself by timely warnings in case of high gas concen- trations in a manner that does not cause the gas to explode. The circuit is based on sensor type TGS109 which is sensitive to gases enumerated in the accompanying table. Power is provided by an 8-volt bell transformer which is tapped at 5 V. The voltage developed across the 5 V winding is rectified by D 3 , smoothed by C u and regulated by R 2 , D 4 , and C 2 . The resulting direct voltage of about 5.6 V is used to supply 1C,. The 3 V alternating voltage is used to operate the sensor, which needs 1 V at about 0.5 A. Resistor /?, provides the necessary voltage drop. The mutual inductance between the two windings of the sensor increases with rising gas concentrations. Note that there is no difference in the two windings: the sensor may therefore be inserted into the socket in any way it fits. In practice, a rising gas concen- tration will cause an increased alter- nating voltage in the secondary winding of the sensor. This voltage is rectified in D, and smoothed by C 3 ; its level (= sensitivity) is preset with P,. Diode D 2 protects one of the inputs of N, against too high input levels. Gates N,-N 2 and N 3 -N 4 are buzzer to operate when there is too high a concentration of gas. Resistor /? 3 serves to counteract changes in sensitivity caused by tem- perature variations. The detector can be built into a small case, but bear in mind the heat dissipation in /?,. Finally, in case of an alarm, be careful in the inspection of the relevant room or space for which the alarm is sounded. Inorganic gases: Organic solvents: hydrogen H ammonia NH 3 carbon monoxide CO ethanol CH 3 CH 2 OH C 3 H 6 0, CH 3 COCH 3 8.75 E ii E swell pedal more than a potentiometer operated by the foot pedal via a toothed bar. The mechanics, however, make home construction a rather more daunting task. The swell pedal described here avoids the mechanical intricacies. The circuit is entirely contained in a flat case of about the shoe-size of the user — see figure 1. A wedge-shaped, hollowed-out piece of foam rubber is glued onto the lid of the case. A light- emitting diode, D 5 , and a light- dependent resistor, LDR, protrude from the lid. A small sheet of metal or plastic, the underside of which is covered with white paper or card- board, is then glued onto the foam rubber. The top of the metal or plastic sheet may be covered (glued) with a small rubber mat. When the foam rubber is compressed by foot pressure, the reflective white paper or cardboard comes nearer to the LED and LDR, which causes the resistance of the LDR to diminish. Reminiscent of the accelerator pedal in a car, a swell pedal enables mu- sicians to alter the sound volume by foot, since they invariably need both hands to play their instrument. Elec- tronic organs have the swell pedal nor- mally built into the front near the other pedals. Guitarists have to buy this almost indispensable aid for get- ting the right blend of accompaniment and solo voice(s) as an optional extra. From an electronic point of view, such commercially available devices are simplicity itself: normally nothing Because of the amplifying, inverting, and compensating action of 1C,, a voltage is applied to IC 2 which is used to control the drive current provided by transistor T, for OTA (operational transconductance amplifier) IC 3 . After the pedal box has been glued together, so that the electro-optical components are in a light-proof chamber, adjust P, so that with non- operated pedal the sound volume is just at the right level for accompani- ment. For solo playing, the pedal is depressed as required to obtain the increased sound volume. It is advisable to fit P, in the side of the pedal case as shown, so that it can be re adjusted at a later date if required. s a a spot frequency receiver Monitoring a number of frequencies in the short-wave band, such as the international shipping distress fre- quency, is a fascinating pastime. Since only a limited number of stations is normally monitored, and their frequency is invariably fixed by international treaty, the receiver needs only to be capable of being switched between those spot frequencies. The receiver works on the direct con- version principle, i.e., the oscillator frequency is equal to the received fre- quency, so that the intermediate fre- quency is zero. The aerial signal is fed to tuned RF amplifiers T, and T 2 via a switched preselector. The RF amplifiers are coupled to an S042P type mixer. There are three crystal-controlled local oscillators, which are switched into circuit in accordance with the pre- selector. The output of the mixer is the audio signal, which is fed to AF ampli- fier IC 2 via low-pass filter R „ . . . R, 3 - C 2 8 . . .C x . The gain of IC 2 is about 60 dB. Part of the output of IC 2 is rectified in D, and D 2 and used for AGC (auto- matic gain control) of T, and T 2 . The output of IC 2 is fed to power amplifier IC 3 which drives a loud- speaker or headphones. There is also a tape output. Volume control is pro- vided by P,. Inductors L, . . . L 3 are each wound on a T50/2 toroid as follows: ■ L, = 115 turns enamelled copper wire of 0.15 mm dia. with tap at 11 turns; ■ L 2 ,L 3 = 90 turns enamelled copper wire of 0.2 mm dia. with tap at 9 turns. If different frequencies from those shown are required, one or more of the crystals must, of course, be replaced, but at the same time L „ L 2 . or L 3 , as appropriate, must also be modified. The change in the number of turns and the tap is directly pro- portional to the change in frequency. If, for instance, a frequency of 2600 kHz instead of 2182 kHz is wanted in position 1 of switch S,, the number of turns, n, of should become n =115(2182/2600) =97 turns and the tap should be at r =11(2182/2600)= 9 turns. Oscillator capacitors C 4 , C 6 , and C 8 should have a higher value if the fre- quencies are chosen at the low end of the short-wave band. When the receiver has been built cor- rectly in accordance with HF requirements (short connections, ample decoupling), it should work up to about 18 MHz. The dashed lines in the circuit diagram represent earthed screens between the various sections. The receiver is calibrated by adjusting C 5 , C 7 , and C 9 for zero beat, and then adjusting C,, C 2 , and C 3 for maximum audio output. 3 3 1 two-frequency clock Many computer systems use one clock signal, from which all other timing signals are derived. The fre- quency of the clock signal determines, among others, the maximum number of characters per line the video con- troller can display on the monitor screen. This is normally 32 or 40. If more characters per line are required, the clock frequency has to be increas- ed. The clock generator described here makes it possible to switch between frequencies which are related in a ratio of 2:3. The switching is car- ried out synchronously, so that no bits are lost. The clock oscillator, T,, is controlled by an inexpensive 3rd overtone 27 MHz crystal, XL,. The LC circuit connected to the collector of T, is tuned to 54 MHz. The 54 MHz signal is converted to logic bits by field- effect transistor T 2 which are then applied to the Q inputs of dual J-K bistable 1C, < = FF,/FF 2 ). The ring counter formed by these bistables can be changed over by T 3 . When T 3 is on, the J input of FF, is logic high, and the 54 MHz signal is divided by 2. When T 3 is off, the_J input of FF, is connected to the Q output of FF 2 and the 54 MHz signal is then divided by 3. The output frequency can thus be switched synchronously between 18 MHz and 27 MHz. If a fundamental crystal is used in the XL, position, the oscillator can be modified as shown inset. hi-fi headphone amplifier This 1-watt amplifier lends itself par -| excellence for use as driver for a low impedance headphone or as output stage in a hi-fi preamplifier driving an active loudspeaker. Many preampli- fiers do not permit long, unscreened leads to be connected to them, but the present amplifier accepts these happily. The circuit - figure 1 — consists of an opamp type LF 356 and a push-pull transistor output stage. Low-pass filter /?,/C 2 at the input limits the slew rate of the input signal. In conjuction with the relatively fast LF 356, this results in very low delay distortion. The fixed quiescent current of 30 mA drawn by the output transistors, and set by diodes D, . . . D 4 in conjunction with emitter resistors R 1 and R s , ensures very low crossover distortion. Feedback resistors R 3 and R t fix the gain at about 15 dB. The consequent overall distortion with a — 3 dB band- width from 10 Hz to 30 kHz is only 0.1 per cent. 8.78 eleklor in The amplifier delivers a maximum power of 1 watt into 8 Q for an input signal of about 500 mV iml . High- impedance headphones and 4 Q loudspeakers may also be connected without detriment. The amplifier is best built on the printed circuit board shown in fig- ure 2. To enable it surviving a short circuit at the output, the two tran- sistors should be mounted on heat sinks - do not forget the insulating washers and the heat conducting paste! The power supply need not be more than a simple affair, consisting of a mains transformer with a centre- tapped, 6. . .8 V, 0.5 A secondary, a suitable bridge rectifier, and two 1000 pF/16 V electrolytic capacitors in Resistors: R, = 10 k R 2 .R 4 = 100 k Capacitors: C, = 22 n C 2 = 330 p C 3 = 1 p C 4 .C 5 = 100 n D, ...D 4 = 1N4148 T, BD 135 or BD 139 T 2 = BD 136 or BD 140 1C, - LF 356 Miscellaneous: PCB 85431 Heat sinks for T, and T 2 For Components Sources See Page 9-38 a conventional arrangement. To drive high-impedance headphones at high volume, you need a +15 V regulated power supply: in some cases, this may be derived from the preamplifier supply. In this arrange- ment, care must be taken not to short- circuit the output terminals. send/receive ident Some radio amateurs like to give an identification signal at the beginning and end of a message; others frown upon this practice which they find disturbing. If you belong to the first group, you may find this circuit useful as it gives an ident signal automati- cally when the transmit/receive key is pressed and just after this has been released again. The two signals are identifiable by being slightly different in frequency. XOR gate N, functions as a monostable, whose output is high for a short time after its inputs either change from high to low (at the onset of a transmission), or from low to high lat the end of a transmission). Its out- put is applied to an oscillator, N 2 /N 3 , and to the transmit/receive switching section. When the input pin 6 of N 2 is high, this XOR gate functions as an inverter, so that the oscillator generates a short tone in the medium audio range which is fed to the micro- phone via limiter D 4 /D 5 . The fre- quency determining network is earthed via Cl and D, or via C, and /?,. depending on whether the transmit/receive key is pressed or has just been released. During transmission, the rx/tx output is low: this output is intended to be connected to the corresponding input of the transceiver. Transistor T 2 is on, so that relay Re, is actuated: its con- tacts) may be used, for instance, to disconnect the loudspeaker during transmissions. Current consumption, ignoring the relay current, amounts to about 15 mA. Many electronics hobbyists combine all sorts of digital circuits into works to be marvelled at. However, even they sometimes have that uncertain feel- ing: must they all be powered by one unit or should there be more or can there be more? And in what sequence should they be switched on? Printer first, or computer first? In digital engineering, which by defi- nition embraces computers, inputs are driven by outputs: information is being transferred. When the 1C that drives has a power supply, but the receiving one has not, a current will ensue, whether the circuits are TTL or CMOS. This is an undesirable situ- ation, although it does not normally lead to damage. But the ensuing cur- rent may be so large that the 1C pro- viding the current does not operate efficiently any more, because its out- put voltage, owing to the large cur- rent, becomes too low. Particularly bistables can become disorganized by this. It is, therefore, possible that a certain equipment does not work properly because another circuit con- nected to it does not have a power supply. That situation can become really critical when several outputs of an 1C are terminated in that manner. Nor- mally, an 1C can withstand a short at one of its outputs, but if that happens at several outputs, the 1C will probably give up the ghost. This may happen, for instance, in the case of a Cen- tronics interface, of which the eight data lines are normally driven by one 1C. And what happens to the 1C that is provided with the current? CMOS cir- cuits are generally well protected against this, and TTL devices normally stand up well to them also. But other types may not take so kindly to these currents. Semiconductor manufacturers have, of course, also been confronted with these problems and have found sol utions to them. Anyone designing and building his own circuits should, therefore, heed their experiences and observe the following rules. ■ Driver ICs, whether TTL or CMOS, must have an open-collector output. ■ All inputs should be provided with additional resistance (pull-up resistors) to the positive supply line. If these rules are adhered to, current can only flow from input to output (see figure 2). This does not matter, because the collector of transistor T, can stand quite a high voltage and nothing will, therefore, go wrong. Make sure that the pull-up resistor is connected at the input side, otherwise it has no effect. As to the question at the beginning: it does not matter which unit is switched on first, because the 1C . manufacturers have made sure that the input and output circuits are pro- 8.80 video distribution amplifier 319 1 To feed a number of high-resolution monitors from one source (computer, video recorder), an amplifier is required that, apart from a reasonable gain, has a wide bandwidth. Unfortu- nately, these two requirements are not completely compatible, but the design presented here offers a fair com- promise. The circuit is capable of driving five 75 S loads simultaneously: the band- width at each of the outputs is 30 MHz. It consists of a differential amplifier, T,-T 2 , which is followed by a fast emitter follower formed by T 4 and T 5 . The gain of the differential amplifier is about 23 dB. Transistors T 3 and T 6 are current sources of 30 mA and 200 mA respectively. Feedback network R,-R 6 -R s -C 3 -C a ensures a bandwidth of around 50 MHz and a level pass band. Capacitor C 5 stabilizes the amplifier in the high-frequency region. When all five outputs are loaded, the bandwidth reduces to about 30 MHz, and the pass band then shows vari- ations of around 2 dB. The overall gain is a respectable 8 to 10 dB. The printed circuit board (which is not available ready made) is shown in fig- ure 2. Because of the high current through T 5 and T 6 , both these tran- sistors should be fitted onto a suitable heat sink. As the total current consumption of the circuit is around 250 mA, a separate power supply will be required in almost all applications. Resistors: R, = 6k8 R 2 = 2k2 R 3 » Ik R 4 - 33 Q R 6 = 100 fi R e .R, • 330 B R a = 120 B R 9 -- 680 B R l0 - 4B7 R,| ...R, 6 - 82 B P, = 100 B preset potentiometer C, ,C 4 ,C 7 = 1 n C 2 = 22 p/10 V C 3 = 100 p/10 V C 5 = 18 p C 6 = 470 n C 8 = 470 p/10 V Semiconductors: T,...T 4 = BF494 T 5 = 2N3866 T 6 = BC140 D, = LED. red Miscellaneous: 2 heat sinks T039 For Components Sources See Page 9-38 3 3 3 floppy centring unit In modern disk drive mechanisms, as, for instance, the TEAC FD55x, the motor starts automatically when a disk is inserted into the drive. When the lid is closed, the motor stops again. This arrangement ensures bet- ter centring of the disk. Better cen- tring means less wear on the centre fixing hole, the life of the disk is extended, and read/write errors owing to eccentricity off the disk are prevented. Owners of older drive mechanisms, such as the BASF 6106, can incor- porate that facility with the circuit pro- posed here. The signal from the write protect phototransistor is used to determine when a disk is being inserted (this signal is normally gated when the drive is closed), and to start the motor for the total period of monostable MMV1. The SPEED signal is not absolutely necessary: it stops the motor direct when the lid is closed. If it is not used, pin 3 must be connected to the +5 V line. The motor will then run for the duration of the period of MMV1, i.e., about 10 s. The monostable period can be re- duced by lowering the value of the capacitor. The points where to connect the cir- cuit in the 6106 are easy to find. Look- ing at the pcb from the front, you will see a cut-out in the front centre of the board. Immediately to the left of this are three ICs (see photograph). The one at the front is a 7474, the one in the middle a 7432, and the one at the back a 7404. The signal SPEED is taken from pin 6 of the 7474, and the signal PI from pin 2 of the 7404. The signal MOTOR ON is applied to pin 3 of the 7404. As all existing connec- tions remain, the connecting wires of the auxiliary circuit can be soldered direct to the relevant 1C sockets. In the same way, it is possible to derive the supply voltage for the auxiliary circuit: for instance, +5 V from pin 14 of the 7404, and 0 V from pin 7 of this 1C. It is important to note that there are two types of pcb used in 6106 drives: the ICs and the 1C function are the same in both versions, but the con- struction may look different from that shown in the photograph. E 3 Q direct-voltage doubler A direct-voltage doubler is particularly useful when from an available supply voltage a higher one has to be de- rived. As the current in most such cases is pretty small, the cost of a suitable circuit can be kept down. Astable multivibrator 1C, is a rectangular-wave generator operating at about 8.5 kHz whose output drives transistors T, and T 2 . When the level at pin 3 of 1C, is low, T, is off and T 2 conducts. As the negative terminal of C 2 is then connected to earth, the capacitor charges via diode D,. When the output of 1C, is high, T 2 is off and T, conducts. Capacitor C 3 cannot dis- charge because of D,, but C a charges to a voltage roughly equivalent to the supply voltage of +12 V and the p.d. across C 3 and D,. In our prototype, this voltage across C, amounted to 20 V approximately. The maximum current should not exceed 70 mA: at that value, the output voltage is 18 V, at an efficiency of thirty-two per cent. We have not tested the circuit with other supply voltages, but it can be safSIy assumed that it can be used over the whole supply voltage range of the NE 555. Construction is possible on a small piece of prototyping board, after which the doubler can be fitted inside the power supply unit. If a regulated output is required, it is possible to connect an appropriate voltage regulator, for instance, in the 78LXX series, but in that case the power requirements of the regulator must, of course, be taken into con- sideration when the maximum load current is calculated. 3 3 S mini amplifier tiometer connected to the PCB via a short length of screened audio cable. Current consumption is 2.5 mA at 3 V or 7.5 mA at 9 V under no-signal con- ditions, and 80 mA at 3 V or 270 mA at 9 V under fully driven conditions: in the latter condition, the output power is 100 mW or 1 W respectively into 4 ohms. The output power for different supply voltages and loudspeaker impedances can be estimated by deducting 1 V from the supply voltage, and raising the result to the power 2. Divide the number obtained by 8 and then again by the loudspeaker impedance. The sensitivity of the amplifier is about 50 mV. This can be reduced by lowering the value of /?,. National Semiconductor Application. This little amplifier, operating from 2 3 ... 9 V, and providing 1 W output into a 4 Q loudspeaker, is one of those circuits of which you never have enough. The amplifier is based on one 8-pin DIL 1C type LM1895N. Electrolytic capacitors C 2 and C 6 decouple the supply lines; C 7 prevents d.c. reaching the loudspeaker; and C 3 and C b provide a low-impedance path to earth for audio frequencies. The input signal is applied to pin 4 of the LM1895N via P, and C,. Resistor and capacitor C 8 suppress any tendency to oscillation, i.e., improve the stability. The amplification is determined by /?, and /? 3 : it is of the order of 50. Capacitor C u in parallel with /?,, ensures that the amplification drops off for frequencies above about 20 kHz. If the amplifier is intended for use with a small AM receiver, it is desirable that the amplification starts falling off at a lower frequency. This is brought about by enlarging C,: for instance, if its value is doubled, the amplification starts dropping at 20/2=10 kHz. On the printed circuit board shown in figure 2 (which is not available ready made), P, may be replaced by a wire link; the volume control is then carried out by an external logarithmic poten- — 1-»-© 3. ..91 4 , ]5 india Aug/ ,8.83 variable 3 A power supply As far as construction is concerned, this is a real mini power supply, but it can deliver up to 3 A at an output voltage of 1.25... 25V. Note, how- ever, that integrated voltage regulator 1C, has on-chip overload protection that comes into operation when the dissipation in the device reaches 30 W. The ADJ(ust) pin of the regu- lator is connected to the junction of potential divider /?,-P, . The output voltage, U 0 , is calculated from U a = 11.25 (1 + P,//?,)l V where P, and ft, are in ohms (the value of P, is measured between the wiper and the junction with ft,, i.e., 0. . .2.5 kQ). Capacitor C, is a conventional filter capacitor, while C 2 and C 3 improve the regulation. Protection diodes D, and D 2 ensure that at switch-off the potential at the output of 1C, is more positive than that at its input. The value of ft, has been chosen to ensure that the minimum load current through 1C, is about 3.5 A. It is essential that 1C, is mounted on a heat sink rated at about 1 K/W - do not scrimp on the heat conducting When only low output voltages are needed, it makes sense to use a mains transformer with a lower secondary voltage (for U a = 5 V, the secondary voltage should be 9 V). When a 24 V secondary is used, and the required output voltage is 1.25 V, the maxi- mum output current is 1 A, otherwise the maximum dissipation of the LM 350 is exceeded, and the internal protection will switch off the regu- lator. When the secondary voltage is 9 V, and U 0 = 1.25 V, the maximum load current amounts to 2.5 A. heat sink monitor R Jacobs In almost any equipment in which a reasonable amount of energy is con- sumed, there is bound to be at least one heat sink that enables power semiconductors to get rid of their excess heat. The rating of a heat sink is normally determined on the basis of the maximum allowable temperature of the silicon chip: a rather haphazard method. The heat sink monitor described here constantly monitors the temperature of the heat sink. When that tempera- ture stays below 50. . ,60°C, the green LED lights; between those tempera- tures and 70...80°C, the yellow (orange) LED lights; and above 70. . .80°C, the red LED lights. There is also the possibility of providing a relay with which, for instance, the load can be disconnected. The circuit is, in essence, a window comparator, in which sensor D, pro- vides a control voltage that rises 10 mV per degree Celsius. If the sensor voltage is lower than the voltage at the wipers of P, and P 2 , the outputs of opamps A, and A 2 are low, and D 2 lights. When the voltage across D, lies above that at the wiper of P,, but below that at the wiper of P 2 , the output of A, is high, so that D 2 goes out and D 3 lights. When the sensor voltage rises above that at the wiper of P 2 also, the output of both opamps is high: only D s then lights and transistor T, is switched on. Zener D 4 ensures that D 5 lights bright- ly and that T, conducts hard. To calibrate the unit, place the sensor, together with a calibrated ther- mometer, in a tray of water, which is then heated. Set P, to minimum and P 2 to maximum resistance. Set the cross over from green to yellow (orange) between 50 and 60 degrees Celsius with P,. Next, set the cross over from yellow (orange) to red between 70 and 80 degrees Celsius with P 2 . The sensor can then be fitted permanently onto the heat sink. 3 3 3 6502 tracer A program that has been written into 1 an assembler will rarely run error free on the first run. It often exhibits blurbs and other ramblings: in bad cases, there is a complete hang up and it is then necessary to start the computer afresh with a RESET. To find such faults in a relatively easy manner, the tracer described here will be found very useful. The circuit layout of the tracer is shown in figure 1. Gate N, is an address decoder, whose output in the address range $F000. . SFFFF is logic 0. NAND gate N 2 is fed with the SYNC signal from the computer and the 0 signal; it is disabled by either the address decoder, N,, or bistable FF 2 . The address decoder disables N 2 when the EPROM is addressed from the CPU. This prevents the SYNC line of the 6502 processor generating an Ml (maskable interrupt). If the pro- cessor passes through a machine program somewhere in the RAM, N 2 generates an interrupt as soon as the processor reads an opcode, which makes the SYNC line logic 1. This non-maskable interrupt directs the processor to an interrupt program in the monitor program. All CPU registers are safeguarded by this inter- rupt program and subsequently displayed on the monitor screen. At the same time, the processor disassembles the next command. R, = 1 k R 2 ...R„ = 10 k R 6 = 220 Q Capacitors: C, = 10 p/16 V C 2 = 100 n Semiconductors: D, = LED (red) 1C, - 74LS22 IC 2 = 74LS74 S, - miniature spring-loaded press-to- make switch S 2 = miniature spring-loaded press-to- rpake switch (see text) For Components Sources See Page 9-38 The programmer can, therefore, see beforehand under what conditions the processor starts with the execution of the next opcode. Since the status register and all its flags are also displayed on the screen, the program- mer can easily ascertain whether a flag in the status register has been set incorrectly. Bistable FF, serves as a debounce stage; FF 2 toggles on receipt of a leading edge from FF,: that is, every time S, is pressed. When the tracer is switched on, D, lights. Resistor /? 4 and capacitor C, form a power-on reset network that automatically switches the tracer off when the com- puter is switched on. The printed circuit board for the tracer is shown in figure 2. If you want to build the tracer into the computer case, the PCB can be cut along the dashed line, so that the section con- taining S, and S 2 may be fitted in the most convenient position. Switch S, must be connected to the tracer via a suitable cable, but S 2 may be con- nected to the manual RgSET of the system. Information on software for the tracer may be found in our books on the Junior Computer. India Aug/Sept 1985 8.85 SIS hexadecimal keyboard There are various ways of producing a hexadecimal keyboard. Normally, it is based on a number of key contacts in a matrix, but here a rather simpler method is used: 16 key contacts (0. . ..F) that are commoned to the positive supply line. Such keyboards are commercially available. Code conversion is carried out by two priority encoders, IC 3 and IC 4 . If one of the inputs l 0 ...l 7 of these ICs is connected to the positive supply line via one of the contacts S, . . S, 6 , i.e., made logic high, the relevant binary code appears at the associated out- put, Q„. . ,Q 2 , of which Qo is the least significant bit (LSB). As the encoders are cascaded, there is a total of 16 inputs. Corresponding outputs of the encoders are combined in OR gates N 6 . . . N 8 to form the lowest three out- put bits D 0 . . .D 2 . the fourth data bit is taken from the GS (group select) output of IC 4 . This output is logic high when one of key contacts S 9 ...S 16 (8...F) is closed. As the GS outputs of the two ICs are combined in OR gate N 5 , 0 3 is active high when a key is pressed. The signal at pin 9 of N 3 is delayed by /?, 8 -C 2 . At the same time, the signal at pin 15 of IC 3 triggers monostable N,-N 2 . During the pulse period of about 10 ms, pin 8 of N 3 is logic low so that, indepen- dent of the delayed signal at pin 9, the output of N 3 remains logic high. If pin 9 of N 3 is still high when the pulse begins to decay, the output of N 3 goes low and remains so until pin 9 becomes logic 0 again. During this time, pin 6 of N 2 remains low, so that the monostable cannot be triggered erroneously. The timing diagram in , figure 2 further clarifies the operation, which results in a debounced strobe or strobe pulse. If more than one key is pressed, the highest is selected, as is to be expected from a priority encoder! The circuit requires a power supply of 3. . .18 V: current consumption is not greater than 10 mA. ESS wah-wah box for guitars In this day and age of electrophonics, a wah-wah box is still a popular means of animating an otherwise tired sounding guitar. Such a box, which is basically a high-Q low-pass or band- pass filter, can be designed in various ways. Early designs were invariably based on active (transistorized) double-T filters. The present circuit, using opamps and operational transconductance ampli- fiers (OTAs), is rather more complex but also more efficient and more reliable. Three pairs of opamps, each con- sisting of an OTA and a buffer ampli- fier, in conjunctions with capacitors C 2 . C 3 , and C 4 , form a low-pass filter. Since the usual series resistances have been replaced by voltage- controlled current sources (OTAs), the roll-off frequency of the filter is deter- mined by the currents flowing into pin 5 of the 3080s. These currents are themselves directly proportional to the input control voltage, U c , which has been converted in A, and T,. This voltage, which is derived from a swell pedal, can have any value between 0 V and about 12 V. The negative feedback from output to input enables the Q of the filter to be set with P 2 . The swell pedal may be constructed as described elsewhere in this issue: it can actually be installed in one case together with this wah-wah filter! As it is difficult to describe sounds, and we are sure that the guitar players among our readers will in any case experiment themselves, we will not dwell on what to expect from this musical adjunct. No calibration is needed: the box works or it does not! We hope the former. ,8.87 SIS jumbo displays Although this project will not be of interest to everybody, it has many possible applications. The name refers to the respectable dimensions of the seven-segment displays: 280x140 mm. These sizes immedi- ately indicate that the displays are intended to make alphanumeric infor- mation legible at a distance. This is of import, for instance, for score boards, speed indicators, lap counters, digital church clocks, etc. These displays have a number of advantages: ■ they are entirely solid state, which prevents segment failure since the life of LEDs is much longer than, for instance, that of incandescent lamps; ■ they do not need intricate reflector constructions; ■ if any one LED fails, they remain fully legible by virtue of the special segment construction; ■ they are easily arranged in a variety of colours — red, green, blue, yellow, orange; ■ they work from 24 V with relative high efficiency, which keeps heat dissipation low. It may be said that the large number of LEDs required is a disadvantage, but, in our opinion, this is largely negated by the advantages. The seven-segment display, shown in figure la, is based on a type 74LS248 decoder, which has the same features as the well-known type 74LS47/247, but has in addition internal pull-up resistors and inverted output signals, so that external transistors can be used to cope with the large currents drawn by the segments. The inputs and outputs to the decoder, the read- outs, and the additional functions are correlated in figure 2. All input and output controls have been arranged external to the decoder, so that they can be used in the same way as with normal displays. Wire link R-S serves to interconnect the earths of the +5 V and +24 V supplies. At the output of the decoder there is a switching stage for each segment that switches the relevant segment on or off. Each segment consists of four parallel groups of eight or nine LEDs in series with a current limiting resistor. The displays can be powered from a non-stabilized 20 . . .24 V supply. The current drawn per segment varies from 50 mA to 100 mA. Figures 1b and 1c give the diagrams for displays with a "1" and a respectively. Both can be used for a 12-hour clock. The "1" display has provision for a lamp test (LT); open inputs are considered active, i.e., the display lights. This is in contrast to the seven-segment display which treats inputs that are not connected as logic high, that is, inactive. As mentioned earlier, read-out boards consisting of several figures may be composed by mounting a number of displays side by side on a flat base. The whole may be protected by translucent red perspex: this also acts as a light filter, which improves the legibility considerably. As you need a large number of LEDs, shop around for these because many dealers are prepared to allow a quan- tity discount. Uniformity of brightness of these diodes is not so important for this application, because at the distances for which these displays are intended, differences in brightness do not show up. Seven-segment display: R,...R 7 = 100k R 8 . . R,| 17X1 = 270 Q (with 9 LEDs) - 330 « (with 8 red LEDs) = 390 Q (with 8 green LEDs) 1C, - 74LS248 T,...T 7 = BC517 C, = 100 n 232 LEDs, 5 mm, colour as required I I ® ® Figure 3. Printed circuit boards for (a) the “ " display: lb) the seven-segment display; and Ic) the "1" display. rK* "1" display: R, = 47 k R 2 - 1 M R 3 ,R 4 = 470 k R s . R 8 I2X) = 270 C D,.D 2 = 1N4148 T, = BC517 T 2 = BC547B 72 LEDs, 5 mm, colour as required display: R,.R 2 = 270 Q 18 LEDs, 5 mm. colour as required For Components Sources See Page 9-38 w L_ J SSI sync separator Many monitors have separate inputs for the line (horizontal) and field synchronization pulses. If your com- puter only provides composite sync pulses, the circuit described here makes it possible to split the com- posite sync_signal, CSYNC. into proper line, US, and field, VS, puls es. It is possible to feed the CSYNC as line sync pulses direct to the monitor, which is the reason that the CSYNC input is connected direct to the HS output terminal. To derive the field sync pulses from the composite signal, a dual retrig- gerable monostable type 74LS123 is needed. The first mono period is slightly longer than the distance between two line sync pulses. As the monostable is retriggered by each line sync pulse, it only resets in the absence of a line sync pulse, that is. at the onset of a blanking interval. The first monostable then triggers the sec- ond, which generates a VS pulse at its Q output. When the second mono period has lapsed, the first monostable has already been provided with more line sync pulses, so that monostable 2 is not triggered again until the next blanked interval. The overall result is that all line sync pulses are suppressed, while monostable 2 provides field sync pulses. “T'!! flashing light with twilight switch The special feature of this flashing light is the optical switch, which auto- matically switches the light on when it gets dark, and switches it off again at dawn. This makes the light ideal as a warning light near obstructions. It may also be used for educational pur- poses to show the operation of tran- sistors in conjunction with optoelec- tronics. Assuming that it is light, the LDR (light dependent resistor) has a low value so that there is sufficient base current through T, for the transistor to conduct. Its collector voltage is then small, so that T 2 , an n-p-n dar- lington, is off, and lamp L, stays out. When the ambient light reduces, the resistance of the LDR increases until the base current in T, becomes insuf- ficient and the transistor switches off. Its collector voltage then rises, T 2 conducts, and L, lights. This process takes place quite quickly, because when the collector voltage of T 2 sud- denly becomes nearly 0 V, this poten- 6. ..10V tial is immediately applied to the base of T, via capacitor C u which really cuts off T,. The capacitor then charges via PI and the LDR that is now being illuminated by the lighted lamp. Owing to the optical feedback, the value of the LDR diminishes, the voltage across ff 2 increases, and T, conducts again. The darlington switches off, and the lamp goes out: a new cycle has started. The flashing frequency is primarily dependent on the value of C,: with 47 pF, it is rather low; reducing the capacitance increases the frequency. The BC 517 darlington may be replaced by two BC 547B transistors or a VN10KM MOSFET. The only thing that needs watching is the cur- rent through L,: the maximum per- missible with two BC 547Bs is 100 mA; with a BC 517 it is 400 mA; and with a VN10KM it is 500 mA. Cur- rent consumption of the circuit, with lamp L, out, is about 6 mA at 6 V and around 10 mA at 10 V. The light-dependent resistor may be one of the usually available types: LDR 03, 05, 07. To ensure that the optical feedback works, the LDR must be fitted near lamp L,. The onset of operation is set with P,. or india Aug/Sept 1985 8.93 model railway monitor panel N Koerber Many railway modellers would love to have a track monitor panel, but, unfortunately, the few commercially available types do not justify their cost. It is, however, not too difficult to make one yourself. The reproducing of the track diagram and the mounting of the monitor lights on the panel can be ac- complished without too much trouble. There is a problem, however, in indicating the position of turnouts and colour-light signals, because these elements are normally operated by spring-loaded switches to prevent the burning out of the solenoids. After the push-button on the control panel has been released, the supply line is no longer live and can, therefore, not be used for lighting an indicator. This problem can, fortunately, be solved by a couple of R-S bistables (NOR gate latches). The ' push-button switches and solenoid coils shown in figure 1 are those already contained in the railway set-up. Note that the system is as- sumed to operate from a 9. . .15 V AC supply. Each signal normally requires three lines: one for each of the two coils and a common line. Terminals A, B, C, and D in figure 1 are connected to the rel- evant outputs of the control panel. The circuit as shown is suitable for monitoring two turnouts or two colour-light signals via A-B and C-D respectively, but can be extended as required. The voltages used to energize the coils are rectified and applied to an R-S bistable. This NOR gate latch is set or reset, depending on the nature of the input signal, and this causes the relevant LED to light. If, for instance, pin 8 of N 2 is high, pin 10 of this gate is low, and D e lights. \ The circuit as shown has a current consumption of about 30 mA per R-S bistable branch. Not all monitor LEDs will correctly show the position of the relevant turn- out or light signal immediately upon switching on the supply. Briefly press- ing one of the two push-buttons of each turnout or light signal will correct this situation. The circuit is most conveniently built on the printed circuit board shown in figure 2. This board can accom- modate two monitor channels as shown in figure 1. If more are required, these can be built on additional PCBs. The section contain- ing C 5 , IC 2 and D 9 may be cut off subsequent boards, but if many additional PCBs are used, make sure that the power requirements are still met! The +5 V and 0 V terminals on all boards should be interconnected. - indis Aug, Sepl 2 Resistors: R,...R 4 = 4k7 R 5 ...R 8 = 2k2 R 9 ,Rl„ = 330 Q Capacitors: C,...C 4 .C 6 ..C g = '00 n C 5 = 220 )./40 V Semiconductors: D,...D 4 = 1N41 D 5 . ,0 8 = LED D 9 = 1N4001 Dio- - D ,3 = zer 1C, = 74LS28 IC 2 = 7805 green, as required) 3, 4V7/400 mW Miscellaneous: Tr, = mains transformer, 9. . .15 V second- ary (if not already available from the existing system) For Components Sources See Page 9-38 set pointer Aneroid barometers invariably have two pointers: one that is operated by the mechanics, and one that is set manually. The manually set pointer is really nothing but a mechanical memory that enables variations in barometric pressure to be ascertained. The set pointer can, of course, be made electronic, for which a slide potentiometer is ideal. Such a pointer is not restricted to a barometer: it can also be used with a thermometer, a hygrometer, a battery that needs to be monitored; in short, with any sensor that delivers a slowly varying voltage. The circuit consists of an amplifier, 1C,, and a display stage, IC 2 . The display consists of between 3 and 9 LEDs, the centre one of which, D s , is yellow and represents the point of origin. Potentiometer P, can be adjusted to make this LED light. When the input voltage rises slightly, D 6 (the colour of which depends on the application) lights; when it drops, D 4 (again, the colour depends on the application, but it should be different from D 6 . . . D 9 ) lights. Greater vari- ations in input signal cause D 7 . . .D 9 or D 3 . . .D„ respectively to light. It is at all times possible to adjust P, in a manner which causes the centre LED to light. The potentiometer could be provided with a graduated scale to enable the input voltage to be read direct. It is not difficult to produce such a scale. Apply voltages of 0.1 V, 0.2 V, and so on in steps of 0.1 V, and for each voltage turn P, till the centre LED lights. At each of the positions of P, so found, draw a thin line. The sensitivity of the circuit is of some 1 elektor India Aug/Sepl 1985 8.95 import, because about 1 V is necessary at pin 5 of IC2 to make D, and D 9 light. As the amplification of 1C, is unity I RJR 3 ), about 1 V is, therefore, also needed at the input of the circuit for these LEDs to operate. Opamp 1C, deducts the voltage at the wiper of P, from the input signal, and adds the potential at the junction of R b and R g to the result. Since P, is connected to the refer- ence voltage (1.28 V), only this voltage can be compensated for. Strictly speaking, there is no reason why P, should not be connected to the positive supply line in series with a suitable resistor. In that case, the display is only stable if the supply line is well regulated. If the input sensitivity is too low, the values of R t and R 2 may be increased; note, however, that these values should always be the same. Current consumption is determined primarily by the current through the LEDs, and that in itself is about ten times the current through R b and R e . The latter current is equal to the on- chip reference voltage of 1.28 V divided by the total resistance of R g + R e . The maximum current through the LEDs is about 40 mA (the current via pin 7 must not exceed 4 mA!) so that the total current does not exceed 50 mA. O "*r yellow 9'“" r n 1 r 1 oooomoooo H-H fll M I + B B B MOSFET power amplifier The output power of an operational amplifier is often increased by a com- plementary emitter follower. It can also be done with a MOSFET, but it is not a good idea to connect such a device as a complementary source follower because the maximum out- put voltage of the opamp is then reduced appreciably by the gate- source control voltage of the MOSFET, which can be a couple of volts. Another approach is to connect two MOSFETs as a complementary drain follower. The (alternating) output cur- rent provided by the MOSFETs is limited by the level of the supply voltages and the saturation voltages of T 3 and T 4 . Resistor R g , together with fl g , provides feedback for both the opamp and the MOSFETs. The open-loop amplification of the opamp is, therefore, increased by (1 + Rg/ R$)- The closed-loop amplification of the complete amplifier is (1 + R 3 IR 2 ), i.e., 11 . The current source formed by T , and T 2 is required for arranging the quiescent current of T 3 and T 4 at 50 mA. The values of resistors R t and R g are such that, without the current source, the voltage drop across the resistors resulting from the direct cur- rent through the opamp is not suf- ficient to switch on T 3 and T 4 . With the current source, and depending on the setting of P„ the voltages across R a and Rg rise, which increases the quiescent current through T 3 and T 4 . In view of the temperature dependence of the quiescent current, T 2 must be mounted on the common heat sink (c. 5 K/W) of the MOSFETs. The output power is not less than 20 W into 8 Q, at which level the har- monic distortion amounts to 0.075 per cent at 100 Hz and to 0.135 per cent at 10 kHz. Source: Voice coil drives using com- plementary power MOSFETS by M Alexander in Motor-Con proceedings. April 1984 l 8.96 elekior india Aug/Sac' 'S S 9 2 “on the air” indicator In radio and television studios it is customary to indicate to all concerned when the microphone or camera is "on the air". This is normally done with a red light at or near the relevant camera or microphone. The circuit described here is intended as an auxili- ary for a DIY mixer unit. To make the circuit automatic in action, stereo slide potentiometers are used at the audio inputs. When one section of these potentiometers is connected to the +-15 V line, the potential at the wiper of this section is a measure of the potentiometer set ting. This potential is amplified in opamp A, and applied to the invert- ing input of A 2 . The latter opamp tog- gles as soon as the level at its inverting input exceeds that at its + input, which has been set with preset P,. The slide potentiometers for this pur- pose are always logarithmic types, so that the voltage rise at the beginning of their travel is always pretty small. To ensure correct operation of the circuit even at these settings of the poten- tiometers, the gain of A, has been arranged fairly high, about 26 dB. Opamp A, also serves as a summing amplifier that monitors a row of audio inputs. If it is required that each audio input has its own monitor, the two opamps must be repeated for each input, but P,, of course, continues to provide the non-inverting potential for all opamps in the A 2 position. The output of the indicator is provided by a type BC 547B transistor, which can switch up to 100 mA. This current is sufficient to light a signal lamp or light-emitting diode (LED) with bias resistor, or to drive a relay. Current consumption with the BC 547B off amounts to not more than 10 mA. If low-resistance stereo poten- tiometers are used, the direct current through the "indicator section" may be too high; if that is so, it is advisable to use a dropping resistor in series with the section. LED direction indicator from an idea by M Miller An LED indicator with a difference: three alternately lighting LEDs indi- cate a direction, for instance, in a model railway, or to an emergency exit, or to a door on badly lit stair- ways, and so on. When the supply voltage is switched on, the inputs of gates N 4 . . ,N 6 are logic 1, their outputs logic 0, and all LEDs light. One of the RC networks (/?, + P,/C,; R 2 /C 2 ; R 3 /C 3 ) will reach the trigger threshold first. Let us assume it is /?, + P ,/C,. The output of N, then goes low, the output of N 4 goes high, and D, goes out. There is then no voltage for R 2 /C 2 , the output of N 2 remains logic high, and N 5 remains logic low; D 2 then lights. Subsequently, the output of N 3 goes low, the output of N 6 becomes 1, and D 3 goes out. The logic 0 of N 3 is, after a delay in R , + P t /C,, again at the input of N,. The output of N, goes high, that of N„ goes low, and D, lights. This process repeats itself, so that first one, then two, and then one LED again lights. At every step. the light pattern shifts one place to give the impression of a running, flashing light. The running speed is set with P,. India Aug/Sept 198S 8.97 3. ..15V 0- Tl> IC1 IC2 - It does not really matter whether you use inverting gates (40491 or non- inverting ones (4050) in the IC 2 pos- ition, as long as you connect the unused gates to the positive or -c[r)^=-{£x^ negative supply rail. The RC networks may also be modified to taste or if special effects are desired. If you want to make the circuit even smaller, forget IC 2 and use the three remaining inverters in 1C, as LED drivers, provided you are using a type 40106. The LED currents are then only 5. . .10 mA, so you have to use high output LEDs (that are bright at low currents). The current consumption of the cir- cuit without LEDs and operating from 15 V is about 100 pA. With LEDs, it depends very much on the LEDs and the supply voltage: with standard LEDs and at 15 V, each LED draws up to 30 mA. fast opto-isolator When a computer drives external equipment, it is often required that the earths between them are electrically isolated from one another. The simplest way of effecting this is by an isolating transformer. When, however, the system works at high frequencies, it is much better to use an opto- isolator as proposed here because that is capable of following the fast data transfer. The opto-isolator is driven via a TTL gate. The transistor in the opto- isolator drives comparator 1C,. The trigger threshold of this device is set with P,. Low-pass filter R 2 -C y prevents spurious triggering of the comparator by noise pulses. “ video amplifier for B/W _ _ television sets It appears that the use of portable, mains operated television receivers as monitor in a computer system has become very popular. The article use your TV receiver as a monitor ( Eiektor , December 1984) described an all- embracing amplifier, but here we pro- pose a much simpler one. To raise the standard video signal of 1 V pp to the level required by the tele- vision receiver, a preamplifier with a bandwidth of not less than 10 MHz is required. With careful construction of the present amplifier, this bandwidth is guaranteed, and should actually be of the order of close to 20 MHz. With a supply voltage of 12 V, the direct- voltage output is 4 V. If different supply voltages are used, the DC out- put is retained at that level by suitably altering the values of /?, and R 2 (which form a voltage divider). How- ever, the supply voltage should not be lower than 10 V, nor higher than 15 V. The amplification depends on the ratio Rj : /? 8 ; if higher amplification is needed, the value of R-, should be increased. The respectable bandwidth is achieved by low value base and col- lector resistors: with this arrange- ment, even audio transistors may be used in this, essentially HF, circuit. In any case, the cut-off frequency of a BC 547 is 300 MHz, and that of a BC 557 is 150 MHz. The input impedance is strictly deter- mined by R 3 ; its value of 82 Q is near enough the required impedance, but if you really want to be a purist, there are 75 Q resistors available at some stockists, or you can connect a 100 Q resistor in parallel with a 330 Q one. sis time stretcher From an idea by S Gulikers Anyone with a fascinating hobby must have felt at one time or another that there is not enough time available for his hobby. Any circuit that can stretch those few hours once or twice a week must, therefore, appeal to The time stretcher is a small circuit that can be built into almost any digital clock and makes the hobby evening(s) last an hour longer. The three diodes, D, . . . D 3 , together with /?,. form an AND gate. D, is con- nected to segment g of the tens-of- hours display, and D 2 and D 3 to segments e and g of the hours display respectively. When the clock shows 22.00 h, the common line of D, . . . D 3 becomes logic 1, because the three segments to which the diodes are connected are and the clock signal of the digital clock is divided by two. The clock twin dimmer then runs at half speed only so that it will take two hours before it shows 23.00 h. For the circuit to work correctly, it is essential that the clock signal is divided by two exactly, and this means that resistors R 2 and R 3 must be 1 per cent types. This is also the reason that a BS 170 is used as the switching gate; this MOSFET has no saturation voltage. Using a normal transistor with a certain saturation voltage would not cause the clock signal to be divided by two exactly, so that the clock would be fast or slow by minutes within a few days! The circuit as drawn is intended for common-anode displays; if it is to be used with common-cathode displays, simply reverse the connections of diodes D, . . . D 3 . Dimmer circuits are always popular and this one offers two independent controls in one. Control of each section of the circuit is provided by a type S576 which is an improved version of the S566. This type of 1C controls the phase gating by short or long command pulses emanating from a touch pad. Pulses shorter than 60 ms are treated as Short pulses between 60 ms and 400 ms cause the lamp to be switched on or off, depending on whether it was off or on respectively. If the touch pad is touched for more than 400 ms, the appropriate lamp is dimmed at a certain speed. If the finger is held on the touch pad, the lamp will go out completely and will then slowly light up again: when it reaches full brightness (and the finger is still on the pad), it will begin to dim again, and so on. The S576 is available in three versions: A, B, and C. With the A and C ver- sions, the lamp is always switched on or off half-way between maximum and minimum brightness, and it first attains maximum brightness before it can be dimmed. The B version is interesting in that it remembers the last brightness level, so that the lamp is always switched on or off at the last brightness setting. These various possibilities are summarized in The circuit of the twin dimmer is shown in figure 2. Power for the ICs is provided via R 2 , C 4 , D,, and D 3 . The supply is smoothed by C,. Capacitors C 3 and C s determine the speed with which the lamps dim or get brighter. The twin dimmer is best built onto the printed circuit board shown in fig- ure 3. This board is intended to be fit- ted into a standard round junction box. Because of this, it is, of course, important that the components used are of the correct size as shown on the board. 1 ,8.99 For Components Sources See Page 9-38 two-frequency oscillator Not so long ago, when semiconduc- tors were still quite expensive, it paid to make a transistor serve more than one function. Although this is no longer necessary because of cost con- siderations, it is still fun to do so — and it may even have its uses! The circuit presented here is an LC oscillator that changes frequency through reversal of the supply voltage. When the supply voltage is positive, D, conducts and short-circuits L,C,. Oscillations are then maintained by crystal XL; and L 2 C 2 . The DC operating point is set by P, in a way which ensures a compromise between faultless starting of the oscillator and low distortion of the output signal. When the polarity of the supply voltage is reversed, transistor T, operates in its inverted mode, i.e., the functions of emitter and collector are U = +10V-f« U = —10 V -* fxi interchanged. This means that the amplification is reduced, but, of course, an oscillator needs an amplifi- cation of only just above unity to operate. Crystal XL 2 and L 2 C 2 are effectively cut out by D 2 , and the fre- quency is now determined by crystal XL, and L,C V The circuit lends itself, for instance, for use as BFO switched between USB and LSB. The crystals may have values of up to 1 MHz. Current consumption in either mode does not exceed 45 mA. From an idea in the Master Handbook of 1001 Electronic Circuits. 8.100 ktor India Aug/Sept DEB automatic car alarm Even the best car alarm is useless if you forget to set it upon leaving your car, whence this circuit. The relay has a make and a break con- tact: the former is necessary to delay the switching in of the alarm after you have got out of your car, and the latter serves to switch on the car alarm proper. Immediately on re-entering your car, you must press the hidden switch, S,. This causes silicon-controlled rec- tifier Th, to conduct so that the relay is energized. At the same time, the green LED lights to indicate that the alarm is switched off. As soon as the ignition is switched off, T, is off. T 2 is on, and the buzzer sounds. At the same time, monostable 1C, is triggered, which causes T 3 to conduct and the red LED to light. The silicon-controlled rectifier is then off, and D 4 is reverse biased, but the relay remains ener- gized via its make contact for a short time, preset by P,. As soon as this time has lapsed, the relay returns to its quiescent state, and the alarm is set can be set to a maximum of about 1 via the break contact. The delay time minute. BBS garage stop light A novel use of solar cells makes pos 2 itioning your car in the garage rather easier than old tyres, a mirror, or a chalk mark. The six solar cells in figure 1 serve as power supply and as proximity sensor. They are commercially available at relative low cost. The voltage developed across potentiometer P, is mainly dependent on the intensity of the light falling onto the cells. The cir- cuit is only actuated when the main beam of one of the car's headlights shines direct onto the cells from a distance of about 200 mm (8 inches). The distance can be varied somewhat with P,. 1 Linder those conditions, the voltage developed across C, is about 3 V, which is sufficient to trigger relaxation oscillator N,. The BC547B is then switched on via buffer N 2 so that D 3 begins to flash. Diodes D, and D 2 provide an additional increase in the threshold of the circuit. The total voltage drop of 1.2 V across them ensures that the potential at pin 1 of elaktor india Aug/Sept 1986 9.01 the 4093 is always 1.2 V below the voltage developed by the solar cells. As the trip level of N, lies at about 50 per cent of the supply voltage, the oscillator will only start when the supply voltage is higher than 2.4 V. The circuit, including the solar cells, is best constructed on a small veroboard as shown in figure 3, and then fitted in a translucent or transparent man- made fibre case. The case is fitted onto the garage wall in a position where one of the car’s headlights shines direct onto it as shown in fig- ure 2. The LED is fitted onto the same wall, but a little higher so that it is in easy view of the driver of the car. When you drive into the garage, you must, of course, remember to switch on the main beam of your headlights! A descriptive article on tne operation and use of solar cells appeared in the July 1985 issue of Elektor: solar bat- tery. twin keyboard for Apple II W Arends and H G Scholz The keyboard supplied with com- puters is for many applications not the neplus ultra it is claimed to be. Unfor- tunately, deficiencies normally do not become apparent until the machine has been in practical use for a while. Retailers have long since realized this and often stock improved keyboards that are fully compatible with the computer in question. It is, however, not always clear how the new keyboard can be attached to the com- puter. One possibility is, of course, to open the computer, remove the existing keyboard, install the new keyboard, and put the computer together again. It is, however, much better to use the solution suggested here, which is aimed at the Apple II and compatible machines. The accompanying circuit makes it possible to connect the additional keyboard in parallel with the existing one. Basically, it is just an electronic switch-over unit, designated MUX in the diagram. Both keyboards are connected to the input of MUX by their data lines. Which keyboard data are applied to the computer is from now on deter- mined by MUX. When a key is struck, the keyboard does not only generate data bits, but also a strobe pulse. Depending on whether the strobe pulse emanates from the original or from the additional keyboard, the G output (pin 14) of bistable IC 2 is set or reset. This pulse, therefore, serves as a sel- ect signal for the MUX. The electronic switch consists of two type 74LS157 ICs. Each of these ICs contains four 2-to-1 multiplexers, so that all eight input data are available at the output. If the select input of both ICs is logic 0, outputs 1Y. . 8Y contain the data present at inputs 1A...8A. If, however, the input to the ICs is logic 1, the data from IB. . .8B is available at 1Y. . 8Y. The Apple II requires a positive strobe pulse, and inverters N 2 and N 3 are. therefore, provided to ensure that this condition is met whatever the strobe pulse from the additional keyboard. 0 3 2 blow that synthesizer! Circuits for generating electronic popularity because they are con- music are usually controlled by key sidered to be easier to learn to play switches. Not only do keyboards offer than string or wind instruments, the simplest technical solution for pro- Because of that, we have not tried to ducing fast changing, reproducible create an electronic oboe, flute, or tones over a wide frequency range, clarinet with the present circuit. In any but they also enjoy tremendous case, the technical intricacies associ- ated with such instruments would make their electrophonic counterpart prohibitively expensive. So. what we have got here is the rela- tively simple facility of converting breath power into a proportional analogue voltage with which the volume of a music synthesizer can be controlled; the tones remain con- trolled by the keyboard switches. No doubt, many of you, ingenious readers, will be able to think of various other applications of the converter. The circuit does not operate direct from the exhaled breath, but from the noise generated by this. A thin, flex- India Aug/Sept 1985 9.03 ible tube, to which a mouthpiece may be attached, leads into a closed box, in which not only the circuit, but also an inexpensive microphone have been fitted as shown. The noise received by the microphone is amplified in 1C,, the gain of which can be adjusted with P,, and sub- sequently rectified by IC 2 -D,-D 2 . An active low-pass filter removes most of the ripple from the output voltage. To keep the circuit as simple as poss- ible, we have opted for a compromise between input sensitivity and output ripple: the relation between these two properties can be adjusted with P 2 . If you have an oscilloscope with slow sweep, calibration of the converter should present no problems. First, adjust the value of P, so that tfi£ output voltage with hard blowing into the tube just does not cause full drive (dependent on the sensitivity of the following instrument). Second, adjust P 2 so that the output signal is relatively free of ripple, while the converter still reacts to normal breathing. A steeper filter would have been better here, but that would have increased the cost. fl 3 3 automatic sliding door Nobody pays much attention to auto- matically opening and closing sliding doors nowadays. In view of the com- plex mechanics involved, not too many people have so far attempted to fit an automatic sliding door in their living room. If you are happy with a relatively slow movement, such a door can, however, easily be realized with the aid of a DC motor and a small electronic control unit. The basic mechanical construction of the automatic door is shown in fig- ure 1. A suitable length of stranded nylon wire is attached to the left- and right-hand sides of the door and strung across four nylon roller guides as shown. The wire is attached to the spindle of a DC motor, the rotational direction of which depends on its polarity. Such motors are available in variety in many model building shops or from electrical suppliers, and should be suitable for operation from 6... 18 V. It will be sufficient to loop the nylon wire a couple of times round the motor spindle. Correct tension is obtained by incorporating a tensile spring in the wire as, for instance, shown in figure 1 . A small push button switch is fitted in the left and right-hand door frames so that when the door is fully open or closed, a switch contact is closed. You also need a light barrier or similar device that transmits a positive pulse of suitable length on the approach of a person. Such devices have been published in Elektor before, and there is also one elsewhere in this issue. The diagram in figure 2 contains a bridge circuit, consisting of transistors T, . . .T 4 which, depending on the logic level at the bases of T,-T 3 or T 2 -T„, determines whether the motor is at standstill, rotates clockwise, or turns anti clockwise. When the circuit is being tested, the motor may be replaced by D, and D 2 (with limiting resistors R a and /? 9 respectively). The choice of transistors depends on the current drawn by the motor, which should not exceed 500 mA. T,-T 3 and T 2 -T 4 form complementary pairs, for instance, BD239-BD240. A short pulse at pin 6 of bistable FF, sets the door in motion: the first time. it may be necessary to reverse the connections to the motor! When the door is fully open, it touches switch S 2 . It does not matter whether it is just a touch or whether the door keeps the switch depressed: the motor stands still for a short time, which is adjustable with P,, and then rotates in the opposite direction so that the door closes. If, while the door is closing, the light barrier is actuated, the motor changes direction again, and the operation repeats itself. When the door is closed, switch S, provides a pulse which causes the motor to be switched off until the next time the light barrier is actuated. 9.04 digital joystick interface C G Mangold The BBC and Electron computers pro- duced by Acorn have a joystick port to which only analogue joysticks can be connected. For many purposes, a digital joystick, i.e., one with four con- tacts, is much more suitable. The interface suggested here enables a digital joystick to be used with the two computers mentioned. The joystick port is provided with a voltage of 1.8 V when the analogue joystick is set to the left or top pos- itions, 0 V with the joystick in the right or bottom positions, and 0.9 V with the joystick in neutral. The 1.8 V is the reference voltage of the analogue-to- digital converter in the computer. As can be seen from the circuit diagram in figure 1, the various voltages can simply be provided by four sets of contacts or switches. Each of the sets of contacts controls an electronic switch. The 0.9 V for neutral is obtained from a potential divider. The electronic switches are required because the contacts in the joystick have a common connection and can, therefore, not be used direct Table 1. for shorting resistors in the potential The interface is calibrated with the aid divider. The fire button is connected of a small auxiliary program: REPEAT to the +5 V line by a junction in the PRINT ADVAL(I) ADVAU2): UNTIL® joystick, and thus produces a logic 1 Potentiometers P, and P 2 should be when it is pressed, whereas the com- set to the centre of their travel, puter expects a 0. The signal is. Connect the joystick and the interface therefore, inverted by transistor T,. to the computer, start the auxiliary 1 or mdia Aug/Sept 1985 9.05 [] E u GHz prescaler In the 12 GHz input stage (February 1985) for the microprocessor- controlled frequency counter, we used an SB8755 prescaler in the IC 7 pos- ition. This 1C, which divides the 100. . .1200 MHz signal at input C by 512, is perfect for the purpose, but is rather expensive. Just recently, another prescaler, which is much cheaper and more sensitive, has come onto the market: the U665B from Telefunken. The U665B is a -F1024 prescaler with integral pre-amplifier. Its sensitivity is better than 10 rnV (ms for frequencies between 80 MHz and 900 MHz. It is fully usable up to 1200 MHz, but its sensitivity drops to about 30. . .40 rnV, ms at that frequency. To fit the U665B onto the PCB, first remove existing IC 7 , IC 8 , and P 3 . No other components should be removed because, although they may look superfluous, they are needed for the interconnection between the compo- nent and track sides of the board. The new 1C is fitted so that its pin 1 coincides with pin 8 of the previous IC 7 . Next, solder capacitors C m , C m , C m , and C 104 direct to the relevant pins of the new 1C and to the earth plane. Then, solder pins 4 and 6 direct to the earth plane and place a wire link between pin 8 of the U665B and the hole where pin 1 of IC 7 used to be (see drawing). Finally, solder a wire link between the holes where pins 1 and 11 of IC 8 used to be. So much for the hardware; now something about the software. The U665B divides the input frequency by 1024, while IC 7 + IC 8 divided by only 512. This difference means that one byte in the EPROM must be altered: address $627 reads $09; this should be amended to $0A. Finally, note that the U665B may not yet be available everywhere. 9.06 eleklor india Aug/Sept IS Digi-Course Chapter 4 Solving an old puzzle with electronics. Circuits based on NAND and NOR gates were discussed in the last chapter. The truth tables make the functioning of these gates very clear and easy to understand, by relating every input combination to a unique output state. As we do not have five NOR gates on our Digilex Board, one of the inverters can be replaced by an inverter obtained from a NANO gate. The solution is very simple. By connecting the circuit on the Digilex Board and studying the input-output relations we know that the output indicates whether both the inputs are equal. The function is known as Inclusive- OR or simply EXNOR. The complete truth table is given below for those interested in the theoretical aspect of the circuit. 0 0 In the last chapter, we had given two problems for you to solve. Let us first discuss their solutions. The first problem was to construct a circuit functioning as an AND gate using only NOR gates. The truth table is given below. NOR AND A B A + B A • B 0 0 1 0 0 10 0 10 0 0 110 1 Similar to the OR circuit built using NAND gates, here also the input states must be inverted. The inverters are obtained by using NOR gates with shorted inputs. The second problem was to find the function represented by the following circuit. Wolf. Goat and Cabbage. The old puzzle about the wolf, goat and cabbage can be solved with the help of electronics. This is how the story goes: A farmer comes with a wolf, a goat and a cabbage to the bank of river. He has to cross the river in a boat. But there is a problem - he can take only one object with him in the boat at a time. How does he cross the river, without the wolf gobbling up the goat or the goat gobbling up the cabbage? We can connect the circuit on our Digilex Board to solve the puzzle. We need all the three ICs for this purpose; and all the eight indicators. For solving the puzzle, we need indication for the presence of the animals and cabbage on the left and right banks of the river, and of course - indication if there is a risk on any of the two banks, that either the wolf gobbles up the goat or the goat gobbles up the cabbage. Definitions At the begining. we must define certain things - because the gates can only recognise only two states "O" and "1" represented by 0V and 5V. ug/Sept 1985 9.07 4t@ 8,8 0. t 8. 8,8- 4l © (• 1 ® Our circuit has four inputs - one for each participant, namely the wolf, the goat, the cabbage and the farmer. They can be either on the Right bank or the Left bank of the river, so it's easy to represent by "0" Let us denote their presence or the "Right" bank by "0" and presence on the "Left" bank by "1". The circuit can be connected in such a manner that LED indicators D1. D2, D3 will glow when cabbage, goat and wolf are present on Lett bank and D6, D7, D8 will glow when they are present on the Right bank. When the Digilex Board is placed on the top edge of the magazine, the symbols show the meaning of LEDs. LEDs D4 and D5 glow if there is risk on Left or Right bank. The inputs are provided with four connectors, at the input terminals: K13 for cabbage (C) L10 for goat (G) M4 for wolf (W) N2 for farmer (F) Connecting the circuit. Connect the indicators first. The LEDs on Left side must glow when the corresponding inputs are "1 ", Hence they are connected directly as follows: Both these circuits require a triple input nand gate, which is not available on the Digilex Board. Hence we must look for another circuit configuration. Rather than proceeding mathematically, we want to solve the problem as a game. Mathematical approach will also require detailed knowledge of Boolean Algebra. Let us tackle the problem from the other side. Instead of looking for combinations when there is risk, let us look for the conditions when there is no risk. This step is not as arbitrary as it may appear. In fact we need an inversion at the end because we have only NAND and NOR gates available on the Digilex Board. The goat plays a dicisive role, as it can eat the cabbage or can be eaten on by the wolf. Let us start with the left bank. There can be no risk on the left bank when cabbage and wolf are both on the right bank. This is represented by the following circuit. As we have only eight LEDs, no indication is provided for the farmer. When the inputs are "0", the LEDs on right side must glow. This requires an inverter between the output and input. Hence the connections (k12-K13, L9-L10, M4-M5, N1-N2 are shorted so that the NAND gates function as inverters.) The remaining circuit must be connected in such a way that LED D4 glows when there is risk on Left bank and LED D5 glows when there is risk on the right bank. The risk condition is present when either the cabbage and goat or the goat and wolf are present on the same bank without the presence of farmer on the bank. Let us begin with the left bank. LED D4 must glow when cabbage and goat are on left bank and farmer is on right bank, and it must also glow when goat and wolf are present on left bank and farmer is on right bank. * M *5 j| 'if ♦ f , . » ' ■ t H ' •* << itu m v** India Aug/ Sept 1985 The connections are as follows: K11 - R13 M6 - R12 R11 - V12 VII - 0 VI 3 - E (For testing the output) Because of the size of boat the farmer cannot leave the goat on one bank alone and take cabbage and wolf together with him to the other bank. This means that there can be risk on the left bank when goat is present but farmer is not present. This can be represented by the following circuit. "1" except when G=" The connections are as follows: L10 - S9 (change over B - L10 to B - S9) N3 - S10 S8 - D (for testing the output). i££i s; u v I™ ■A _ © c ! But these circuits let us know when there is no risk on the left bank. We need an' indicator to glow when there is risk on that bank. So the outputs of these two individual circuits are placed on the inputs of a NOR gate and the output of the NOR gate is connected to the indicator circuit D. This makes the LED D4 glow when there is risk on the left bank. Remove S8 from D and VI 3 from E. and connect S8 - W9 VI 3 - W8 W10- D. Thus we have got a circuit which indicates risk on the left bank. A circuit to indicate risk on the right bank is now very simple. The point to note here is that a logic ”0" state at the input represents presence on the right bank. The connections are as follows: K13 - U2 (previously reverse K13-A to A-U2) M4 - U1 (previously reverse M4-C to U1-C) U3 - Y3 Y2 - 0 Y1 - X6 T6 - X5 X4 - E The circuit makes the LED D5 glow when there is risk on right bank. Now. you are ready to solve the puzzle on the Digilex- board! Note: Do not leave any inputs in open states, connect India Aug/Sept 1985 9.09 Everyone remembers how the natural phenomenon of ECHO fascinated us in childhood, RADAR also functions using the principle of ECHO. A special transmitter transmits a radio signal which is reflected, when it hits an obstacle like a ship, a plane, a car or even a mountain. The returning signal is received by a receiver. The time taken by the signal to return tells us the distance between the Radar and the obstacle. The frequency of the radio signal used by Radar is between 0.3 GHz and 30 GHz. (1 GHz = 1 Billion osciallations per second). Since the radio signal can also travel at night and under fog. Radar can "see" even under these conditions which are impervious for the human eye. Two of the most well-known possibilities for the application of Radar are: 1. Radar for locating ships, aeroplanes, coasts etc. 2. Radar for mointoring speed in road traffic. This articles deals only with the first type-Locating Radar. Radar can be recognised easily from the large rotating antenna. One such antenna is shown in figure 2. The Radar antenna transmits short pulses of radio signal. The construction of the antenna ensures that the signal is beamed in only one direction. The receiver then "listens" for the echo in the same direction, after every pulse is sent. If the beamed signal pulse encounters an obstacle, the signal is echoed back and received by the receiver. The Radar evaluates the time lag between the transmitted pulse and the received pulse. The longer the obstacle distance, the greater is the time lag. The Radar antenna continuously rotates around its axis, thus covering all directions in each rotation. The screen of the receiver is also circular in shape and the center of the screen represents the location of the 9.10 elektor India Aug/Sepl Antenna and Display unit of a small Radar equipment Radar station. The complete screen can be considered as the map of the area being covered by the Radar beam. The Radar signal pulse can be thought of as an invisible point moving on the screen from the center to the border of the screen. As soon as an echo is received by the receiver it makes this invisible point visible momentarily and the screen is illuminated at this position which corresponds to the location of the obstacle from which the pulse was echoed back. As the antenna is rotated uniformly at 20 to 30 rotations per minute, the illuminated spot on the screen also indicates the exact direction of the obstacle with respect to the Radar position. The pulses are sent out at the rate of 5000 per second, and thus forming an invisible line on the screen between the center and the border which continuously points in the same direction at which the antenna faces. Thus this invisible line scans the complete screen during each revolution of the antenna. The point representing the obstacle location is illuminated during each rotation of the antenna. If the obstacle happens to be moving, the movement is also recorded by the illuminated spot on the screen. As the fixed Radar station locates the moving obstacles, like ships and aeroplanes etc., conversely ships and aeroplanes with built-in Radar systems can determine their own location with respect to a fixed point, or various features of the landscape around them. (Mountains. Coastal strips etc.) These mobile Radar equipments consist only of two units — the antenna and the screen, as shown in figure 3. As against this, the modern Radar systems, for instance the control apparatus of flight safety, are considerably more elaborate and complex, and they are mostly computer controlled. The computer collects and processes the information from the Radar receiver before feeding it to the screen. The illuminated spot does not fade out during each rotation and the computer can superimpose a map with important information on the screen. A secondary Radar equipment shown in figure 4 can even identify individual aeroplanes. For this the plane must have a transponder on board, which receives the Radar signal and transmits its own signal containing a characteristic features. The secondary Radar equipment of the flight controller receives the transponder signal, decodes the characteristic features code and shows it on the screen. From this the traffic controller knows, as to which illuminated spot corresponds to which aeroplane Without this type of computer-controlled Radar equipment it would be impossible to manage the monitoring of the modern air traffic at large Airports. Radar screen at the Air Traffic Control ;9.1 1 4.5 V Battery Eliminator A simple battery eliminator circuit for 4.5V output is presented here which will be quite useful for the hobbyist. The circuit must meet two basic requirements. 1 . To convert the mains supply voltage of 230 V AC to 4.5 V AC. 2. To rectify the AC voltage. The first requirement is met by the transformer. The mains supply cord brings the mains voltage of 230 V to the transformer, through a fuse Si and the main switch SI . SI is used to switch the power ON or OFF, where as the fuse Si serves to interrupt the supply of current to the transformer in case of a short circuit. As soon as a short circuit develops, the current through the fuse shoots up, the fuse wire melts and the supply is cut off. The neon lamp La glows when the mains voltage is available across the primary winding of the transformer. When the mains voltage of 230 V AC is present across the primary, an alternating voltage of 4.5V induced across the secondary winding. This 4.5V AC is supplied to the rectifier bridge, made of four diodes. A diode allows current flow only in one direction, as if it was a closed switch. (Figure 2) If the voltage changes polarity, the diode behaves like an open switch and blocks current flow. (Figure 3). Now let us see the effect of having four diodes in bridge connection. At first let the plus pole be on the top of the bridge input. Diodes D2 and D3 behave as closed switches. Diodes D1 and D4 behave as open switches, (see figures 2 and 3). This combination gives a plus pole at the top even at the output of the bridge. As the voltage supplied at the input of the bridge is an alternating voltage, the poles will reverse after one hundredth of a second and now at the input of the bridge we have minus pole at the top. (Figure 5). As the voltage polarity has reversed, diodes D2 and D3 behave as open switches and diode D1 and D4 behave as closed switches. Even this combination gives rise to plus pole at the top on the ouput side which means that we have a direct voltage at the output of the bridge. 2 ©— ►kg -e 83615X-2 This direct voltage at the output is however not comparable to the battery voltage. Even though the direction of voltage is steady, the value is not. This gives rise to a humming noise if an audio apparatus like a radio is connected across this power supply. An electrolytic condenser of 1000 jjF/ 16V must be connected across the output terminals of the bridge circuit to get over this problems. (Remember the polarity of the Electrolytic condenser) 3 0 - 00 — © - ©— «''•—< © 83615X3 An LED in series with a 180(1 is shown in figure 1 across the output. This can serve as the "Power ON" indication instead of the neon lamp La. 1 230V 9.12,l.ttorin vase 4 Component List B1 = Rectifier bridge Trl = 4.5V/500mA Transformer La = Neon Lamp (230V) Si = 200mA Fuse SI = Double pole switch 230/1 5A Other components: 1 Mains cord with 3 pin plug. 1 Enclosure Box. 2 Sockets (Red and Black) 1 Fuse holder and fuse 5 Construction Details: Assembly of the circuit can be simplied by using plastic box. While four leads of the rectifier bridge are directly soldered. Input pair to the secondary of transformer and the output pair to the + and - sockets. Double check all connections and polarities before switching ON. If all connections are correct and components good, circuit should work at the first attempt. Figure 6 shows a laboratory prototype. Computer Tomography In the year 1895. a photograph was taken, showing something that had never been seen in a photograph The Bones of a living human hand! The photographer was Konrad Rontgen. He proved with this that a ray exists, which is indeed related to light, but works much finer than visible light. This ray, called Rontgen Ray (X-Ray) can pass through the human body and expose a photographic plate. Bones and thicker tissues produce shadows on the photographic plate by blocking the X-Rays. Prolonged exposure to X-Rays can cause damage to the human body Even the inventor of X-Rays himself became a victim of the over exposure. In spite of this risk, the X-Rays are one of the most important diagnostic The conventional X-Ray process has two main disadvantages: 1 . Organs lying one over the other are not clearly distinguished in the X-Ray photograph, as their images overlap each other. For example, during a scan of the lungs, the ribs, the lungs the vertebral column and heart are simultaneously photographed on the plate. 2. Many organs block the X-Rays with equal intensity and appear on the plate as same shade of grey. A simple method for overcoming this problem is to take the photographs from two different directions so that the X-Ray specialist can accurately localise the conspicuous tissue, for instance a tumour. Unfortunately even in the second photograph the images do overlap in another direction, and the process still does not offer a totally clear picture. However this gives us an important clue that each additional photograph contributes additional useful information. This itself is the basic principle of the Tomograph. Tomograph The usual X-Ray equipment transmits one cone of rays, with which the entire surface of the photographic plate is exposed. The X-Ray tube of the tomography equipment transmits the rays only in one plane, (see figure 3). If these rays were used to expose a photographic plate as before, they would form only a thin line image on the plate. These rays are taken up by a row of X-Ray sensitive electronic detectors, which measure the intensity of the rays reaching them. An electrical signal corresponding to the intensity is sent to the computer by each individual detector. For preparing a tomograph, the total equipment, including the X-Ray apparatus and the row of detectors, rotates once around the patient. Figure 4 shows that the source of rays and the array of detectors must rotate in the same plane. The X-Ray eleklor India Aug/Sept 1985 9.14 Detector with 256 or 512 individual detector elements. apparatus emits the rays at short intervals, so that during a full rotation a number of sets of values are collected. Because of the pulsed radiation of the rays, effect of the rays on the patient is kept within limits. The computer collects all the measured values and puts together a picture from them, which shows the cross section of the body in the plane of rotation. Figure 5 shows a typical tomograph. The scan level is at the height of kidneys. Even a layman can recognise the liver on the left side, vertebral column in the middle and the two kidneys next to it. (The tomograph shows the section from below.) Since every point is scanned several times, this apparatus can also distinguish between less differentiated tissue types, which appear equally grey on the conventional X-Ray plate. The Computer The computer carries out the following functions: — Collecting the detector values. —Composing a picture from the collected values. — Controlling the operation of the complete equipment. How the computer produces the picture from the measured values can be explained with a simple example. Figure 6 shows a cross-section, which consists of nine squares, some are white and others black. Like the organs of the body the squares are not seen from outside. A simplified tomograph should find out the distribution of the squares. At first the structure is scanned from the top. The detectors lying below detct the shadows of the squares. From this the layout of the elements cannot be recognised. The second photograph is taken from the side. Even this measurement (b) does not allow us to draw any conclusion about the distribution of elements. The third photograph taken along the diagonal shows the presence of three black squares along the diagonal. This corresponds to the third possibility among the five possibilities for the distribution presented by the first two photographs (see figure 6c). It is not just a coincidence that in our example three measurements were necessary. From three tests with three values each, we obtained nine observations which are sufficient to establish the distribution of elector india Aug/Sopl 1985 9. 1 5 5 In case of the real tomograph the number of squares (fields) is much greater and the size of squares shrinks to a point in the picture. (about 1 square mm.). Their number can be calculated in the same fashion. The computer tomograph SOMATOM from Siemens has a row of detectors with 512 elements. The frequency of measurement per rotation can be set to 240, 360, 480, 720 or 1440. If the number of detectors is multiplied by the frequency, we obtain the total number of points in the picture. It lies between 122880 and 737280. (For comparison, consider a normal TV picture which has 440833 such points.) This flood of data is processed by the computer during each rotation, which can last for 1 .4 to 3.2 seconds. Apart from this the modern tomograph can also produce longitudinal sections and even three dimensional pictures. For this purpose a series of sectional pictures are taken, shifting the bed of the patient a little between successive scans. The computer combines all these sections to present the total picture. elektor indis Aug/Sapt 1985 9.16 Batteries in series connection We have already seen that the standard voltage available from a Zinc-Carbon cell is 1.5 V. However we also know that batteries with even higher voltages are available, for example, the 9V battery, which is. most popularly known. I The second illustration in figure 1 discloses the secret of this higher voltage available from a single battery. Six individual 1 .5 V cells are packed inside the casing to give 9 V output. Some maufacturers also offer battery packs with 3V or 4.5V output. First and the third illustrations in figure 1 show the details of these battery packs. This principle of increasing the total output voltage is know to everyone who has a torch. More than one battery cells are inserted into the torch casing in such a way that plus pole of every cell has a connection with minus pole of the previous cell. This arrangement functions in a way similar to that of a train having two engines. Naturally, both the engines must pull in the same direction. In case of batteries, the voltages must pull in the same direction. This "direction of pull" of the cells is shown in the illustrations by arrows. Whenever batteries or other electronic elements are connected one after another like this, it is known as a series connection. The output voltage of a series connection of batteries is the total of individual cell voltages, as can be seen from the arrows in the illustrations. We can build up a battery pack using battery holders or battery boxes readily available in the market. Two such battery holders are shown in figure 2. The voltage available will always be a multiple of 1 .5V; 4.5 V with 3 cells, 6 V with 4 cells, 9 V with 6 cells and so on. Today the transistorised equipments do not need very high voltages but when the Transistor technology was still in its infancy, battery packs as large as a shoe box were available, containing 60 or 80 individual cells connected in series and delivered 90 V or 120 V at the output. &sm Dry Battery Charger Although the rechargable accumulators have been with us since long, hobbyists, inventors and scientists have always been interested in inventing a process for charging normal Zinc-Carbon batteries. Even today, no true recharging process exists for this type of batteries. However a part of the emitted charge can be replenished, when the batteries are not strongly discharged. This can substantially increase their service life. Circuit of recharging Many of the transistorised equipments like Cassette Recorders, Radios, etc. can be operated from mains as well as batteries. These equipments can be fitted with a few extra components and easily converted to operate in such manner that the built in eliminator of these equipments can replenish the charge of the batteries. These equipments have a switch (S in figure 1 .). which changes over from batteries to the built-in eliminator and vice versa. Mostly this switch is incorporated in the mains socket and is automatically switched from batteries to mains when the mains plug is inserted into the socket. The modifications to achieve this are shown in figure 2. This requires a resistance Rr and a few diodes Or. Components The batteries can be charged at the most with 1.7V per cell, hence the built-in eliminator voltage cannot be used directly to recharge the cells. If this voltage ( U NT) is less than the battery voltage, it is not possible to recharge the batteries. If it is more than 1.7V « n (n=number of cells used) then it must be brought down using diodes Dr. First the voltage U NT is measured with a multimeter under no load condition. (To achieve this, keep volume control on minimum and switch off the drive motor of the recorder.) The difference between this measured voltage and the value 1 .7V x n must be taken up by the diodes, each of them taking up about 0.6V. — Op o> b— OP o> Example A transistor radio with 9V batteries is being modified for recharging. The built-in eliminator output on no load is 11V. The voltage during recharging should not be more than 1.7V >6 = 10.2, which means that the difference of 0.8V must be taken up by the diodes. As one diode can take up 0.6V, we need two diodes in series. Thus 1 .2 V will be taken up by the diodes and we are left with 9.8V as the recharging voltage, which means a voltage of 1 63V will be available across each cell. For resistance Rr the genrally used values are as given below: Battery Voltage Resistance Rr 12V 6811 9V 4711 7.5V 3911 6V 3311 4.5V 2211 9.18 i Experiments witii batteries in series The following experiments will give you a practical idea of what we have so far studied about batteries in series connection. The components used in these experiments can be obtained from any electrical goods shop. — 1 Battery box suitable for 9V output. — 1 Torch bulb. (3.6V or 6V) — 1 Holder for bulb. — 2 Pieces of insulated copper wire. — 6 Battery cells to be fitted inside the battery box. The battery box should be preferably fitted with a detachable end plate, repositioning which converts the box into 6V or 3V battery box. Before starting the actual experiments, you have to fit the bulb into its holder and connect two pieces of wire to the two terminals on the holder. (Don't forget to remove the insulation from the wire ends!) Now to check that the battery cells are in order, touch two poles of any individual cell with the two wires connected to the bulb holder at the other end. Does the bulb light up? Then everything is in order. We can medium brightness Four stages of the experiment. Voltage across the bulb is provided by the number of ceils in circuit. Brightness of the glow of bulb depends on the voltage available across The battery box has two rows of 3 cells each. For our first experiment we need only one row of three cells. Connect one wire to the minus pole end of the row and second wire to the plus pole end. This covers all the three cells - effectively giving the total voltage of 4.5V available from the series connection of three cells having 1.5V each. The bulb glows brightly. Chhotani Building. 52-C, Proctor Road, Grant Road (East), Bombay 400 007 Now, without removing the wire at the minus pole end, touch the other wire to the plus pole of the second cell instead of the third. This time there are only two cells in circuit, giving a total series voltage of 3V. The bulb glows, but not as brightly as before. Take the wire end away from the plus pole of the second cell and touch the plus pole of first cell with it. The bulb now has a very dim glow. Naturally because the voltage available in the circuit is now only 1 ,5V. If you now connect both the wire ends to the minus pole of the first cell, the bulb does not glow at all! By changing the point at which we tapped the voltage, we were able to get voltages of 4.5V. 3V and 1 ,5V across the bulb. This time we used only one row of 3 cells from the battery box. If all the 6 cells are fitted into the box, the arrangement is like the one shown in figure 3. Two rows of 3 cells each, connected again in series by the end plate. From this arrangement it is now possible to tap different voltages from 0 to 9V as illustrated in figure 3. QUIZ: Finally a quiz question for you to solve by trying out. What is the voltage U in the following circuit? (To try out this circuit, shift the end plate in the battery box forward so that now it can accomodate 4 cells instead of 6.) ju|od qoea ie sseum6uq eqi Buijedtuoo 'q|nq aqi qjiM UMOqs eq os|e ueo smi AE = S'l -S'l + ST + ST pejonpep eq isniu e6ej|OA si| Apjeiod pesjeAej qi|AA pepeuuoo si sgeo eqi euo S V A9 °l d n PP e l0U op seBej|OA jjaqj jnq 'seues u| S||eo p eAeq e/w eteH : U3MSNV

] i-o-o X X The Digilex-PCB is now available! The Digilex-PCB is made from best quality Glass- Epoxy laminate and the tracks are bright tin plated, the track side is also soldermasked after plating. Block schematic layout, of components and terminals is printed on the component side. Price: Rs. 85.00 4- Maharashtra Sales Tax. Delivery charges extra: Rs. 6.00 Send full amount by DD/MO/PO. Available from: precious 0 ELECTRONICS CORPORATION Components Diodes Diodes are like one way streets of the electronic circuits. They conduct the current only in one direction. When voltage is applied in such a way that current can flow, a voltage of approximately 0.6 V is dropped across the diode. This value is the threshold voltage of the diode. The two terminals of a diode are known as cathode (represented as bar in the symbol) and anode (represented by the arrow head). Mostly the cathode terminal is marked by a coloured band, a dot on the body or by tapering the shape of the body itself. If the polarity is unknown, it can be tested using a battery cell and a suitable lamp. Transistors Transistors have three leads known as emitter, base and collector. Depending on the construction, the transistor can be NPN or PNP type. In case of an NPN transistor, the emitter should be negative compared to the collector, and in case of a PNP transistor, the emitter should be positive compared to the collector. ©Me © — H © The lamp lights if the diode is connected with its anode at the plus pole of the battery and cathode connected to the lamp. Maximum allowed reverse voltage and maximum allowed forward current are the most important characteristics of a diode to be considered when selecting a diode for a particular application. The principle of operation of a transistor is such that a small current made to flow from base to emitter inside the transistor causes a much greater current to flow between collector and emitter which is proportional to the base-emitter current, thus resulting in current amplification. In the SELEX circuits, mostly types BC547 (NPN) ‘and BC557 (PNP) are used, however if these particular types are not available, the following types can be interchanged: ■Hector. Collector. - ©I Emitter NPN: BC 547,8,9, BC 108,8,9 BC 237,8.9. PNP: BC 557,8.9, BC 177,8,9, BC 251,2,3. Light Emitting Diodes. LEDS are encapsulated in' a transparent casing and emit light when current flows through them. The threshold voltages in case of LEDs is not 0.6 V like a normal diode but lies between 1.6 V and 2.4 V depending on the type. The current should be between 15 and 25 mA. Integrated Circuits. Today there are so many different types of ICs, that only a few general remarks can be made here. Most of the ICs are moulded in DIL (Dual In Line) casing with two rows of pins. The rows of pins usually stand far apart from each other and must be bent slightly towards the centre line of the 1C. before inserting it into a socket. Pin No. 1 is generally marked with a dot or a notch. Testing batteries with an Ammeter. Frequently we throw away batteries which may not be really exhausted. When the battery is being used up, its voltage gradually reduces and a stage comes where the battery can supply about 60% to 70% of the rated value. Many electronic gadgets stop functioning at such low voltage levels. The batteries are replaced with new ones and the old cells are thrown into wastepaper basket. However, these batteries can still be useful for the electronics hobbyist for experimental purposes. How do you make sure, whether a battery is really exhausted or not? There is no point in measureing the battery voltage with a multimeter, because when no current is being drawn from the battery, even an exhausted cell shows 1 V on the multimeter. The best way is to switch the multimeter on to 2 A or 4 A DC range and test the current supplied by the battery. The reading on the multimeter should be at least 0.5 A. This measurement should be carried out very quickly- otherwise the battery will be exhausted by the test current itself. A good 1 .5 V 'Large' cell should supply about 5A. a 'Medium' cell about 3 A and a 'Pencil' cell about 1.5A. 9.22 jg/S«pt 1985 i mM GRAPHICS PLOTTER Anlka Instruments have successfully developed a new digital graphics plotter. The series 2600 graphics plotters are compatible with almost all micro, mini and mainframe computers. These microprocessor controlled plot- ters can be used to plot, on paper or film, high precision graphics generated by the computer system. The series 2600 plotters are high speed plotters with writing speeds of 45 cm/sec. They have the plotting resolut- ion of the order of 0.05 mm per step and a very stable line quality. A built in ASCII character set is provided along with various other graphics capabilities. * 7 For further information, write to : Anika Instuments Pvt. Ltd. 24-Housing Society, N.D.S.E. (1) New Delhi 110 049 UNIVERSAL PROM PROGRAMMER Professional Electronic Products have developed a Universal PROM prog- rammer PP-81, which is suitable for programming, copying and testing a wide range of PROMs. Additional features like clear RAM, Complement RAM and Partial Programming are also provided Personality modules suitable for programming EPROM, EEPROM and Bipolar ROMs are available. Professional Electronic Products, Post Box No. 316, Delhi Road, Meerut 250 002. PCB DRILLING MACHINE Elmech PCB drilling machine is a high speed PCB drilling machine which uses no pulleys, gears or belts. It has life lubricated ball bearings for smooth operation and is fully protected against overloads. The design is very compact and can also be operated on a battery. The drilling capacities available from various models are from 0.5 to 2 mm. The machine is suitable for phenolic based as well as glass epoxy boards. For further information, write to : Electromech Engineers India 882, Industrial Area A, Ludhiana 141 003 (Punjab) DELAY LINES FOR COLOUR TV Indian Television Industries offer lumi- nance delay lines for use in colour TV receivers. The standard delay line has a time delay of 330 ns and input/output impedance of IK ohms. This has 10% tolarance and can be used with chroma decoder 1C type TDA 3561. The delay lines can also be supplied in other specifications to meet other require- ments against specific order. For further information, write to: Diplomat Pvt. Ltd. 49/2, Gurjaipal Nagar. Jalandhar 144 001 DIGITAL CALIBRATOR A/D/I introduce a digital calibrator lor the instrument manufacturers and maintenance departments of the pro- cess industries. This unit can measure or feed voltages calibrated in terms of mV or °C. In the measuring mode the instrument has very high input imped- ence and in calibration mode it has a very low output impedance. The range available is 0 to 200 mV. This display is available in LED as well as LCD version, and the instrument can be supplied as portable battery operated unit or as a mains operated unit. Analog and Digital Instrumentation 962, G.I.D.C. Makarpura Industrial Estate, Vadodara 390 010 GRAPHIC TABLETS A new line of graphic tablets has been introduced by Preh. These graphic tablets are adaptable to various graphic systems and are claimed to be ideal entry devices for a large number of applications. For further information, write to: Shilpa International, 107, Park lane, Secunderabad 500 003 UDT PHOTOPS Photops Hybrid Detector/Amplifier Combinations from UDT, U.S.A. are high speed photodetectors with inte- gral electronics. Their ultrasensitive photodiodes detect light over a wide range of intensities. The built in JFET Op amps allow low level measurements at high speed and ensure low noise output. Applications range from medi- cal diagnostic instrumentation to bar code readers for commercial and industrial use. For further information, write to: Toshni Tek International 267, Kilpauk Garden Road, Madras 600 010. TOUCH SWITCH Patron Electronics have developed a bistable touch switch, which can be used to operate any electrical or electronic device with a single sensor. The sensor can be as large as 300 square cm in area. The switch is available as an OEM module or as a complete unit. For further information, write to: Patron Electronics 2421, Ghee Walon ka Rasta, Johari Bazar, Jaipur 302 003 i Aug/Sept 9.23 mM PLANIMETER Toshni-Tek International have market- ed a new Planimeter KP-90 manufact- ured by Sokkisha Co. Ltd. Japan. This computer planimeter is a precise instrument (or fast and accurate meas- urement of plane areas of any shape and contour. An electro optical shaft encoder which generates 1000 pulses per revolution is used for the basic measurement. The instrument works on rechargable NiCd batteries and has an auto shut off function when the instrument is not in use for more than 3 minues continuously. For further information, write to : Toshni-Tek International 267, Kilpauk Garden Road, Madras 600 010 PROGRAMMABLE LOGIC CONTROLLER Advani-Oerlikon have developed a programmable logic controller based on a single microprocessor chip. This equipment, ADOR PC-4896, is simple to program and easy to operate. It can accept a maximum number of 96 Input/outputs. The ADOR PC-4896 is ideal for use in continuous process plants where there are many applica- tions involving sequence control, firing, interlocking and precise speed control. The equipment finds appli- cation in cement, steel, petrochemical, thermal power and paper & pulp industries. Advani-Oerlikon Limited, Post Box No. 1546, Bombay 400 001. UNIVERSAL COUNTER Trans Marketing Pvt. Ltd. have intro- duced a new universal counter from Racal-Dana. The instrument has full GPIB program- mability, direct frequency measure- ment to 1.3 GHz and instantaneous nine digit resolution. Single shot time intervals down to a nanosecond can also be measured. The microprocessor-based design, combined with powerful custom-LSI provide a comprehensive maths capa- bility and a range of special functions. The input conditioning circuits provide maximum flexibility and include a choice of AC/DC coupling, input inpedance. trigger slope and common or seperate mode Amplifiers used have exceptionally flat response and wide dynamic range over the entire frequency band. The instruments are available under O.G.L. lor R & D laboratories. For further information, write to: Trans Marketing Private Ltd. Sterling Centre, 16/2, Dr. Annie Besant Road Bombay 400 018. SUB-MINIATURE TOGGLE SWITCH 'Comtech' T-7 is a sub-miniature PCB mounting toggle switch having overall dimensions olio > 7 > 15 mm (excluding lever) and a rating of 125 V at 0.5 Amps. (Resistive) or 0.5 VA at 28 V AC/DC. It is offered with either two positions (change over) or with three positions (with centre-off). The gold plated shorting and bar terminals ensure a faithful operation at low contact resistance of 25 miliohms. Body moulded in ABS plastic is designed to remain away from the PCB to help easy and fast soldering. For further informal ion, write to: Component Technique 8, Orion Apartments, 29/A, Lallubhai Park Road, Andheri (West). Bombay 400 058. AC TO DC REGULATOR The MAX 610 family of AC to DC Regulators has been marketed by Malhar Corporation. These regulators accept 8 V RMS AC input and need onlyafilter capacitor to make a complete 5 V DC/100 mA regulated power supply. With the addition of a current limiting resistor and capacitor the MAX 61 0 can directly connect to 110 V to 220 V AC line and give a 5 V DC output. The MAX 610 fmaily is ideal for applications where size, weight and component count must be reduced. The reliable power up reset and over/under voltage detection makes it well suited for application in micro- processor based controllers. For further information, write to : M/s. Malhar Corporation 43, Wheeler Road Cox Town, Bangalore 5 60 005. INDUCTIVE PROXIMITY SENSOR TKPS 4XX Inductive Proximity Sensors are non contact, solid state devices. These can be used as direct input interface for Programmable Logic Controllers and other Solid State Logic Systems. The sensor components are comple- tely sealed in epoxy to resist the effect of oils, coolants, dirt and grime, and have greater resistance to shock and vibrations. The sensor head may be immersed in non corrosive liquids without impairing its operation. Repeat accuracies of over 0.01 mm are available for accurate size control. Sensing distances upto 10 mm can be For further information, write to: Teknix International 2 Bhagat Nivas Bhagat Marg, Jaipur 302 001. DIGITAL OHM METER Economy Electronics have introduced a bench type instrument for the measurement of various resistances during manufacturing process of Electrical Heaters, Coils, Relays etc. The instrument is also useful for the inspection, production and R&D departments. 7 segment red LED display is provided for direct reading of the resistance value. Measurement ranges are available from 2 ohms to 2 M ohms in seven steps. Accuracy is + 0.1% of range *0.1% of reading + 1 digit. For further information, write to: Economy Electronics 15, Sweet Home, Pitamber Lane, Off Tulsi Pipe Road, Mahim, Bombay 400 016. 9.24 elektor india Aug/Sept EACH NEW DAWN BRINGS THE POSSIBILITY OF A NEW PROGRESS. • ICs: TTL. CMOS. MOS. LSI. LINEAR. MICROPROCESSOR. MICRO COMPUTER etc. • 1C SOCKETS: MOLEX & SMK MAKE INCLUDING MOLEX PINS & TEXTOOL O INSERTION SOCKETS. • TRIMPOTS MULTI TURN AS WELL AS SINGLE TURN OF DOURNS. VRN & DAKEMAN MAKE • DISPLAYS: 3".. 5" & 1" IN COMMON CATHOD & COMMON ANODE INCLUDING 3 Vi DIGT LCD. TANTALUM CAPICITORS OF ALL VALUES. ZENER DIODES. SILICON DIODES. POWER TRANSISTORS. PHOTO TRANSISTORS. PHOTODIODES. LED'S CRYSTAL, etc. INDENTOR FOR ALL TYPES OF ELECTRONIC COMPONENTS. INSTRUMENTS. GREEN PHOSPHER AS WELL AS RGB MONITORS ETC. Our Principles: PHONES: OFFICE: 369192 5137225, 5135645 M /s. Z,EX TRADING PTE LTD ELECTRONICS CORPORATION PUSHPDANT NIWAS 3 CHUNAM LANE 3RD FLOOR. OFF LAMINGTON ROAD GRANT-ROAD, BOMBAY-400 007 SINGAPORE/HONGKONG classified ads. ZX 81 users, RAM expansion available. Internal expansion up to 16 K and external RAM pack (or 48K memory available. Also available in kit form. Contact: PLS. phone 5618875 or write to P.O. BO 17770 Mulund (W). Bombay- 400 080 For 24 electronic kits as MW Transmitter, Musical horn for scooter or car. Running light (360 Watts) etc. Contact : PERFECT ELECTRONICS 453. Ganapati Ali, Wai - 421803. Available data sheet and application of any electronic components. Minimum charges Rs. 1 5/-. Write to: DATA BANK, Plot No. 16, Bldg. No. 3, Flat No. 17, Bhavani Nagar. Maral Maroshi Road. Andheri (East). Bombay - 400 059. Components are normally available with the following companies: V1SHA ELECTRONICS 1 7, Kalpana Building. 349, Lamington Road Bombay • 400 007 Phone: 362650 DYNALOG MICRO SYSTEMS 14, Hanuman Terrace, Tara Temple Lane, Lamington Road Bombay - 400 007. Phone: 353029, 362421 ELECTROKITS 20, Narasingapuram Street (First Floor) Mount Road Madras - 600 002 INTEGRATED ELECTRONICS INSTRUMENTS 8-2-174 Red Cross Road Secunderabad 500 003 Phone: 72040 Advertisers Index APEX 9.28 | COMPONENT TECHNIQUE 8.16 CONNECTRONICS 8.11 I DEVICE ELECTRONICS 8.09 j DOMINION RADIOS 9.26 I DYNALOG 9.40 ELCIAR 9 30 ELCOM 9.33 ELECTRICAL INSTRUMENT CO 8.14 ELECTRO MICRO COMPUTER 9.35 ELECTRONICS UNLIMITED 9.33 ELECTRO SYSTEM 9.35 GENERAL INSURANCE 8.17 GRAPHICA DISPLAY 8 14 HIOKI 8 10 IEAP 9 28 INDUSTRIAL RAOIO HOUSE 8 13 INSTRUMENT CONTROL 9 35 ION ELECTRICALS 9 30 JETKING 9 28 LEONICS 9 34 LUXCO 929 MACMILLAN INDIA LTD 8 08 MICROSIGN PRODUCTS 8 06 OSWAL 9 32 PADMA 8 14 PARVAIL ELECTRONICS 8 06 9 30 PEARL ELECTRONICS 9 34 PLA 9 27 PRECIOUS 9 36 PULSAR 9 34 PULSECHO 8 08 ROCHER 8 16 SAINI ELECTRONICS 9 26 SCIENTIFIC MES TECHNIQUE 8 08 SOLDRON 8 10 TARGET MARKETING 8 06 TECHNOMATIC 8 02 8 05 TEXONIC 8 16 UNLIMITED ELECTRONICS 8 07 VASAVl 8 15 VISHA 9 39 ZAC ELECTRONICS 9 33 ZODIAC 9 25 9.38 KN No 39881/83 LIC No. 91 Dynalog’s Leadership in 8 and 16 Bit Microprocessor training/development kits.... is only a part of the Dynalog Success Story! MICROFRIEND-68K 16/32 Bit Microprocessor training/development kit Continues the tradition of Dynalog’s Leadership N MICROFRIEND-68K Highlights: ■ 68000 CPCJ operating at 8 MHz. ■ 20K FIRMWARE with single line assembler and disas- sembler. ■ 1 28K Dynamic RAM. ■ Onboard EPROM Programmer for 2716, 2732, 2764, 27 1 28. ■ Video Controller using MC 6845 CRT Controller Chip. ■ RS 232C Serial Port with complete baud rate control. ■ Centronics compatible Printer Port. ■ Audio Cassette l/F with file management. ■ ASCII Keyboard l/F. ■ 40 I/O lines and 3 timers ( 1 6 bit) available on connectors. ■ Detailed informative documentation. Dynalog Micro-Systems 14, Hanuman Terrace, Tara Temple Lane, Lamington Road, Bombay 400 007. Tel: 353029, 362421 Telex: 01 1-75614 Gram: ELMADEVICE Sales Representatives TECHNICS, 4731/21 Dayanand Road, Daryaganj, NEW DELHI-110 002. Tel : 276988