elekte? Volume 3-Number 10 PUBLISHER: C.R. CHANDARANA EDITOR: SURENDRA IYER EDIT ASSISTANCE: ASHOK DONGRE ADMINISTRATION: J DHAS PRODUCTION: C.N. MITHAGARI ADVERTISING & SUBSCRIPTIONS eIeIf the BAEC is "Dickens" 26 Forrest Road Pennarth South Glamorgan Telephone: (0222) 707813 The Amateur Computer Club can be contacted at Andy Leeder Church Farm Stratton St Michael Norwich NR15 2QB 10-15 intruder alarm a combination of infra-red and electronics technologies J With burglaries on the increase in | most parts of the world, there is I a growing demand for good, reliable, yet inexpensive, security | equipment. Manufacturers of burglar alarms are convinced that there are many millions of people prepared to spend money — in many instances a great deal — to protect their property. The j intruder alarm described in this article is intended for use with a | number of infra-red movement [ detectors featured m the July 1985 issue of Elektor India, but can also work with other types I of sensor. The alarm uses a minimum of two printed circuits: one for the control circuits, and the other for the interfaces. The latter may be repeated a number of times if required. The interface board is designed to connect either two infra-red sensors or one infra-red sensor and an anti-tamper circuit to the control board. Other than infra-red sensors may also be used. To this end, each interface offers two operating conditions: (a) NO, in which the switching contact is normally open and closes when the alarm is triggered, and (b) NC, in which the contact is normally closed and opens when the alarm is triggered. A delay has been incorporated which allows time between setting the alarm and vacating the property and between enter- ing the property and disabling the alarm. In the latter case, a pre-alarm buzzer sounds and an LED lights to indicate that the alarm has been triggered. This arrangement is also very useful for testing the alarm. The LED remains on when the alarm has been set off. The intruder alarm works off the mams, but a 12 V battery is included as back-up during mains failure. Basic configuration As soon as switch S, (preferably a key- operated type) in Figure 1 is closed, diode D 4 lights and monostable MMV) generates a reset pulse that triggers MMV 2 whose (J output then becomes logic 0. This logic level prevents an alarm pulse from inter- face I reaching monostable MMV 3 , at least for the time being. When the alarm is in this delay phase, diode D 9 (see Figure 3) lights, but the alarm is not yet triggered. When MMV 2 toggles, D 9 goes out and MMV 3 is enabled. In this condition a pulse from interface I will trigger MMV 3 which causes its Q output to go low. This results in bistable FF , being set, diode D 3 commencing to blink, and a buzzer start- ing to sound: this is the pre-alarm phase. Only when MMV 3 returns to its stable state, ie„ after a further delay, is monostable MMV 4 triggered. Tl\e second delay makes it possible to enter the prop- erty and switch off the alarm before it has sounded. As soon as S, is switched off, D 3 stops blinking, MMV 3 is disabled, and the alarm is reset. The various pulses men- tioned here are shown diagrammatically in Figure 2. Circuit details The pulses mentioned in the previous paragraph are also shown on the circuit diagram in Figure 3. Terminal 2 on this diagram is the input from the anti-tamper circuit. This line is one of the wires in the multicore connecting cable between the sensor and the interfaces board. This arrangement ensures that cutting this cable by a potential intruder does not disable the alarm, but rather the contrary, 1 0- 1 6 elektor in because the anti-tamper circuit is con- seconds and 4 minutes by P 2 . If link B has nected direct to the board, bypassing been made, the siren will switch off after switch S(. this time has lapsed; the unit can only be When a pulse from one of the sensors has reset by switching off S, and then closing arrived at the output of MMV 4 , or bistable it again. If, however, link A is used, the FF 2 has received a clock pulse from alarm will sound again and again until the MMV 3 , bistable FF 2 is set. Output 0 then unit is switched off. This arrangement becomes, and remains, logic 1, which ensures that the unit conforms to regu- prevents MMV 4 being triggered again. At lations in many areas where an alarm may the same time, the output pulse is not sound longer than three minutes and amplified in T, and then used to energize may not automatically switch itself on relay Re,. At this instant the siren will again. sound. Reset circuit MMV, generates a pulse at The length of time that the relay remains its Q output whenever S, is opened (rising energized may be preset between 10 edge at TR) or closed (falling edge at TR). ,10-1 7 Power supply The power supply — see Figure 3a — is rather more elaborate than usual, because it includes protection against mains failure. To this end, it contains two LEDs: an amber one, D l5 , to indicate operation from the mains, and a red one. D l7 , to show when the unit works from the battery. During mains operation, the potential at the input of IC 5 is higher than that at the output, so that D| 5 lights. As the voltage across C l2 is about 19 V, the potential at the junction of J? le and J?, 9 is around 14 V; T 4 is then off, and D, 7 does not conduct. When the mains fails, T 4 conducts and D 17 lights. It is best to use a 12 V lead-acid battery with a capacity of about 1 Ah: diode D 16 may then be omitted, and R ? o should be replaced by a wire link. It is, however, also possible to use a 12 V NiCd battery with a capacity of 0.5 Ah: in that case, D 16 must then be connected as shown, whereas R 20 should be omitted. When the batteries are not in use, they are trickle charged automatically. Interfaces The interfaces, the circuit diagram of which is given in Figure 3b, offer two operating conditions, as stated before. NO contacts should be connected in parallel; NC contacts, in series. A secondary func- tion of the interfaces is to prevent noise generated in long leads from reaching the control circuits. A number of interface boards may be con- nected in parallel: an AND gate is then formed by diode D 5 on the interface board and R { or R 2 . as the case may be, on the control board. This means that when one of the interface boards provides a logic 0, the relevant input of the control board is also logic low. It is thus possible to use quite a number of sensors — see also the wiring diagram in Figure 4. Each interface has its own positive and negative supply connections, which are used in the first instance for the NO or NC contacts. They can, however, also be used for sup- plying power to the infra-red sensors. It is advisable to fit a 200 mA miniature fuse in the negative supply line to prevent short circuits between the two supply lines in case the cable is cut. Switch S, is normally fitted at the control unit; if it is required to be located elsewhere, a separate interface as in Fig- ure 3b has to be used. The switch, still in series with D 4 , is then connected between the NO terminals on the interface (anode of D 4 to +!). Interface terminals 0, output, and -i- should be connected to control board terminals 8, 1, and 7 respectively. The value of /? 2 in Figure 3b must be lowered to 1 kQ. Construction and test The printed-circuit boards for the control unit and the interfaces are illustrated in Figures 5 and 6 respectively: note that the latter is intended for two identical interfaces. If diode D 3 is not required to blink, solder a 100 nF capacitor across the buzzer. The power supply has been designed for use with a 12 V 6 W siren; if a 240 V siren is used, the rating of Tr, may be reduced to 15 V; 0.5 A. Interconnections between the boards are shown in the wiring diagram of Figure 4 which illustrates the use of l'/s interface The control unit should be housed in a suitable, robust metal case as shown in the photograph. Before fitting the battery, adjust P 4 to give a voltage of 13.8 V across terminals 7 and 8. Then fit the — fully charged! — battery. Close S, when D 9 should light and shortly afterwards go out again. The lighting time is preset between 10 seconds and 4 minutes with P 3 . Test the anti-tamper circuit by opening an NC contact or closing a NO contact: relay Re, should then be energized; D 3 should blink (unless a 100 nF capacitor has been provided across the buzzer); and the buzzer should sound. The holding period of the relay, ie., the length of time the alarm sounds, is preset with P 2 . Note that P, . . .P 3 have minimum value when they are turned fully anticlockwise. When a normal alarm group receives a pulse from a sensor, nothing should hap- pen immediately; if, however, the anti- tamper circuit receives a pulse, the alarm should be set off immediately. A normal alarm group can only trigger the alarm when S, is closed: this happens after a delay of between 10 seconds and 4 minutes — preset with P, — when relay Re, is energized. M 10-21 J Steeman solid-state relay In general, mains-operated loads are switched by electromagnetic relays. Such relays have been in use for many years and are, in the main, reliable and sometimes ingenious. None the less, they are slowly but surely being superseded by their even more reliable electronic counterparts. A further advantage of these solid-state devices is that they are becoming less expensive than electromagnetic relays. This article for switching ohmic loads of up to Electromagnetic relays have one or several sets of contacts which open or close when a soft-iron core is magnetized by a coil around it. Solid-state relays involve no mechanical movement whatsoever, as switching is effected by a single silicon-controlled rec- tifier (SCR) or two SCRs in a common envelope — normally called a triac. This type of relay is of great importance in digital circuits. Note that the SCR was originally called thyristor. Solid-state relays have a much better lifespan than electromagnetic types, particularly at high rates of switching. They also exhibit far less electrical noise and may be used in explosive environments since there are no contacts across which arcs can form. And, of course, they are completely free of mechanical noise. The basic concept of a solid-state relay is shown in Figure la. The switch may take the form of an SCR in a bridge circuit as in Figure lb. This configuration enables both the positive and the negative halves of the mains voltage to be switched. describes a simple solid-state relay 600 watts. Switching noise When the moment of switching does not coincide with a zero crossing of the mains, the sudden change in current causes high-frequency impulse noise, which, for instance, may result in unwanted signals appearing on the screen of a television receiver, or becoming aud- ible as clicks in the loudspeaker of a radio receiver. The magnitude of the noise depends on the frequency of switching, on the instant relative to the zero crossing when switching takes place, and on the type of load being switched: it is generally greater with inductive or capacitive loads than with ohmic loads. Switching at zero crossing In a silicon-controlled rectifier, SCR, the forward anode-cathode current is con- trolled by a signal applied to a third elec- trode, called the gate. When a positive current is applied to the gate, and the anode is positive with respect to the cathode, current flows through the SCR from anode to cathode. The magnitude of the gate current determines the breakover point, is., the anode voltage at which the SCR switches from the blocking to the conducting state. The SCR conducts as long as the current through it is greater than the so-called holding current. This means that in a.c. applications the SCR switches off when the mains passes through zero. Consequently, a positive current (pulse) must be supplied to the gate at every half period of the mains voltage to ensure continuous conduction. For ohmic loads, the correct moment of applying the pulse to the gate is when the mains voltage passes through zero, since for such loads the voltage and current are in phase. With inductive and capacitive loads, voltage and current are not in phase, and it is, therefore, not correct to trigger the gate when the mains voltage passes through zero. The optimum trigger moment for such loads is rather more dif- ficult to determine. Solid-state relay The circuit of the solid-state relay is given in Figure 2. It is triggered at the zero crossing of the mains voltage and is, therefore, only suitable for use with ohmic loads. The relay is driven via an opto-isolator, so that the control unit (computer, time switch, and so on) is electrically isolated from the mains. The main current loop is closed via load La, bridge rectifier D, . . ,D 4 , silicon- controlled rectifier Th,, and fuse F,. The maximum forward current through the diodes is 3 A, while the SCR is rated at 5 A. When the SCR is in the blocking state, the entire full-wave rectified mains voltage is present across it. At every zero crossing of the mains voltage, transistor T , produces a positive current pulse, provided switch S| is open and the phototransistor in 1C, is off. When the instantaneous voltage at the gate is sufficient to switch the SCR on, the voltage across the SCR, and consequently that at the gate, drops to zero. As long as the above provisions are met, this action repeats itself at every zero crossing of the mains voltage. The various situations and associated voltages are shown in Figure 3. With both S, and 1C, off, the SCR con- ducts. ie., the relay is actuated. It may be switched off by closing S,. or by applying a voltage of + 5 V to the series combi- nation of and the LED in IC,. The opto- 10-23 representation of various associated voltage /I <1 <1 Resistors: Ri = 100 k; 1 W R2 = 2k2 R 3 - 220 k; V, W R 4 = 180 Q Rs = 10 k Rs = 100 O Capacitor: C, = 100 n: 400 V isolator may be controlled by TTL (transistor-transistor logic), but also by other signals. Note that the maximum for- ward current through the diode should not exceed 100 mA, while the reverse bias should be not greater than —3 V. Construction The relay is most conveniently built on the PCB illustrated in Figure 4. This board is intended for fitting in a 120x65x55 mm plastic case, which results in a neat and compact unit. The connections to the switch, control unit, mains, and load are best made with the aid of nylon dual ter- minal blocks. This makes it possible for the PCB to be fitted, or removed, without the need for soldering. The SCR should be mounted on a small heat sink. Because the unit works from, and with, mains voltages and relatively high cur- rents, great care should be taken in the wiring. Do not forget the earth connection between the mains outlet and the unit! Switch S, and the input connectors for the control voltage are mounted at the top of the case. Remember that the switch should always be open when external control is used. Finally This solid-state relay is intended for use with ohmic loads of up to 600 W. It cannot be used for switching transformers, neon tubes, and other inductive or capacitive loads, because this would upset the trig- gering system. M fields In recent years there has been consider- able interest in the effects of atmos- pheric electric and magnetic fields on living organisms, and in particular in their effect upon human health. For example, experiments carried out in West Germany into the effects of electric fields on motoring fatigue seem to indicate that the presence of an electric field inside a motor vehicle can reduce driver error. The interior of a motor vehicle, because of its metal construction, is largely screened from external electric fields. Researchers from the West German Defence Ministry, the Max Planck Institute, and the Munich Institute for Biomedical Technology co-operated in developing a device to generate an electric field inside a car. It was found that, with the device operating, drivers made 8 to 10% less errors than normal. Furthermore, the more fatigued a driver was, the greater was the improvement in his performance when the device was switched on. Professor Konig, of the Munich Technical University, writing in the German Motoring magazine ‘ADAC- Motorwelt’, stated that, ‘ . . . electric and magnetic fields exert a biological influence upon the human organism'. On the other hand, Prof. Dr. Ir. Justus Bonzel, director of the Dusseldorf Research Institute of the Cement Industry, in reply to criticisms regarding the screening effect of concrete buildings, wrote, ‘The question of the influence of electric fields upon humans and animals still remains unanswered, and most scientists do not accept that a clear link exists. In spite of this, it is often asserted (and even pseudo-scien- tifically explained) that living in a concrete building has a negative influence on the health of the occupants, as a result of their being screened from electric fields which are present in the open air. (...) As far as the screening effect of building materials is concerned, it can be proven that materials such as high-quality concrete, brick, lime/sandstone and wood all screen or let through electric fields to virtually the same extent, and that the interiors of buildings made of these materials contain electric fields similar to those found in the open air.' Which of these two conflicting view- points is true? Certainly, in view of the automobile experiments, it would appear that there is positive evidence that electric fields do have an effect upon health, and that the subject bears further investigation so exactly what are atmospheric electric fields? The ionosphere, which is a region of electrically charged air molecules, begins at a height of approximately 70 km above the surface of the earth, and has a positive potential of 300-400 kV with respect to the earth. The ionosphere and the earth's surface thus act as the plates of a gigantic capacitor, which inciden- tally has a 'leakage current’ of about 3 x 10"'° jiA/cm J due to movement of ions between the ionosphere and earth. Figure 1 . Th« ionosphere begins at a height of approximately 70 km and extends to about 1000 km. The ionosphere is charged to about 300 400 kV with respect to earth and the Figure 2. Electric field strength is greater at the tops of hills than in valleys, as can be seen from the bunching or expansion of the Between the ionosphere and earth there naturally exists a DC electric field, and in addition there is an AC field with a frequency of 10 Hz. The field strength is not uniform at all points between the ionosphere and earth, but at ground level in the open air the average field strength is about 130 V/metre. A diagrammatic representation of the ionosphere is given in figure 1 . Terrain and buildings have a considerable effect on local field strength at ground level. Figure 2 shows how the equipo- tential lines are ‘cramped’ closer together on hilltops, which means that the potential gradient and hence the field strength is greater there than in the valleys, where the equipotential lines are more widely spaced. The potential difference between the ionosphere and earth causes a constant movement of ions between the ionospere and earth. Near ground level positive ions predominate, there being approximately 2500 positive and 450 negative ions per cubic centimetre of air, although at sea these figures may be reduced by a factor of 10, and in urban areas may be increased by a factor of 10. The ion concentration may also vary considerably with weather conditions. For example, before the onset of a thunderstorm there is a heavy ionic concentration with a predominance of positive ions. When rainfall occurs the concentration of ions quickly falls and the negative ions predominate. It is believed that negative ions have a beneficial effect on health and positive ions a detrimental effect. This may explain the oppressive atmosphere that attends the onset of thunderstorms, and the subsequent relief when the rain In conclusion it is fair to say that there is sufficient evidence to warrant further research into the effects of electric fields and ions on human and animal health. M 10-25 Design considerations Designing a loudspeaker box is no sinecure, since there is no detail that is not important or does not play a role in the finished product. The loudspeaker frame, the shape of the enclosure, the volume of the box, the positioning of the frame in the box, the damping, the filter, impedance correction networks, filter components — all these have to be care- fully matched to one another. KEF realized all this a long time ago. Their loudspeaker designs never have just one outstanding property, as is often the case with those of other manufacturers: the a loudspeaker for music lovers The pleasure derived from music is heavily dependent on clarity. Although we may sometimes listen to live music, most of what we hear nowadays is recorded, so that even the finest musical sounds will be spoilt if the repro- duction lacks fidelity or if they are accompanied by irritating noises and distortion. The item that most often lets the sound reproduction equipment down is the loudspeaker. But not the PL301, which has been designed by KEF Electronics Limited, the well-known loudspeaker designers and manufacturers from Maidstone. KEF has an international reputation for high- quality products and has, moreover, been active successfully in the DIY market for many years. This means that their products are readily available in most of the western world. Cogent reasons for building the PL301, which may not be the cheapest KEF design, but is certainly among the best! 301 PL complete design is good. And that is cer- tainly the case with the PL301: no out- standing strong individual points, but no weak ones either! Technical character- istics are given in Table 1. The PL301 is a good-size three-way system in, as usual with KEF designs, a closed acoustic box. The cross-over between bass, middle, and treble frequencies is provided by a Linkurtz-Riley filter with an initial slope of 18 dB per octave. The enclosure contains two separate com- partments: one for the bass speaker, and one for the middle frequency speaker and tweeter. At first sight, this basic design looks very similar to the RR105, the most pretentious of KEF’s loudspeaker enclosures. Even the speakers themselves are the same as in that model. On that basis you might think that the present design is a sort of updated or perfected (is that possible?) RR105, but you would be wrong, because the PL301 has a number of characteristic details which put it a little outside the normal KEF family. Those details are concerned with the precise positioning of the loudspeakers, the shape of the enclosure, and its freedom of resonance — together resulting in a neutral, uniform reproduction and tightly controlled bass performance. Building the enclosure(s) takes, of course, quite some time, but this is where the DIY enthusiast has the edge over the industrial producer. The home constructor does not consider the hours he puts into the work as part of the overall cost, and he is, therefore, able to carry out labour- intensive work that a manufacturer could not tackle at a profit. Drive units KEF have chosen what is probably their best three-way combination: the T52B tweeter, the B110B for the middle fre- quencies. and the B300B for the bass. These units are shown in the photograph in figure 1. The bass speaker is one of the most recent additions to the KEF family. In earlier top-of-the-range designs, the 200 mm Type B200 or the well-known oval Type B139 was normally used. The B300B (SP1071) is a robust 300 mm unit with a resonance frequency of 23 Hz, and is rated at an impressive 150 watt sine wave or 200 watt music. The T52B (SP1072) tweeter has been in KEF's range rather longer. It has a rela- tively large cone (52 mm diameter), whereas the speech coil is somewhat smaller (37 mm diameter). Moreover, it has a pleasantly low resonance frequency of 650 Hz. The old guard of the three is the B110B (SP1057) middle frequency unit. This 130 mm speaker has been produced by KEF for many years, and this makes it all the more amazing how well it still stands up to modem competition. Particularly as regards pulse performance, the B110B is second to none, as our own tests indicate. Crossover network Although KEF still used 12 dB loudspeaker dividing networks some years ago, nowadays all crossover networks have an 18 dB per octave characteristic. Modem computer-aided design has made it poss- ible to tune these networks precisely to the relevant loudspeaker combinations. At the same time, these computer calcula- tions give the required frequency and impedance correction factors. The circuit diagram of the dividing net- work is shown in figure 2. The crossover points lie at 400 Hz and 3000 Hz respect- ively. The terminal voltages across the article, note that we have designed two different printed circuit boards. The first version is intended for KEF components only, and this b9ard is part of the crossover network kit available from KEF. The second version, shown in figure 6, is of more interest to the pure home con- structor, since this is intended for com- ponents normally available from most electronics retailers. According to KEF, bipolar electrolytic capacitors are not nearly as bad as is gen- erally believed. Be that as it may, but note that they have taken the properties of such capacitors into account during the design calculations of their networks. This means that it is not possible to just replace these bipolar devices by foil capacitors. At low and middle frequencies, this is not much of a problem, since the audible dif- ferences between various types of capaci- tor in this range are virtually nil. Matters are quite different, however, at high fre- quencies: in our opinion a good foil capacitor here is definitely better than an electrolytic type. We feel, therefore, that capacitors C 7 and C B should be polyester, polycarbonate, or (best) polypropylene foil types. The smaller losses in these capacitors, as com- pared with those in electrolytic types, cause the output of the tweeter to rise by almost 20 per cent, while the impedance characteristic descends somewhat at high frequencies. These effects can be negated by connecting a 0.S ohm, S watt resistor in series with both C 7 and C 8 . Enclosure The enclosure is divided into two com- partments: one for the bass speaker, and the other for the middle and high fre- quency speakers, as shown in figure 7. As stated before, the PL301 is typified by its robustness, the individual placing of the drive units, and the shape of the enclosure. The strength and rigidity is, of course, vital. Everything possible has been done in this design to prevent disturbing panel resonances. The material chosen is 22 mm plywood: there are numerous reinforcing struts, and, last but not least, the sides are double-panelled and the resulting hollows are filled with sand. It could not be better! The remaining details worth mentioning reflect, without exception, the aim of obtaining optimum radiation of the sound. Noteworthy in this respect is the placing of the middle frequency speaker above the tweeter. Why this is done is illustrated in figure 4. When both these speakers are mounted in one plane and a line is drawn between the centre of the tweeter cone and the acoustic operating point of the middle frequency speaker, it is seen that in the conventional positioning shown in figure 4b the axis of radiation lies a little below the horizontal, while in the PL301, as shown in figure 4a, it lies above the horizontal. Except when the loudspeaker 5 Figure 4. Axis of radiatii with (al middle frequerv O0(D a b e 0 O C 0 (a) lb) 10 is at ceiling level, upward radiation is, on the whole, better than downward, because it is along a more direct line to the listen- ing position, so that the detrimental effects of damping by floor covering, particularly carpets, are prevented. It should be men- tioned here that in practice the direction of radiation deviates from that shown in figure 4 because of the effects of the phase shift caused by the crossover network. Another noteworthy feature of the PL301 is that the upper compartment is displaced forward with respect to the bass chamber. This shift of about 450 mm may, depen- dent on the listening position, be reduced slightly. The forward shift permits the design of the crossover network to be optimized. The necessary phase equaliza- tion for inter-unit time delay is incor- porated in the crossover design. The interchanging of the middle and high frequency loudspeakers is closely associ- ated with the shape of the cabinet. Most acoustic engineers are agreed that the shape of the enclosure has a vital bearing on the reproduction. Figure 5 correlates twelve differently shaped cabinets and the associated frequency response character- istics; it is derived from the well-known standard work Acoustical Engineering by W H Olson. Spherical shape a is clearly the ideal, but practical spheroids j and 1 are a good second and third; particularly shape j is hardly inferior to the spherical shape. Noteworthy is that conoids f. . .i score badly, whereas old faithful shape k comes out quite well. The PL301 uses enclosure shape 1, although the forward placing of the upper compartment makes the middle frequency characteristic approach that of shape j very closely. The partly slanting shape of the j and 1 variants is, therefore, the most conspicuous aspect of the PL301. That shape is continued in the front grille. From a practical point of view, this is an ideally shaped cabinet which has only one disadvantage: you need to be pretty good at woodworking to make it. Construction Starting with the dividing network, it seemed sensible to split the PCB, like the cabinet, into two parts: that for the bass section is shown in figure 6a, and that for the middle and high frequencies in 6b. The inputs of the two board are simply connected in parallel. Although building the PCBs is a fairly simple job, it is essen- tial that you stick to the stated component values! Inductors L x and L 2 use ferrite cores, but all others are air-cored. The wire diameter for L 3 , and L s should be 1 mm, and that for L s , 0.5 mm. The capacitors, with the exception of C 9 and possibly C 7 and C e (see crossover network above) are bipolar electrolytic types. Because values of 60 ( which is part of the computer system and a Data Circuit Terminating Equipment popularly called a modem, which is connected to a trans- mission line. See Figure 1. The exchange of data between the DTE and DCE must be governed by clear agreements as to their format. For instance, as the bits are transmitted sequentially, their timing and that of the complete characters must be accurately known. There are two types of transfer: synchron- ous and asynchronous. In the former, a continuous data stream is transmitted, and the receiver is synchronized to the transmitter by a clock signal derived from the data, or by a specially transmitted clock signal. Asynchronous transmission is, however, far more commonly met. This method has its origin in telex engineering. Since the drive motors of the transmit and receive terminals could not be synchron- ized precisely over long periods, each group of data bits was preceded by a start bit and closed by a stop bit. This type of 10-54 ele synchronization is perfectly satisfactory for the relatively short duration of the data An example of such a serial signal is shown in Figure 2. First is the start bit, fol- lowed by eight data bits (for instance, an ASCII character), a parity bit for error con- trol, and Finally the stop bit. The stop bit enables checking whether the trans- mission and reception speeds are the The correct processing of the data stream exchanged between the DTE and DCE is based on a protocol, i.e., a set of conven- tions or regulations. The two most com- mon, official protocols are the RS232 and the V24. Apart from the data, there is a number of signals for the individual control of the DTE and the DCE. All these and the rel- evant connection to a standard D-type connector are shown in Figure 3. DTR. DSR, and DCD are typical signals used for the establishing and terminating of a communication. The remaining signals are used, as required, during the exchange. Assuming the communication is full duplex, i.e., both the TxD and RxD lines are in use, the DTE activates the DTR line to indicate a request for communication. The modem responds to this by activating the DSR line. If the DTE is to transmit, it activates the RTS line. The DCE acknowledges that it can process the data by activating the CTS line. Note that in full duplex operation the DTE can also receive at all times, provided that the modem has activated the DCD line. This takes place during the establishment of the communication. See also data com- munication by telephone and direct- coupled modem in the September and October 1984 issues of elektor electronics respectively. The "secondary” connec- tions are relevant to the main channel — back channel separation in modems. Translating all these actions to a ready-to- use serial connection to a computer is effected by an intelligent peripheral chip: asynchronous communications interface adapter (ACLA) Type 6551. This device is simply accommodated in the microprocessor system to provide com- plete RS232 and V24 compatible communi- cation with the outside world. Interface circuit As may be seen from Figure 4, the ACIA does not need much more than a crystal and a number of gates for signal level matching to carry out its task. To the left of it are shown the usual connections to the computer system: here, the slot con- nections of the I/O bus. The 1843.2 kHz crystal connected to pins 6 and 7 is used to provide a number of baud-rates, which are selected by the soft- ware. The R x C pin (5) is a bidirectional input and output respectively for an exter- nal clock to provide non-standard baud- rates, and for outputting the internal baud- rate generator. In either case, the clock frequency amounts to sixteen times the baud-rate. To the right of IC ; are the familiar RS232 lines with buffered inputs and outputs. Seen from the IC, all signals are inverted, which means that all control signals: DTR, RT& CTS, DCD, and DSR a re ac tive h igh, whereas data signals TxD and RxD are active low. These levels are standard for 10-55 RS232 connections. There are two poss- ible, corresponding voltage levels: TTL, ie., high= +5 V; low =0 V, and RS232, ie., high = + 3 . . . + 25 V (nominally 12 V) and low = —3 . . —25 V (nominally —12 V). It is intended that one of the groups of parallel-connected ports is selected: N| . . .N 3 provide RS232 levels, and N 4 . . .N 6 , TTL levels. The receive buffers are suitable for both TTL and RS232 levels. It is, therefore, necessary to oheck with which levels the system to be connected operates. The printed-circuit board in Fig- ure 5 shows both IC 2 and IC 3 , which are required for TTL or RS232 levels respect- ively. The ± 12 V supply is not needed in TTL operation. Operation As stated, the circuit is based on a Type 65S1 ACIA, a block diagram of which is given in Figure 6. Communication at the DTE end is effected by five registers, four of which are detailed in Figure 7. Since the inputs are connected to address lines A 0 and A,, the registers are located sequentially in the address range of the I/O bus. If, for instance, the interface card is placed in slot 1 of the universal I/O bus, and the start address of the I/O range is set to 4000 hex , the Transmit and Receive Data Registers are at 4000 hox , the Status Register at 4001 hex . the Command register at 4002 hox , and the Control Register at 4003 hex . Transmit and Receive Data Registers During transmission, bit 0 (LSB) is sent first. The bits not used, for instance, 5. . .7 where a 5-bit format has been chosen, are treated as "don’t care". In the receive mode, the first received bit goes to location ft and subsequent bits to 1, 2, and so on. The highest, not used, locations are given a ft Status Register This register can only be read. Bits ft 1, and 2 indicate respectively whether dur- ing reception errors in parity, framing, or overrun occurred. The more important bits 10-56 el.kto-.nd, a are, however, 3 and 4, which indicate whether a complete character has been sent or received. These bits determine in the control program whether the next character can be processed. The final three bits enable the reading of the DCD and DSR lines and of the interrupt status. If an interrupt structure is used for the control of the interface card, the status register must be checked after detection of an interrupt to decide what the next operation should be. If such a structure is not used, link ^ should be left open; the interrupt is then still generated but not passed on to the computer system. An arbitrary write operation to the Status Register gives rise to a program reset. The effect of such a reset is indicated in Fig- ure 7 for each register. A dash indicates indeterminate. Command Register Bit 0 determines the DTR signal and the receiver status. Bit 1 is used to decide whether- an interrupt indicating a full Receive Data Register should be given or not. Bits 2 and 3 control the RTS signal and, therefore, the operation of the transmitter. They are also used to decide whether an interrupt indicating an empty Transmit Data Register should be given or not. Bit 4 is normally 0 Bits S . . . 7 are used for parity control and control of the transmitter and the receiver. Control Register This register determines the format of the serial data. Bits 0. . . 3 determine the baud- rate; there is a choice of a number of stan- dard baud-rates, all derived from the crystal frequency. In position 16 x external clock, an external clock signal may be applied to pin 6 of IC, (pin 7 open); the baud-rate will be one sixteenth of the external frequency. The internal baud-rate generator is connected to the receiver when bit 4 is logic 1: transmitter and receiver then work at the same baud-rate, while pin 5 of IC, functions as the output terminal of the internal generator; the fre- quency is sixteen times the selected baud-rate. This arrangement makes it possible to connect several ACIAs in tandem. When bit 4 is logic 0, pin 5 func- tions as the input for the receiver clock via bridge J 2 . Bits 5 and 6 determine the 10-58 8a length of the data word. Bit 7 enables selection of 1, l'/t, or 2 stop bits. It is clear that there are a great number of programming possibilities. With a little knowledge of machine language and some experience in this type of work, almost anyone can establish his own specific serial connections. As far as connections to peripheral equip- ment are concerned, distinction should be made between DTE-DCE (interface card — modem) and DTE-DTE (interface card — another computer, terminal, or printer) combinations. In the latter, the connecting cable must make it possible that the func- tion of the DCE is simulated as far as feasible. Figure 8 gives a number of examples of the connection between the interface card and peripheral equipment: in a the peripheral equipment is a modem; in b. . .d the modem function of the connected unit is more or less simulated by the cable connection. Figures 8b and 8c are simple versions of the so-called cro-s or zero-modem con- nections, whereby the transmit and receive data lines are interconnected crossways, while the two systems generate their own control signals. Any control characters are exchanged via the data lines. The local mode connection illustrated in 8c is most suitable for the present 6SS1 card. Rather more control signals are used in the arrangement shown in 8d. The mutual DTR-DSR inter- change is effected at start-up. When one unit gives RTS, thereby effecting its own CTS, the other activates DCD, so that it switches to the receive mode. How the various connections are made is, therefore, entirely a matter of application. The sample program shown is a simple RTTY receive program for the present card. It was originally intended for the Acom Atom, but is easily adapted for use with any 6502 computer (which must, of course, be fitted with the universal I/O bus and the present interface). H Figure 8. Some examples of RS232 connections: b. . .d are intended for use between two DTEs. A -simple RTTY receive program for the Acorn 10-59 design ideas The subject of "speech and com- puters” has been discussed in these pages on several occasions, the last one being in the April 1985 issue. The accent then was mainly on the speech processing aspect, which is technically easier than voice recog- nition. It is, therefore, fascinating for all those interested in this subject that General Instrument now have available two chips - the SP1000 and the VSR1000 - that, in combi- nation, tackle the problems of both speech processing and voice recog- nition. The SP1000 is a voice recog- nition and synthesis 1C, and the VRS1000 is an 8-bit microcomputer to control the SP1000. The combination of the two chips provides the designer with complex, ready-to-use recognition and syn- thesis algorithms Typical applications include robotics, aids for the handi- capped. security devices, and voice dial telephones coefficients as well as relevant infor- mation as to audio signal amplitude: all this information is then stored on board the SP1000 and sent to the microprocessor every 20 ms. Each spoken word is divided by the SP1000 into twelve frames, for each of which a time-weighted average of the coefficient is calculated and stored. This means that for each word a storage requirement of 108 bytes is required. A single word should last no more than 2 seconds, and the space between two words should be not less than 200 ms. In this context, it is important that the audio signal level is matched to the input of the analogue-to-digital converter, so that the latter works under optimum con- ditions. To this end, the SP1000 has three ' gain'' outputs, which are fed to an external analogue gam control The LPC coefficients calculated by the VHS1000 are converted by the from the text provided by the host computer; during voice recognition, it compares the LPC coefficients pro- vided by the SP1000 with the stored vocabulary. The VRS1000 can recognize twenty words simul- taneously. Other functions of the VRS1000 are: retraining of the vocabulary; creating vocabulary subsets; storing templates on disc; and rejecting a word that is found not to be a member of the recognition vocabulary. Accuracy of voice recognition is claimed to be better than ninety- eight per cent according to the Doddington-Schalk standard test. The search time, i.e., the time required to recognize or reject a word, depends on the size of the vocabulary: as a guide. 45 ms per word. It seems, therefore, sensible to sub-divide a vocabulary of, say, 100 words into five subsets of 20 words each; the search time will then always be less than 1 second. It should be borne in mind that the recognition accuracy is bound to suf voice recognition & speech processing The nucleus of the SP1000 is a lat- tice filter that can be programmed for synthesis and recognition modes. In the synthesis mode, the VRS1000 feeds the filter speech data it has computed: these data control the filter characteristics. In the recog- nition mode, the filter extracts from an incoming audio signal the LPC (linear predictive coding) information required for the digital speech pro- cessing. To this end, the audio signal is scanned every 160 ft s by an 8-bit analogue-to-digital converter; the resulting eight bits are fed to the SP1000, as shown in Figure 1. The filter extracts eight significant audio filter in the SP1000 into a digitally speech-modulated signal. Only a low-pass filter and a power amplifier are then required at the output to complete the synthesis process. The VRS1000 is functionally equiv- alent and pin-by-pin compatible with the General Instrument Type PIC7040, which is a licensed, second-source of the Texas Type TMS7040. These ICs are 8-bit microprocessors with internal 4 K ROM. This ROM contains the entire program for the control of the SP1000. During speech processing, the VRS1000 deals with the computation of the LPC coefficients extracted An advantage of the system is that a vocabulary has to be learnt: it is specific to a speaker, but can, none the less, be modified at any time. Furthermore, a number of speakers may enter their own specific vocabulary. Since articulation, depth of voice, and speed of speech even in the same word spoken by the same speaker vary from time to time, it is possible to enter a word up to 255 times, and store the average. This, of course, results in very high recog- nition accuracy. ccess to a 20-word vocabulary Iz sampling; 8 bits por sample on ol 8 LPC and 1 amplitude coefficient mi word duration 2 seconds programmable rejection threshold selectable number of training passes during th the vocabulary » synthesis oi 10 LPC coefficients • synthesis of sound effects and mu • real-time interpolation of energy, p 1 0-60 elekio Communication with the host com- puter is kept simple by the use of the ASCII code, so that it will nor- mally not be a problem for individual control programs to be used. H Literature: S PI 000: preliminary information Application note AN -0504 General Instrument Times House Ruislip HA4 8LE Telephone: (089 561 33355 General Instrument Corporation International: 600 West John Street Hicksville New York 11802 Telephone: 516-733-3107 of the SP1000/VRS1000 design ideas Moving Coil Meters a modified A moving coil meter basically consists of a through which the test current flows, and horse shoe magnet. The current flowing through the coil produces a magnectic field. As the coil is pivoted between the two faces of the permanent magnet, (modified hors shoe) it tries to realign itself by rotating about its Figure 1 shows a most commonly used design of the meter movement. In order to keep a uniform and minimum air gap. the coil is wound around a cylindrical piece of magnetic material. The test current is supplied to the coil through spiral springs, which also serve to return the moving coil to its orignal position as soon as the current flowing through it becomes zero. All the forces acting on the moving coil are balanced in such a way that the deflection of the coil is proportional to the current flowing through it. To indicate this deflection of the moving coil, an indicator is directly connected to the coil and indicates the deflection on the scale. A damping paddle is sometimes connected to the moving coil to prevent sudden movement of the coil and indicator. This paddle moves in a damping chamber, which appears at the bottom of the illustration of figure 1 . Figure 3 shows the symbol of the circuit element 'Moving coil meter". The moving coil instruments ar so sensitive, they can be seldom used alone. A sensitive meter movement needs just 25 uA current for full scale deflection of the indicator. For most of the applications, these meter movements must be supplemented by resistances. Figure 4 shows the function of the parallel resistance (shunt): It bypasses a part of the test current away from the measuring instrument and thus protects it from overload. The amount of current passing through the shunt depends on its value. The remaining current passing through the movement decides the amount of deflection of the indicator. The meter scale must be accordingly marked to read the test current. Voltage measurement with a moving coil meter is possible only with an additional series resistance. When a resistance is connected across a voltage source, the current flowing through it is governed by the Ohms law. Since we have introduced a fixed resistance in series with the test voltage and meter movement, the current varies with the voltage. This produces a deflection of the meter movement proportional to the test voltage. The measuring instrument in figure 5 measures the current flowing through the moving coil, but the scale can be marke in Volts! Figure 2 shows the schematic diagr the magnetic field. 1 0-62 elektor india October 1985 selex^ Resistors "... You know a lot about pocket radios, isn't it?" "Well yes. what do you want to know?" "This afternoon I had taken apart the old pocket radio. From that I have brough along a few parts, look The resistor brakes the current by offering resistance to the voltage. The higher the voltage, the lesser is the effect of resistance and the more is the current which is allowed to flow through the resistor. Let us take an example of a car on a downhill road. The driver must apply the brakes, otherwise the car would run faster and faster and come off the road. The speed of car depends on how hard is the braking. The same thing is true about the current. The higher the resistance, the lower is the current. As the slope of the road is important to decide the speed of the car, level of voltage is important to decide the flow of current through a resistor. A high voltage level is like a steep road and a higher resistance level is like braking harder." Current Paths In our June '85 issue, we had seen that current can flow only in a circle (a circuit) and that it can no longer flow through this path if it is interrupted at any position. What happens when more than one path are available for the current to flow? We can see what happens, with a small experimem. For this we need the following: — 2 Flat batteries of 4.5 V each — 2 Resistors of 22011 1 /8 Watt — 2 LEDs All these components can be obtained from any electronics component shop. First, connect the circuit as shown in figure 1 . A battery, a resistance and two LEDs are connected in the circuit. The LEDs glow when current flows through them. The brightness of glow depends on the level of current flowing through them. The series resistance serves only to limit the current flow in the circuit so that LEDs are not damaged due to excessive current. The components can be connected by twisting the leads together. Polarity of LEDs must be observed correctly. The shorter lead of the LEDs is the cathode (minus pole). 1 U B = 4 5 10-63 2 selex When the circuit is connected, a current flows through it as shown in figure 2. Both LEDs glow with equal brightness because the same level of current flows through both. Now with the remaining components, the circuit is connected as shown in figure 3. Will the second battery also drive current through the LEDs? and if so. through which path will it flow? Observe the circuit properly and it will give us the answer. LED 3 also glows, and this means that the second battery is also driving current through the circuit. To complete the circuit the current which flows through LED3 must also flow through LED2, which is confirmed by the fact that LED2 glows more brightly than LED1 or LED3 Figure 1 . The two LEDs light up, showing the presence of current in the circuit Figure 2. The current path in the simple A second circuit is added.LED 2 is included in both the circuits. through LED 2. As a double check, disconnect the circuit at point x in figure 4. This extinguishes LED3 and LED2 glows less brightly than before. This also proves our observation that both the batteries drive current through the circuit paths available to them, independantly of each other They take up exactly the same current at the minus pole which they discharge or drive out through the plus pole. Both the currents find the correct paths back to the voltage sources from which they emerge This fact was established by the scientist Gustav Robert Kirchhoff (1824-1887) who proposed the theorem that all the currents which flow into a nodal point, must always flow out of the same nodal point again. This is known as the Kirchhoff's Law. Let us examine the two nodal points in our circuit of figure 3 & 4. Figure 5 shows the current paths meeting at the first nodal point. Currents II and 13 flow into this node and current 12 leaves this node This gives us the relation 12 = II + 13. Based on this theoretical relation, we can explain the fact that LED2 must glow with more brightness compared to LED1 and LED3 as the current flowing through LED2 is the sum of two individual currents flowing through LED1 and LED3 respectively. 10-64 eleklor india oclober 1 985 I Although the terms "theorem", "nodal point" etc. sound theoretical, we can see the practical importance of the Kirchhoff's Law in studying various current paths in a circuit. A complex circuit can be understood easily when individual current paths are studied. I selex The Ohm’s Law "Resistors" as we have just seen, are the brakes for the current. In fact these resistors can be also thought of as the intermediate levels between good conductors (like copper wires) and non-conductors (insulators like glass, ceramics, plastics etc.) These intermediate levels are of great importance to the Electronics Engineer, since he can control currents and voltages with their help. We already know - "When a voltage is applied across a resistor, a specific current flows, not more, not less. The current depends upon the level of resistance offered by the resistor." Greg Simom Ohm (1787 to 1854), the discoverer of this interrelation, formulated from this the law which was later named after him as Ohm's Law: The voltage (U) across a resistor is equal to the product of the resistance value (R) and the current (I) flowing through the resistor. U = R . I Which is also stated as E = R . I This was once called the "Basic law of Electrical Engineering". Without going into the theoretical background the significance and utility of the Ohm's law can be seen with the help of a few examples. Figure 1 shows a 4.5 V battery connected across a 100 Ohms resistor. As the circuit is complete, the current flows through the resistor. By substituting the values of voltage and resistance into the Ohm's formula, the unknown current can be calculated as follows: 4.5 V = I x 10011 4.5 V or I = — — — = 0.045 A = 45 mA ! A milliampere (mA) is one thousandth of an Ampere. } We can practically confirm this result by measuring I the current on a multimeter in the 100 mA DC range. This small experiment makes clear, how a resistor I can set the current in a circuit for a given voltage [ value. Another interesting detail can be studied from a similiar experiment by adding one more 1 .5 V cell to the circuit in series with the 4.5 V which is already there. Now the voltage increases to 6 V and the Ohm's Law tells us that the current flowing in the circuit should be 6 V J I = — — - = 0.06 A = 60 mA This result can again be practically confirmed by measuring with a multimeter. The current increases with increase in voltage level, for a fixed value of resistance. Now try another variation. Increase the resistance value to 120 Ohms by substituting the 100 Ohms resistor by a 120 Ohms resistor. Keep the voltage fixed at 6 volts. The current should now be 12011 = 0.05A = 50 mA The current decreases with increase in resistance, for a fixed value of voltage. Let us once again go back to the example of a car on a downhill road. The steeper the slope, the faster is the speed of the car. The same is true for the circuit the higher the voltage, greater is the current. When the brakes are applied harder, speed of the car goes down. In case of the circuit, increase the resistance and current goes down. When the voltage in a circuit is fixed, the resistance regulates the flow of current. A 100 W bulb consumes about 0.45 A current, when it is lighted, (see figure 2). The Ohm’s law gives the value of resistance. Unfortunately this indicates the resistance of the filament in the lighted condition. (When it is hot.) When we switch on the bulb, it is still cold, and the filament resistance at this time is approximately 40 Ohms. This gives us the value of current that passes through the bulb for a moment at the time of switching it on as: 10-65 6 7 selex 230 V 4011 = 5.75 A This is more than 10 times the steady state current in the lighted condition. It is not surprising, that most of the bulbs are lost while switching them on. Note that in our calculations so far we have not used the original form of the Ohm's law U = R . I. but we have used the variations. The original statement of the Ohm's law is used when we know the current flowing through a branch of the circuit, and the resistance in that branch, for example the collector current of a transistor, and the collector resistance. (See figure 3) The voltage drop across the resistor can be calculated using the original statement of the Ohm's law. U = R . I Two resistors in series offer more resistance to the current than a single resistor. Two resistors in parallel offer less resistance to the current than a single resistor. Figure 4. Resistor R1 prevents the current in the circuit from becoming very high. The current is limited to 20 to 30mA. Figure s. Increasing the resistance reduces the current, thus reducing the glow of the LED. 4 >® Experiments: • Experimenting is better than so much of theory! Let's turn our attention to some interesting experiments based on the Ohm's law. For this we need: — 1 Flat battery of 4.5 V — 1 Resistor, 10011 — 1 Resistor, 22011 — 1 LED All these will be available in an electronic components shop. Colour of the LED is immaterial for these experiments. Figure 4 shows the first circuit with the 4.5 V battery, 10011 resistor and the LED. All connected in series. (Be careful about the LED polarity) The resistor is used for limiting the current through the LED within the allowed limits. In absence of this resistor the full 4.5 V battery voltage will be applied directly across the LED and it will be destroyed as a high current will pass through. The LED is used in these experiments to give a visual indication of the level of current flowing through the circuit, because the LED glows more brightly when more current flows through it. Observe the brightness of the glow of the LED in the first experiment and then substitute the 10011 resistor by a 22011 resistor. (See figure 5) Now observe the brightness of glow, it is less than that in the first experiment. Naturally so. because with increased resistance, the current has reduced. Connecting both the resistors together in series in the circuit as shown in figure 6 will further reduce the current and the LED glow will be dim. A further possibility is connecting both the resistors in parallel as shown in figure 7. In this case the LED glows more brightly than all the previous combinations. The result initially appears surprising. However as the two resistors in parallel offer two paths to the current and effectivelt the total current driven by the battery increases, and LED glows more brightly. The distinction between the series and parallel connection of resistors must be noted here - a series connection strengthens the' effect of resistance, whereas a parallel connection reduces the effect. selex Digi-Course Chapter 5 Gates, Logic Circuits, The Previous chapters in our Digi-Course series mainly covered the basic gates and the possibilities of processing logical signals. In this chapter we shall look into circuits based on gates, which allow the signals (logical signals) to pass through or block them, depending upon the circuit conditions. The gates can be said to be some kind of "Logic Switches" The T ruth table of an AND gate also shows this feature. >>f One of the inputs, designated here with S, can be considered to be the control input. With S=1 the AND gate output is same as the input A, where as with S=0 the output is constantly "0". With S = 1 the AND gate allows the signal at input A to pass through to the output. With S=0 the gate blocks whatever signal may be at input A. To try this on Digilex-Board. use a NAND gate with an inverter (a NAND gets with one input unconnected). 2 An OR gets also behaves in the same manner. I OR S A A + S With an OR gate, a 0 on the S input allows the signal at A to pass through to the output, and a 1 on the S input blocks the gate and makes the output constantly "1". This also can be tried on the Digilex-Board, using a NOR and a NAND gate. NAND and NOR gates, when individually used in this fashion, also behave as logic switches with inverted outputs. Changeover Switch A mechanical change over switch can be constructed from two switches with common actuating lever. Unfortunately this is not possible with the digital switches directly. (See figure 5) 6 We have seen how an AND gate with a control input S behaves as an ON/OFF switch. Here the control signal S is taken through an inverter to the first AND gate and is directly connected to the second AND gate. This prevents the condition that both the "AND- switches" are ON and OFF at the same time. The outputs of the two AND gates are taken through an OR gate. The truth table of the entire circuit is given below : Truth Table 10-67 Unfortunately the circuit of figure 6 cannot be directly wired on the Digilex-Board, which has only NAND and NOR gates. But there is no reason to panicl We already know how to use NAND and NOR gates to simulate the required circuits. Replacing the AND gates with NAND and Inverter combination, we obtain the circuit shown in figure 7. Input A : M4 Input B : N1 Control Input S : L9 AND-OR universal gate Figure 9 illustrates the possibility of application for the logical Changeover Switch, an AND-OR universal gate. Inputs are named as C and D for the purpose of differentiating the circuit from the previous one. Compare the circuit enclosed in the dotted lines with the AND equivalent, we had derived in chapter 3. A NOR gate was used there instead of the OR in the present circuit. This means that the present circuit is the "NAND" equivalent. Now this circuit can be easily wired up Digilex-Board. lIMMMMtt Here we have two NAND — NOR equivalent combinations for the AND and OR functions followed by the circuit of figure 8. Thus we have deviced a circuit which behaves as an "OR” gate when there is a "O" on S input. The same circuit behaves as an "AND" gate when there is a "1" on the S input. The connections on the Digilex-Board are as follows: Input C : K13 Input D :K12 K11 — R13 R11 — N1 (Input B) K13 - VI 2 K12 — VII VI 3 — S9 S8 — M4 (Input A) The Changeover switch can also be realised as shown in figure 10. We shall not go into the detailed explanation of this circuit. It is left to the reader to prove the equivalance. The truth table shows clearly that the signal on input A appears on the output in an inverted form, when there is a "1 " on the control input S. In case of a "0" on the control input S, the signal on A appears directly on the output. This is also known as a controlled inverter. Incidentally, it will be of interest to go back to chapter 3 again, where we had studied a circuit with reversing logic: The EXOR — circuit. 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 + Maharashtra Sales Tax. Delivery charges extra: Rs. 6.00 Send full amount by DD/MO/PO. Available from: precious ELECTRONICS CORPORATION Chhotani Building, 52-C, Proctor Road, Grant Road (East), Bombay 400 007 immiintifi 10-69 ne w pr oducts DIGITAL CLOCK MODULE ION Electricals offer a digital clock module for OEM applications— like Emergency lights, T.V, Sets. Intercoms, Slide Projectors, Room Coolers etc. The module can be fixed inside the panel with screws. The module is based on a single PCS measuring just 50 * 77 mm. Thedisplay indicates Hours and Minutes, with a flashing dot in between. Bars are provided for AM/PM indications. Alarm output is provided which operates only once in 24 hours. Power required is an external trans- former with 6-0-6 V secondary output. Terminals for providing 9 V DC backup battery are provided. For further information, write to: ION Electricals. 307 Owner's Industrial Premises 505 Gabrial Road, Mahim Bombay 400 016 BOX CAPACITORS Metallised Polyester and Metallised Polypropylene Capacitors in Box configuration are available in wide range to suit the various small Signal Circuits Applications from KEIL. These Capacitors are available in standard pitches of 7.5 mm, 10.0 mm, 15.0 mm, 22.5 mm and 27.5 mm depending on the values. Capacitors with 5.0 mm pitch also can be offered. These Capacitors, are LCSO approved. KEIL also offers special Metallised Film Capacitors for the purpose of Radio Interference Suppression and Surge Protection, meeting all the safety standards. For further information, write to:- Marketing Division Kothari Electronics & Industrial Ltd. 'Kothari Building' 1 14 Nungambakkam High Road Madras 600 034. DATA LOGGER Advani-Oerlikon have developed a Microprocesor-Based Data Logger for efficient supervisory control of process parameters in continuous process industries like steel, cement, petro- chemical, fertilizer and power gene- The Data Logger monitors and records various parameters such as tempera- ture, pressure, density, speed, voltage, weight, force, flow, level, etc,, and displays process information at a centralised place The moment a set parameter is exceeded or a wrong trend is sensed, it gives an audio-visual The Data Logger is capable of monitor- ing special instrumentations with paral lei BCD outputs, transducers with pulse or contact closure outputs or contact or voltage status from alarm points. The Data Logger has reprograming and on-line programing facility. For further information, write to: Advani-Oerlikon Ltd. Post Box No. 1546 Bombay 400 001. HYDROGRAPHIC CHART RECORDER The Hydrographic chart recorder operates on the principle of SONAR (SOUND NAVIGATION AND RANG- ING) and provides complete recording of the bed of water in a river, lake. sea. canal, tank etc. It can also be used for detecting fish shoals or submerged objects under water. The equipment uses latest 1C techno- logy and operates on 12 Volt batteries It provides an instantaneous reading of the contour of the river bed. 100 mm wide heat sensitive paper is used for recording. Six depth ranges can be selected on 0- 160 meter selector switch and the depth can be recorded to a typical accuracy of * 1%. The Special controls used include sensitivity control, shallow time control. Noise Reject Gray line controls etc. The equipment is ideal for Navigatiion, Depth Measurement, Locating fish, Scour Detection, Bottom profile measurement etc. For further information, write to: Electronic Instruments & Controls 4319 3-Ansari Road, Daryaganj, New Delhi 1 10 002. SERVO CONTROLLED VOLTAGE STABILIZER JIVAN Servo Controlled Voltage Stabi- lizer is a stepless Voltage Stabilizer with output accuracy of * 1% over a wide input range. It has output level control for setting the output voltage between 220 to 240V or 400 to 420V Facility for manual operation is also provided. High and low voltages are indicated by the neon provided on front For further information, write to: Jivan Electro Instruments 3 94, G.I.D.C. Makarpura Baroda 390 010. new products SUBMINIATURE SWITCH Elcom have introduced a subminiature DPDT toggle switch which has snap fitting facility. The switch body is moulded in polyamide. Suitable for both low and high current switching applications, the toggle switch is offered with an option of paddle or rocker lever in various colours. The standard rating is 4A, 250V AC. For lurther information, write to: 103, Jaygopal Industrial Estate, B, Parulekar Marg Dadar Bombay 400 028. SWITCH MODE POWER SUPPLY (SMPS) Atron Electronic Industries, have developed Switch Mode PowerSupplies (SMPS) for Black & White Televsion Receivers. Atron SMPS units are provided with mains isolation and are available in two models. Model 'A' gives DC stabilished output of 110V and 16V. Mains input voltage variation of 130V to 270VAC are tolerated. SMPS units are superior in performance and are less expensive than series regulated power supply units. For further information, write to: Atron Electronic Industries 62 A, M.G. Road, Secunderabad 500 003. PCB-REPAIR KIT Eltecks Corporation offer a PCB-repair kit,. containing all the materials required for PCB-repair, in one pack. The kit consists of Eltecks Air-drying Conductive Silver Preparation No: 1228, Paste Thinner 0070, special solder wire, soldering flux, brushes and other miscellaneous materials which are required. The above materials are packed in a decorative wooden box for the user's convenience. The above kit is useful for various Industries, where quick, convenient and reliable repairing/servicing is re- quired for Electronic/Electrical Equipment. For further information, write to: Eltecks Corporation C-314, Industrial Estate Peenya Bangalore 5 60 058 DPM MODULE Bantron Electronics nave introduced a compact 3'h digits DPM Module with 1 " bright LED display This is ideal for instruments with a long viewing range. The accuracy is 0.1% ± 1 digit The module has autopolarity. auto zero and high input impedance Operating vol- tage of *5V DC. Two measurement ranges are available: 200mV and 2V For lurther information, write to: Bantron Electronics 111 A/229, Ashok Nagar Kanpur 208 012. MICROFRIEND— III Dynalog has introduced the first 8085A based trainer/development system available in India having some extra- ordinary onboard features such as EPROM programming. Editor Assem- bler-Disassembler for 8085, Cross Disassembler for 8086. Centronix Com- patiable printer interface. Video Dis- play Controller & ASCII Keyboard interface in adition to the features like Audio Cassette interface, STD Bus. full 64K Bytes RAM/ROM decoding. ROM based BASIC is available as an option. Dynalog Micro-Systems 14 Hanuman Terrace Tara Temple Lane Lamington Road, Bombay 400 007. WAVE SOLDERING SOLDERMATIC Wave Soldering Machines with flat solder wave provide consistently high standards of Solder- ing performance & eliminate many of the installation, maintenance & servic- ing problems. All normal components can be soldered, including coils, switches, connectors, transformers, relays and discrete or integrated components-including SMDs. Range of 3 machines is available with wave widths of 220. 305 & 380 mm. These can handle boards (or row of boards) upto 430 mm long with standard carriers. The machines are compact, and are fully equipped with foam fluxer, built-in air pump, ventilated hood. Base Units, Feed Conveyors, Return Elevators as well as Cleaning Units are available as For lurther information, write to: Hytech Services, E-96, Greater Kailash-ll New Delhi 110 048. ZIF SOCKET 'Comtech' ZIF-40 socket features en- larged contact opening for easier insertion and extraction of ICs. The contacts are on an even 2.54 mm. (0.1") spacing with 15.24 mm. (0.6") between the contact rows. A built in stopper ensures that the 'Locking Lever’ can'L be overstressed. Glassfilled Nylon body and Beryllium-Copper contacts ensure a faithful cyclic operation even in the arduous working conditions, with proper masking-caps the same 40 pin socket can be used for 28 or 24 pin ICS. Component Technique 8, Orion, L.P. Road Andheri (West) Bombay 400 058. 10-71 Distributors: jyvwiviw) Electronics Corporation Chholam Building 52C. Proctor Road Grant Road (E). Bombay-400 007. WE OFFER FROM STOCK l.c.'s : TTL. CMOS, MOS, LSI, Microproccessor. Micro computer etc. Zener Diodes : 400 mw & 1 Watt S • Red, Green, Yellow in 5mm and 3mm dia 1C Sockets I SMK & Memorex make Trimpots : Multiturn Bourn's, VRN & Beckman make Single Turn cermets : EC as well as imported Floppy Discs : 8" as well as mini floppy of memorex, & dyson make Write to : 1 Pushpdant Niwas 3rd Floor. 3. Chunam Lane. Dr. D. Bhadkamkar Marg. Bombay-400 007. Phone : 5137225. 5135B45 Desoldered Without Dia mond Desoldered With Diamond Diamond Desoldering Pump ■ Desolders thoroughly. ■ Sturdy construction ■ Replaceable Teflon Nozzle ■ Largest selling in India ■ Widely accepted in CI.K., CJ.SA, West Germany and Singapore. Export Enquiries Welcome I precious ®, Manufactured by: Industrial Electronic & Allied Products 1423, Shukrawar Peth, Off. Bajirao Road, POONA 41 1 002, India. Phone: 446241, Gram: SEFOTAKE. 10-72 Booster Manufacturers : Now you can prove the superior performance of your products, by providing Booster Gain Meter which reads directly in db. available in a wide range Manufactured by: PADMA Electronics Pvt. Ltd. STOCKISTS Western Region : Precious Electronics Corporation. Chhotani Building. 52-C. Proctor Road Grant Road (East). Bombay - 400 007 Phones : 367459, 369478 Southern Region : Northern Region : Precious Electronics Corporation. Tantia Electronics Co. 9, Athipattan Street Mount Road. 422, Lajpat Rai Market Madras 600 002. Phone: 842718 Chandani Chowk, Delhi - 1 10 006 Phones : 238612, 233856 AlfciKet ’ ssstsftiir a 600 W dissipated by 300 mm H EAT SINK . ESCtrr’*” 470 EC .ztsx lor quick coolinq Afco Industrial & Chemicals Limited (Electronics Division) Kan^ir Marg (East) Bombay -400 078. Tel : 582164, 584913. TESTICA T-3 Rsl 70/ ONLY Afcwsct HEAT SINK 470 EC THE ONLY MULTIMETER WITH PRO MPT SERVICE AFTER SALES ACCURATE! ROBUST! ECONOMICAL! AVAILABLE AT ALL COMPONENT SHOPS MANUFACTURERS : ELECTRICAL INSTRUMENT LABORATORIES, 339/68. RAJESH BUILDING. LAMINGTON ROAD. BOMBAY 400 007. PHONF 36 07 49. 800 Watts Dissipation 10-74 ele classified ads. INDUSTRIAL ELECTRONICS "Please con- tact us for your requirements of Analogue and digital timers, frequency meters and Capacitance meters, Digital panel meters. Digital counter. Speed Switches. Frequ- ency switch. Temp switch. Sequential timers and your specific requirements of Industrial Electronics Items. Contact: M/s. DOTES, 80/60-A, Malviya Nagar, New Delhi-110017. Tel - 654039. 650674 "MICROPROCESSOR ENGINEER" Lead- ing electronic limited company in Bombay depth knowledge of designing micropro- cessor based devices. Knowledge of machine language essential. Good future prospects for the right person. Emoluments:- Salary. House rent allo- wance, Medical Leave, Travel allowance. Bonus, Provident fund, Gratuity, etc. Apply with biodata. Personal Attention. Managing Director. P.O. Box 9122, Bombay - 400 025. Educational Electronics kits available. Price list He. 1/- (We organise All India Universal Electronics Club Membership FREE. Enrolment form Re. 1 /-) - Contact: Renuka Electronics. 18. Ranganathan Street, Nehrunagar, Chromepet, Madras-600 044 Available data sheet and application of any electronic components. Minimum charges Rs. 15/ -. Write to: DATA BANK, Plot No. 16, Bldg No 3. Flat No 17, Bhavam Nagar, Maiol Maroshi Road. Andheri (East). Bombay - 400 059 Imported Solar Photovoltaic Panels 12 V. 55 Watts, suitable for charging lead acid batteries Contact: POLYCOM. 127. Silverlake Terrace, 55 Richmond road, Bangalore-560025 Telephone - 568860 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 - 412803 57.2/ < Advertisers Index ADVANI OERLIKON 10-05 AFCO I & C LTD 10.06 10-74 APEX 10-10 BLUJAY 10-10 COMPONENT TECHNIQUE 10 08 COSMIC 1084 DEVICE ELECTRONICS 10 79 DOMINION RADIOS 10 10 DYNALOG 10-07 ECONOMY ENGINEERING 10 08 ELCIAR 1008 ELTEK BOOKS N KITS 10 76 GENERAL ELECTRONICS 10 77 HIOKI '0-72 IEAP 10-72 INDIAN ENGINEERING CO 10-06 JETKING 10-73 KEJRIWAL 10.04 LABELLA LABORATORIES 10-04 NIPPON INDIA 10-11 PADMA 10-74 PRECIOUS MULTIMETER 10-77 RAJASTHAN ELECTRONICS ... 10-04 RAJRISHI 1002 SELLAIDS PUBLICATION 10-06 SAINI ELECTRONICS 10 76 SIEMENS 10 09 SUCHA ASSOCIATES 10 77 TESTICA 10-74 TEXONIC 10 73 UNLIMITED ELECTRONICS 10 81 VASAVI 10-73 VISHA 10-83 ZODIAC 10-72 wuxxcuuuiia floppy centring unit (080) August/September 1985 Id MOTOR ON to GND c< direct reading digitizer (054) The article does not mention that, with the input open. Pi should be adjusted to give an output of 000 while Pjshould be adjusted to give 900 at the output when the input is 900 mV. Components are normally available with the following companies: VISHA ELECTRONICS 17. 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 8-21 74 Red Cross Road Secunderabad 500 003 Phone: 72040 10-82 MH/BYW-228 UC.No.91 Watts News? After 30 years of communicating our finest efforts to you— we still have more news for you. Cosmic is now breaking every sound barrier in maintaining its sophisticated electronic image by touching perfection in the manufacturing of its T ape decks/ recorders. Stereo Systems. Amplifiers. Turntables. Head-Phones. A single dominant factor has encouraged us to keep expanding and that is consumer satisfaction. Stay tuned to us. cosmic We are Sound!