mm- contents elektor October 1981 — UK 03 selektor 10-01 universal LED display 10-02 Voltage monitoring is not always a simple matter. This circuit has a 'front end' in order to eliminate input offset problems and to provide automatic scale adjustment. shutter speed meter 10-04 The camera shutter speed is fairly critical and it can differ from the indi- cated figure by a large margin. How large can be discovered when using the circuit presented in this article. missing link 10-07 economical fridge defroster 10-08 1 Many fridges are equipped with a defroster but in many cases they may not be as economical as they first appear. This design will enable your defroster to save you money. using the Junior Computer as a voltmeter 10-11 (G. Sullivan) With the addition of the small circuit and program published in this article, the Junior Computer can be used as a digital voltmeter. And a good one at that . . . plug-in EPROM programmer 10-14 (R. Pequet) An extremely simple method of programming 2716 s in situ. EDITOR: P. Holmes UK EDITORIAL STAFF T. Day E. Rogans P. Williams synthesiser ICs 10-18 (H.P. Baumann) The American Curtis company has recently introduced a set of ICs specifi- cally designed for use in synthesisers. We take a close look at them here. 70 cm transverter 10-25 (P. de Winter) This, the second article in the series, covers the construction and cali- bration of the transverter in detail. LCD panel meter 10-32 The use of digital voltmeters as panel meters is becoming increasingly popular. The version described here was originally designed for the Elektor barometer but it can be used for many other purposes. high-speed cassette interface 10-34 (J. van Laren) A baud rate of 4800 is possible with the cassette interface described in this article and it will be extremely useful to any microprocessor owner. R F-test generator 10-38 At one time or another, radio Hams are going to need an RF generator for receiver alignment. The generator featured in this article will produce an output frequency in 9 MHz steps up to the giga hertz range. wide range dark room timer 10-40 The dark room timer published in this article is fully automatic with a wide enough range to cater for almost all photographic needs. As an added refinement, it even controls the safe lights. teletext decoder 10-43 f All the teletext decoders available for home construction suffer from one major disadvantage in that they require modifications to the TV set itself. However, the Elektor Teletext decoder system does not need any modifi- cations at all . . . unless you really want to fit it internally. market 10-51 TECHNICAL EDITORIAL STAFF J. Barendrecht G.H.K. Dam E. Krempelsauer G. Nachbar A. Nachtmann K. S.M. Walraven selektor elektor October 1981 — 10-01 New coinless phones New public payphones which can be used without cash are coming into operation in London. This further step towards the cashless society is being introduced at major railway and under- ground stations throughout London. The phones use a special card imprinted with five pence call units. Cardphones will also appear in Birmingham, Glasgow and Manchester in the next few weeks and the cards will be available in those areas as the phones are installed. To make a call the user inserts the card in the phone and taps out the number being called on a push-button pad. As the call progresses the call units on the card reduce. A digital readout on the phone continually tells the user how many units are left. At the end of the call the card is returned for future use. It will still be possible to make '999' calls free of charge without inserting a card. As well as being used for inland calls, the Cardphones can be used to ring all countries on international direct dialling, which now numbers more than 100 countries including Australia, New Zealand, Canada and the USA. Cardphones have a number of attrac- tions. There is no need to worry about having enough change. There is no need to keep feeding the phone with coins when making international or long dis- tance calls. It is hoped that the absence of cash will make the phones less attractive to thieves and less likely to be vandalised. And there is no costly collection of money. About 200 Card- phones are being installed for the trial and if the public takes to them more will be introduced. A special card — the size and shape of a credit card — is needed to use the phone. These cards can be obtained from post offices and a number of retail outlets, such as railway station book- stalls and Travellers-Fare kiosks. There are two values of card, one of 40 units costing £2 and a 200 unit card at £10. The units are imprinted in strips on the card using holographic technology based on special patterns of light. This offers maximum security against fraud and forgery. Since this process has a whole range of exceptional properties it pre- vents copying or modifying the card. The unit value, holographically memor- ised on the card, is shown on a visual dis- play on the front of the phone cabinet when the card is inserted. Throughout the call, the Cardphone continuously calculates the units used and reduces the value of the card at the rate of the in- coming metering pulses — erasing units by a thermal process. The amount shown on visual display decreases as the call progresses and units are erased. The rate at which units are used up will depend on the distance of the call (ie whether it is local, trunk or inter- national) and the time of day. Twenty seconds before the units on the card run out, the visual display starts flashing and the caller hears a warning tone in the receiver. The call in progress is undisturbed by the warning signal. If the caller wishes to continue the call, he, or she, can press a button on the cabinet. This automatically removes the remaining units from the card, stores them, returns the exhausted card and allows the caller to insert another card. The 200 unit card has two imprinted strips and contains 100 units on each side. When the first side is used up and the warning activated, the same method can be used to return the card so that the second side can be used. When a call is ended and the handset replaced, charging for the call stops immediately and the card is returned after a few seconds with any units remaining. If, after making one call, the phone user wishes to make another call, it is sufficient to tap and release the handset rest. This clears the line for the next call without returning the card. If it is required to know the number of units remaining on a card without making a call, this can be found out by inserting the card without lifting the receiver. The card's unit value will then be shown on the visual display. The steps in making a call are: — Lift receiver — Insert card — Key telephone number on push- button pad - Make call - Replace receiver - Retrieve card with remaining units The Cardphone is micro-processor con- trolled and is programmed so that cer- tain service numbers - such as directory enquiries and '999' emergency calls — can still be obtained free of charge. British Telecom (706 S) 10-02 — elaktor October 1981 universal LED display Although a description of the circuit was published last month, it requires minor modifications for the printed circuit board described in this article. A LED level display using the well known UAA170 has far more appli- cations than the thermometer and baro- meter referred to in the previous article. In many instances, an indication of a level is preferable to an exact and precise reading. With a display of 16 LEDs, the circuit now provides an easily read indicator that will prove excellent for a great many purposes. The 3 ICs, the LEDs and a few other components are all mounted on a printed circuit board that measures just 37 x 82 mm, small enough to be used practically any- where. When a certain voltage is to be measured with a meter, one of two extreme cases can occur: 1. The voltage may be too high for the range of the meter. A voltage divider consisting of two resistors will remedy the situation. 2. The voltage is too low. In this case, since the range is not fully used, the voltage has to be linearly amplified. Due to the input series-resistor to feedback resistor ratio, the amplification factor of an inverting opamp can be determined with accuracy. The inverting effect of the circuit can be cancelled by adding a second, inverting stage. Another advantage is that the inverting opamp with feedback can also act as an adder, that is another fixed voltage can be added to the input signal being measured. This is necessary if the input voltage level has an offset voltage super- imposed on it. Let's take an example. A voltage varying between 8 and 10 V is univcrsit) LEI) display including automatic offset and scale adjustment Another LED display ..??!! Before jumping to the conclusion that our designers are being LED astray allow us to explain why a different version of the LED display published last month was considered necessary. Voltage monitoring is not always a simple matter for the meter must be adapted to the voltage levels being measured. The circuit now contains a 'front end' consisting of three opamps in order to eliminate the problems of offset voltage levels and to modify the measuring and display ranges. This will enable the circuit to be used for a variety of applications and not just for the weather station as described in the September issue. IMii Figure 1. The circuit diagram of the LED display. Thanks to the offset and scale adjustments, the meter has the advantage that improved resolution is possible. universal LED display elektor October 1981 — 10-03 82015 2b U„ 820152a u in CV» Figure 2. The input to output voltage ratio is fixed. Figure 2a shows the amplification factors with different values for R5. Figure 2b gives various offset levels in relation to the setting of P2. IvO O ns IsO °^ r ICO O' F>3 ononnoo IC2 i'oUUUOOP EZH 3 nnonnona. S icn 1 Joouuuuotr h ^ hhhn^ h m m | Figure 3. The component overlay and track pattern of the printed circuit board for the universal LED meter. to be measured. If a meter is used with a range of 0 ... 10 V, only 20% of the dis- play range will be used. But a 0 . . 2 V range would lead to an overrange, since the lowest voltage is 8 V. The answer to this is to offset the display range by 8 V by 'adding' a negative 8 volts. This method is frequently used in offset and scale adjustments. Figure 2 shows the relationship between various input and output voltages inside the circuit formed by A2 and A3. The resistor values shown are relevant when the circuit is used for the tem- perature or humidity display. To display temperature values, connect R5 in the feedback loop of A2 (R5 = 270k). In the case of humidity indications, only R4 (47 k‘, needs to be in circuit. A single pole switch as shown can be used here. The total amplification can be calculated as follows: R5 . R7 = v R3 R6 The amplification factor should be sufficiently high to bring the upper input voltage level to 5,2 V. The re- quired factor is found by dividing the U output by the Uj n put- Since R6 = R7, this ratio can be related to R3 and R5: To avoid problems, choose a slightly greater value than strictly necessary. PI can readjust it later. P2 helps com- pensate the offset for temperature problems. The impedance converter A1 ensures that the circuit has a high im- pedance input. Testing and calibrating the circuit Testing the circuit is very straight- forward. The input at point C is earthed and PI is set with its wiper to the input of A3. If P2 is now turned to the nega- tive supply the LEDs should switch on in sequence. When D16 lights PI can be turned back and the LEDs will 'drop down' the row. Return both PI and P2 to the original settings and carry out the same check with a positive DC voltage at the input. If the display is to be used as a volt- meter, readers are recommended to use one LED for each decimal unit (ranges of 0.16 V, 1.6 V or 16 V in other words). For a 0 ... 0.16 V range, R5a = 270 k For a 0 ... 1 .6 V range, R5b = 27 k For a 0 ... 1 6 V range, R5c = 2k7 Resistors: R1,R6,R7,R8 = 10 k R2 = 1 k R3 = 6k8 R4 = 47 k R5 = 270 k PI = 5 k-Trimmer P2 = 10 k-Trimmer Capacitors: Cl = 10 m/16 V Semiconductors: D1 . . . D16 = LED IC1 = UAA170 IC2 = A1 ... A3 = % LM 324 IC3 - 78L12 Miscellaneous: 1 SPOT switch To calibrate the circuit, connect 0.1 V, 1 V or 10 V to the input and adjust PI until the tenth LED (D16) just lights. The voltage at the wiper of P2 should be exactly 0 V. If the circuit is to be used for other than the barometer display, the input of IC2 (point X) is linked to point A. The supply voltage in this case will then only be 1 5 V. H 10-04 — elektor October 1981 shutter speed meter In recent years some amazing electronic steps have been taken in the field of photography. Modern cameras are crammed with all sorts of electronic components and chips, providing auto- matic exposure control, shutter speed control, etc. In fact our electronic 'brother' keeps such a close check on every photographic enterprise that it has become almost impossible to 'goof up' the shot. The fitting of electronic devices inside a camera is well beyond the powers of any electronics enthusiast, however keen. Apart from that, any photo- grapher is aware of the great risk in- volved in 'fiddling around'. The modern camera is very much 'state of the art' both electronically and mechanically highly suitable for this particular purpose. This is an extremely versatile 1C and we have used it before in circuits such as the mini counter and the revol- ution counter (published last month). As many readers will have guessed by now, it is the well known MK 50398N. This 1C contains a six digit BCD counter (counts both up and down), a latch, a BCD-to-seven segment decoder and all the electronics required to control the display. The MK 50398N is at the heart of the circuit diagram shown in figure 1. Above it are the six LED displays (LD1 . . . LD6) which are common cathode types. In addition, a timebase is needed to provide the counter in IC1 with a clock sliuikr SIKHfl IlK'JlT For various reasons electronics and photography are hobbies that can often go hand in hand. This is not surprising, considering the number of electronic circuits that can be put to use in photography. In any case, many electronics ex- perts and enthusiasts are also keen photographers. In the past Elektor has published circuits that were primarily designed to facilitate the development process, such as dark room timers (a version of which is included elsewhere in this issue) and exposure meters. On this occasion, however, our designers felt it was high time attention was devoted to photographic measurement techniques. After all, the camera has to be able to take a good picture before it can be developed into a reasonable slide or photograph. Shutter speed is critical but it can differ from the indicated figure by a fairly large margin, especially on the cheaper camera. Bearing this in mind, a shutter speed meter with a digital readout would be a very useful accessory. and merely loosening a few screws is likely to send thousands of expensive, microscopically tiny parts flying. Even if you were lucky enough to find everything, you'd have to send the whole lot to the dealer's and then pray they manage to put the camera together again. Fortunately, there are quite a few electronic circuits which are not too difficult to build and which are of great help in photography. Usually, for instance, the camera's operation is not so easy to monitor. All the photo- grapher can do is check whether the shutter is working and whether the diaphragm is still opening and closing, and that's about it. Well, the diaphragm shouldn't give too many problems in practice, but if the camera is several years old the shutter speed may have lost some of its precision. Slowing down the shutter speed will of course cause every photo to be over-exposed. The shutter is a very complicated and delicate mechanism which is easily affected by dropping or knocking the camera. No amount of quartz can remedy such carelessness! The shutter speed meter described here enables the actual period for which the shutter is open to be measured very accurately. The photographer can then decide whether or not the shutter is still working properly and, if it isn't, take it to the dealer's for repair. The circuit diagram In order to measure the shutter speed, the camera itself, ironically enough, can be used; for while the shutter is open light through the lens falls onto the light-sensitive film. By placing a photo diode or photo transistor behind the lens the exposure time can be measured with the aid of an electronic measuring circuit. An 1C is currently available which is frequency. This is done by a crystal oscillator (N7, R4, Cl , C2 and of course a crystal) which produces a very stable 1 MHz frequency. After passing through the Schmitt trigger N8 and the divide- by-ten IC2, a frequency of 100 kHz is left to feed the clock input of I Cl . Furthermore, IC1 has a count inhibit input (pin 26). A logic one at this input causes the counter to be disabled. If, on the other hand, it is pulled low, the contents of the counter are in- cremented by one after every ten microseconds. The count inhibit input in this case is used to enable the counter for the time period that the photo transistor is illuminated for. The photo transistor T1 is connected between the positive of the supply voltage and the input of Schmitt trigger N9 which is also an inverter. When lights falls onto T1 it will start to conduct. The input of N9 will then become logic one and the output of N9 will be logic zero, so that the counter inside IC1 is enabled. With the aid of resistors R1 and R2, T1 is set so that it will only react to a con- siderable amount of light. The counter's contents, as shown on the displays, now indicates the shutter speed in microseconds when the photo transistor is placed behind the shutter and a light bulb is situated in front of the lens. If, for instance, the figure 100 appears on the display, this corresponds to a duration of 1 ms. In photographic terms this means the shutter time is exactly 1/1000 s. Now a method has to be found to reset the counter after every measurement. This is done with the push button S2 which connects the clear input of IC1 to the supply voltage. Seeing as chips form an essential part of the electronic diet these days, the power supply consists of an integrated voltage regulator (7812) and such nutritious ingredients as a transformer, a bridge rectifier and one or two capacitors. elektor October 1981 — 10-05 shutter speed meter 1 LD1 . . . LD6 = 7760 (CC) Figure 1. The shutter speed meter circuit diagram. Nearly all the required functions are carried out in IC1, an MK50398N 1C. A crystal oscillator acts as a stable reference timebase. Construction The printed circuit board on which the whole circuit is mounted is shown in figure 2. It is laid out in two sections, one for the displays and the other for the rest of the circuit. The two sections are carefully separated with the aid of a fine saw, after which the components can be mounted. 1C sockets are defi- nitely recommended to avoid any damage being done to the ICs. IC5 can be soldered straight onto the board and does not require a heat sink, provided the secondary transformer voltage does not exceed 15 V. The two boards are then linked together with lengths of wire (connections 1 ... 6 and a . . . g) so that the boards are at right angles to each other. A piece of red Perspex can be placed in front of the displays to mprove legibility. After this the reset key, the photo transistor and the transformer can be linked to the board. The wires con- nected to the photo transistor should not be longer than 20 cm. The oscillator can be calibrated with the aid of the trimmer capacitor C2. This will require an accurate frequency meter which is linked to the output of N8. The fre- quency is then regulated to exactly 1 MHz. However, readers who do not own such a frequency meter should set C2 at three quarters of its maximum capacity and the frequency will then be sufficiently accurate as well. Figure 3a gives an idea of what a case for the shutter meter could look like. It will house the two boards, the trans- former, the mains fuse and the mains switch. The displays are shown at the top of the drawing with the reset switch above them. The top of the case should be covered with a layer of foam rubber large enough to cover the back of the camera body. A hole is made in the middle of the foam rubber in which the photo transistor is inserted. The transistor should be level with the foam rubber, so that the camera can be placed on the shutter speed meter without being damaged and, at the same time, the layer of foam rubber prevents stray light from entering. Using the meter In addition to the meter and camera we need a desk lamp with a 60 ... 75 W light bulb in it. The meter is placed on a table and the rear side of the camera is opened or, if possible, removed altogether. The camera is then placed on the foam rubber with the photo transistor exactly in line with the centre of the lens. In the case of mirror reflex cameras, make sure that the foam Parts List Resistors: R1 = 100 k R2 = 2k2 R3= 10 k R4 = 1 M R5 . . . R11 = 270 n Capacitors: Cl - 22 p C2 = 4 . . . 44 p trimmer C3= 120 p C4 = 1 000 p/35 V C5 = 330 n C6 = 100 n C7 = 10 p/1 6 V tantalum Semiconductors: D1 . . . D4 = 1 N4001 LD1 . . . LD6 = 7760 (common cathode) T1 = photo transistor FPT 100 or similar IC1 = MK 50398 IC2 « 401 7 IC3 = ULN 2003 IC4 = 4093 IC5= 7812 Miscellaneous: Trl = 15 V, 200 mA transformer X = 1 MHz crystal FI * 100 mA slow blow fuse 51 = DP mains switch 52 1 push button switch Figure 2. The two printed circuit boards involved in the meter. These have to be separated before any components are mounted. shutter speed meter elektor October 1981 — 10-07 82005 3b Figure 3. Drawing a illustrates the case of the shutter speed meter. The top is covered in a thin layer of foam rubber to protect the camera and screen the photo transistor against stray light. Drawing b shows the positions of the camera, the case and the lamp when checking the shutter speed. rubber and/or photo transistor does show zero, it will be necessary to move (do) not come into contact with the the photo transistor and the lamp mirror, as this is both detrimental to around a little. As we mentioned the mirror and to the result of the before, the reading will be in tens of measurement. In any case; it should be microseconds. This means the result locked up if possible. will have to be converted into the The lamp is placed at about 30 cm in normal form of shutter speed indication, front of the lens and is then switched Table 1 shows the display reading at on. Figure 3b illustrates this. The various common speed levels. If the shutter speed button is turned to the reading is to be converted in terms of •equired speed, the distance is set on 1/x, x will represent 10 s /reading. The "'finite, the diaphragm is fully opened, device's measuring range extends from :->e shutter is cocked and the meter is 1/1000 s to 10 s. eset. The meter's display will then In the case of mechanical shutters it is ndicate zero. After pressing the shutter almost impossible to obtain precisely : Jtton, the shutter speed will appear on the same shutter speed at every measure- display. If the display continues to ment. The result is bound to fluctuate slightly. The best solution is simply to calculate the average by measuring the same shutter speed ten times. The reset key is only pressed for the first measurement. After the procedure has been repeated ten times, the final reading is divided by ten and then compared to the table or applied to the formula given above. This gives the average result. With respect to the values measured, these will never be absolutely accurate, as this just is not possible in practice. So don't worry if the result misses the mark a little. Most photographers are delighted if the shutter speeds are within about 10% of the ideal values. Even a value within 20% is still a satis- factory result and a difference of 30% should not affect the exposure times in practice. The shutter speed meter is an accurate device and, provided it is used sensibly, will prove to be worth its weight in gold (or at least, film)! H Infocard 13 (Elektor 72) The package for the TL 074 and TL084 ICs is indicated as IV; this should be V. Infocard 21 (Elektor 74) In the tone and frequency table F octave 2 should be 87.3071 . 70cm transverter (1) (Elektor 74) In figure 6 the pin assignment for the inputs and outputs of IC1 are the wrong way around: 2 should be to the left and 1 to the right. This is correct on the component over- lay and the printed circuit board. LED audio level meter (Summer Circuits Issue '81, no. 23) The text mentions an LM 381 9 1C. This should be LM 3915. scoreboard (Summer Circuits Issue '81, no. 1) The mains connection which is directly con- nected to the triacs also has to be linked to the ground of the rest of the circuit. If triacs are used, IC4 = IC5 = 74248. TV games extended (Elektor 77. p.9-22) To obtain a good amplitude balance between PVI tone and PSG explosion, it has proved better to modify three resistor values: R9: was4k7, becomes 1k5 R1 1 , R18: was 4k7, becomes 2k2 10-08 — elektor October 1981 economical fridge defroster Most fridges are based on the straight- forward principle of compressing and evaporating a volatile liquid (Freon). An electric motor drives a compressor which compresses the Freon into a liquid. During the process heat is pro- duced which escapes into the environ- ment through a radiator (usually at the back of the fridge). The liquid Freon is fed to the cooling plates in the fridge where it is evaporated (into a gas) and in doing so causes heat to be extracted from the inside of the fridge (and there- fore from the products stored as well). for longer periods and therefore con- sume more energy. This ends up costing more than the actual products being stored are worth! The remedy is to stop the cooling process whenever the cooler is covered in a thin layer of ice. This can be done by unplugging the fridge. When the ice has melted the water can be removed and the fridge will start oper- ating more efficiently again. A simpler solution is to provide the fridge with a semi-automatic defrosting unit. The fridge is switched off by pressing the (usually red) button pro- economical fridge defroster takes the ice out of the energy price! Many fridges nowadays are equipped with an automatic defrosting unit to prevent the cooler from icing up. A thick layer of ice will affect operation and raise the electricity bill considerably. This can be remedied by defrosting the unit periodically. The trouble is, automatic defrosting units also consume quite a lot of current and what is more they produce a fair amount of heat which adds to the fridge's energy intake. An electronic fridge defroster, on the hand, manages to do the same work at a lower energy cost. vided. When the ice has melted on the cooling plates the fridge is switched on again automatically. The modern defrosting units with which many fridges are equipped nowadays are fully automatic and so make life very easy. The snag is, such luxury ends up putting up the energy bill as well! A vicious circle . . . What happens is that whenever the thermostat switches off the motor, a heating unit (10 ... 25 W) is switched on thereby heating the cooler and melting the ice. The water produced is led out to a special container outside the fridge. Thus, after every cooling phase the cooler is defrosted. The heating element not only consumes quite a lot of current, but the heat it produces will have to be removed too, which again requires energy. In the end it may be discovered that it is much cheaper to switch off the automatic unit altogether, as it defeats its purpose! Nevertheless, fridges need to be de- frosted from time to time. An economi- cal method is to provide the heating unit with a manual switch. In practice, the cooler can be kept in an optimum condition by defrosting it for about half an hour a day. In some respects, how- Once a certain temperature has been reached the motor is switched off by a thermostat. After a time, depending on how well the fridge is insulated and the size of its internal surface area (not its volume!) and on the difference in tem- perature between the internal fridge temperature and that of the room, the temperature will have risen to such an extent that the thermostat switches the motor on again. More often than not the cooling plates enclose a small ice box in which frozen products may be stored. Whenever the door is opened a certain amount of warm air outside will flow into the fridge and cause condensation in the cooler interior. As a result, a thick layer of ice will eventually form in the ice box and so make it more diffi- cult for heat transfer to take place. Consequently, the motor will operate C - 135 W motor 0 ” door twitch E ■ interior light cut one of that* wires Figure 1 . If the circuit of the electrical system in your fridge looks like this, it is all right to use this economical defroster circuit. economical fridge defroster elektor October 1981 - 10-09 Figure 2. The circuit diagram for the defrosting unit. With the aid of a timer (IC1) the defrosting system is operated for one hour at 7, 15 or 31 hour intervals. ever, this brings us back to square one, for we now have to remember to do this every day, which can be quite a bind. Fortunately electronics has got the answer: add a circuit which switches on the defrosting unit for one hour every 7, 15 or 31 hours, according to choice. Before readers jump in at the deep end and grap their soldering irons, it is advisable to check whether their fridge does indeed have a fully automatic defrosting system with a heating unit that is connected in parallel to the ther- mostat. (This is usually the case in fridges that have a built-in ice box). In other words, the electrical system in your fridge should correspond to the one in figure 1. When the thermostat switches off, the heating unit will be in series with the (low-impedance) motor. The current passing through the heating unit is too low to drive the motor, but produces enough heat to melt the ice on the cooler. Readers who do not have a drawing of their frdige's electrical system had better use an ohmmeter (the heating unit and the wire connections are often well hidden). Plug in to mains. Wait until the motor stops and then pull the plug out. If the meter does not measure any resistance between live and neutral, you can be sure that the circuit diagram described here is not suitable for your type of fridge. This also refers to semi- automatic fridges for such types do not have a heating unit. If, on the other hand, the measurement shows that the fridge can be provided with an electronic defrosting unit (the resistance measured should be several kfl) check to see whether you can find the wires connecting the heating unit. The wires usually connect the thermo- stat to the cooler. One of them (it doesn't matter which) will have to be cut to allow the electronic circuit to be connected in series with the heating unit (see figure 1). Note: The cooler should never be re- moved as this is linked to the cooling motor. The circuit diagram Figure 2 shows the complete circuit dia- gram for the defrosting unit. In fact the circuit consists of a simple time switch, built up around a 4060 binary counter. This 1C contains an internal oscillator which produces a Q10 output cycle time of around 112 seconds with the values chosen for R4, C4 and C5. The voltage at pin 9 will therefore change in level after every 56 seconds. This can be checked with the aid of a voltmeter and a wrist watch. If this parameter is not met, adjust the value of R4. A divide- by-two is situated between every output in IC1 (the first 5 outputs are not used). The output at Q6 has a cycle time duration of about 2 hours (low for 1 hour and high for 1 hour). Output Q7 goes high on the negative-going edge of this signal and so does output Q8. Outputs Q6 . . . Q8 control the triac (Tril) by way of diodes D6 . . . D8 and transistor T1. As a result, thf triac stops conducting for 7 hours and then con- ducts for 1 hour (see figure 3). This triac is connected in series with the heating unit. As a result, the defrosting system is operated for one hour at seven hour intervals, that is, the heating unit is switched on after the motor stops. If the motor is switched on again by means of the thermostat, the fridge will not be defrosted for an hour after which it takes another seven hours before the thawing process is resumed. If after a few days there is no ice on the cooling plate (depending on how often the door is opened!) diode D9 can be connected to the circuit using the wire link B1,so that the intervals now last 15 hours. If necessary, wire link B2 can also be made to create intervals lasting 31 hours. In order to obtain a stable clock fre- quency, IC1 is supplied with a voltage that is stabilised by the zener diode D5. The R1/C3 network makes sure the A - 1 how B - defrott time C - twitched oft 81158-3 O ■ twitched on Figure 3. The pulse flow chart of the levels at the Q6 . . . Q8 outputs of IC1. If all three outputs are logic 0, the defrosting system will operate. 10-10 — elektor October 1981 economical fridge defroster Parts list Resistors: R1 = 470 k R2 = 680 n R3 = 10 M R4 = 3M3 R5 = 220 k R6 = 39 k R7 = 1 k R8 = 47 n/1 W Capacitors: Cl = 100 p/25 V C2,C3 = 1 00 n C4,C5 = 22 m/ 16 V C6 = 100 n/400 V Semiconductors: IC1 = 4060 T1 = BC 547 Tri 1 = TIC206D D1 . . . D4 = 1N4001 D5 = zener 10V MOO mW D6 . . . D10= 1N4148 Miscellaneous: Tri = 12 V/50mA FI - 1 00 m A slow fuse + holder 3 strain relief glands plastic case BOC 435 West Hyde Figure 4. The track layout pattern and component overlay for the fridge defroster printed circuit board. The board should be housed in a plastic case. counter is automatically reset when the supply voltage is switched on, so that the thawing process always starts first. Construction Figure 4 shows the component overlay and the copper tracking pattern of the printed circuit board for the economical defroster. Mounting the components on the board should be straightforward enough. Leave the wire links B1 and B2 until later. The triac does not need to be provided with a heat sink. As can be seen from the circuit diagram, the power supply is not grounded — this is exactly how it should be! The board must be housed in a plastic case (BOC 435 from West Hyde). For safety reasons it is advisable to use plastic nuts and bolts to fix the board to the bottom of the case and also to mount the trans- former onto the lid. In addition, a 3 way connector (choc-block) is attached to the inside of the lid with a plastic nut and bolt. Three holes are drilled in the lid for 3 strain relief glands for the mains leads. The circuit is connected to the fridge in the following manner. Unplug the fridge and cut the mains lead in the position that the defroster circuit is to be con- nected (in other words, outside the fridge). The mains lead with the plug, still hanging on it is passed through the strain relief and wired to one side of the connector. The primary of the trans- former is now wired between live and neutral. Please, under no circumstances connect it to the earth wire! The fuse should also be connected in series with the primary side of the transformer. The mains lead that is still linked to the fridge has to be connected to the other side of the 3 way connector. Finally a 2 wire mains lead is soldered to outputs A and B on the board. The ends of the lead are led into the fridge and connec- ted 'in series' with the cut wire leading to the heating device. It does not matter which way round the wires are connec- ted, but it is important to know if the correct wire is being cut (see figure 1). The case can be mounted on the back of the fridge. After a thorough check the fridge can be plugged in which will start the motor. If after a few days the cooler is still not covered in ice, wire link B1 may be added to make the economical defroster even more economical. If necessary, link B2 can also be fitted, but if ice starts to form on the cooler (several mm thick) B2 should be re- moved. If this doesn't helpBI should be taken out as well. Readers will have to experiment and see what is best, since every fridge is different! Warning! Remember that in spite of the transformer the circuit may well be con- nected to a lethal voltage, since it is connected to mains via the heating unit.M using the Junior Computer as a voltmeter elektor October 1981 — 10-11 Voltmeters are always useful as far as anybody interested in electronics is con- cerned. Junior Computer owners can use this straightforward circuit and pro- gram to convert their computer into an excellent digital voltmeter. The basis of the circuit is an A/D con- verter from Intersil. This 1C has binary outputs and will convert the input signal plete voltmeter circuit for a 3% digit display including automatic polarity indication. This means the whole circuit can be very straightforward. Figure 1 shows the circuit diagram. The voltmeter 1C contains a 1 2 bit A/D con- verter with tri-state outputs. The out- puts B1...B8 are displayed in two bytes. Which byte is displayed depends using th© Junior Computer as a voltmeter With the addition of a small circuit and the aid of the accompanying program the Junior Computer can be used as a digital voltmeter . . . and a good one at that! The voltmeter has 3% digits and an automatic polarity indicator, even though the program is less than 180 bytes long. G. Sullivan level into the BCD code. Since the 1C has tri-state outputs, it is suitable for use with microprocessors. The circuit The Intersil ICL7109 contains a com- on the address decoder IC2. The low order byte contains the eight least sig- nificant bits and the high order byte the four most significant bits plus an over- flow bit and a polarity bit. Using the given specifications, the Figure 1. The digital voltmeter circuit diagram which enables the Junior Computer to measure DC voltages. IC1 takes care of the whole process, converting an analogue input signal into a 12 bit code for the computer. 10-12 — elektor October 1981 using the Junior Computer as a voltmeter input voltage for 'full scale deflection' will be 4.096 V. The conversion speed is about 30 per second. The input range can be varied by changing the value of R2 and by modifying the reference volt- age which is set by PI. Thus, UfuMsc^le 20 pA Ufull scale = 2 • Uref- The value of Cl and C2 is determined by the oscillator frequency used, where Cl 2048 • period • 20 pA 3.5 V and C2 = 2-C1. The 250 kHz frequency that is used for the converter is derived from the micro- processor clock. For this IC3 is connec- ted as a divide-by-four. If required, the converter may be run at a different fre- quency by choosing a different output of IC3. This will also alter the number of conversions per second. Because of the converter's high input impedance, input attenuators can be included fairly easily to provide several voltage ranges. The program The program which enables the JC to be used as a digital voltmeter is shown in table 1. This will read in the two bytes produced by the converter, after which a binary-to-decimal conversion takes place and the result is then shown on the display. In the case of a negative input signal the status of the polarity flag is detected and a minus sign is shown on the display. If the maximum input voltage of the converter is ex- ceeded, the display will indicate OL (overload) together with the polarity. In the circuit diagram in figure 1 the converter is connected in the 'free run mode'. This means it will start with the next conversion as soon as an analog-to- digital conversion has been completed. This is fine under normal circumstances, but it may prove necessary to detect the actual moment a conversion has ended and then read in the data to avoid this happening while the data is changing. This can be done by connecting the status output of IC1 to PA7 of the port connector and by using the negative- going edge of this output to enable an interrupt (IRQ) at the end of a conver- sion. The interrupt routine can then read in and store the two bytes before the following conversion comes to an end. An example of this interrupt routine is provided in table 2. Once the two programs have been loaded beginning with address 0200, the Table 1. LINE LOG OBJECT SOURCE 0001 OOOO ,dig:t6l voltmeter frogrhm 0000 0000 ,FuR INTERSIL ICL 7103. 0003 0000 0004 0000 ■ rtUTHOR 0. 3ULLIV6N 0005 0000 O00C 0000 ♦ = $uu£G 0007 00D0 riCUM 0008 0OD0 SUM ♦=♦+1 0003 00D3 DELhV ■*=♦+1 t 0010 O0B4 tmpx *=*+! 0011 00115 , 0010 O0D5 .DEFINE 0/D CONVERTER 0013 O0D5 * = f 1800 0014 1800 riB *=*+! ,HI0H MIBLE + Fl60S 0015 1801 LB *=*+1 , LOW BYTE 001b 1800 , 0017 1800 ■DEFINE FI0 0018 1800 * = 41680 O013 1880 FR6 #=*♦ 1 , D0T6 n REG. 0000 1081 DDR0 ♦=*»1 ,6 DIRECTION REG. 0001 1080 FRB *=*+1 , D0T0 B REG. 0000 1083 DDRB *=♦+1 £ DIRECTION REG. coo;. 1084 0004 1084 oCMNDl =41DCC , DiSFLRV 1 BYTE 0005 1084 , 0000 1084 * = 40000 0007 0000 , O0O3 0000 ,M6IN DISPL6V ROUTINE 0009 0000 OC 00 IS M6IN BIT HB , TEST 0VER6NGE BIT 0030 0003 70 06 BVS OL 0831 0005 00 04 00 JSR DISVLT .SHOW VOLTS 0030 0008 4C 12 00 JMF NOL 0033 0O0B fiO 0C OL LDX #40C , DISPL0Y OL MESS0OE 0034 0O0B 00 03 lDV #463 0035 020F 00 43 00 JSR DIST6T 0030 0010 FID 06 18 MOL LD0 riB , TEST F0L6RITY BIT 0037 0015 30 07 EMI NOT 0038 0017 80 68 LDX #408 , DISPL0Y MINUS 0039 0019 FI0 00 LBV #400 0040 0O1B 00 43 00 JSR DIST0T 0041 0O1E 00 73 00 NOT JSR HEXBCD , CuNV. BIN6RV TO BCD 0042 0001 4C 00 00 JMF M0IN 0043 0004 t 0044 0204 /VOLT DISPL0V SUBROUTINE 0045 0004 69 7F DISVLT LD0 #47F , SET PI6 TO OUTPUT 0040 0026 8D 81 10 3T6 DDR0 0047 0029 00 0C LDX #40C , 0DDRESS OF FIRST BYTE 0048 0O2B 00 FF lDV #4FF 0043 0201) C8 LOOP INV 0050 0O2E E3 D0 00 LD0 0CUM/V ,GET BYTE 0051 0031 00 CC ID JSR SC0ND1 .LIGHT DISPL0Y 0050 0234 E0 14 CPX #414 /TEST IF TWO BYTES VET 0053 0036 D0 F5 BHE LOOP 0054 0238 0D 00 18 LD0 HB /TEST POL0RITV 0055 023B 30 07 BMI NXT /SHOW - IF NEG. 0050 003D 00 06 le: ; #408 0057 0O3F 00 00 lBV #400 0058 0041 00 43 80 J3R DI3T6T 0053 0044 t6 D3 NXT INC DEL6V /DELAY ON DISFL6Y 0000 0046 do DC BHE DISVLT 0001 0048 60 RTS 0000 0043 0003 0043 /DISPL0Y - / OL 0004 0043 03 7F DIST0T LD0 #47F /SET Pin TO OUTPUT 0085 0O4B 3D 81 16 3T6 DDR0 0000 0O4E E9 6D 00 lD6 CH6RT / V /GET MESS0OE BYTE OO07 0051 CG 14 BMI ENDD OO08 0053 c D 80 16 ST0 PR6 /LIGHT segments -J003 0056 c-E 80 16 SIX PRE /SELECT DIGIT 0070 0053 66 B4 STX TMFX 0071 005B m2 FF LDX #4FF 0070 0O5D Cm DLVO DEX , DEL6Y 0073 025E D0 FD BNE DLVO 0074 0060 rib D4 LDX TMFX 0075 0060 £3 I NX 0070 0063 £3 I NX /NEXT DIGIT 0077 0064 C6 iNV 0078 0065 D0 E0 BNE DIST6T 0073 0067 mS 00 ENDD LD6 #4@0 , DISPL0Y OFF 0080 0063 8D 80 10 ST0 FRB 0081 026C D0 RTS 0030 0061) ?F C06RT .BYTE 47F/4-3F/ 480 0080 026E 3F 0080 026F 86 0083 0070 40 .BYTE 440,447/ 480 0083 0071 47 using the Junior Computer as a voltmeter elektor October 1981 — 10*13 I RQ vector has to be specified: 00S3 0272 30 0034 0273 0035 0273 0036 0273 A3 00 0037 0275 35 D0 0033 0277 35 D1 0033 0273 AE 01 IS 0030 02 7C F0 0B 0031 02 7E A0 01 0092 0280 93 0033 0231 35 D2 0034 0233 20 3E 02 0035 0236 LH 3036 0237 D0 F5 0037 0239 AD 00 13 0033 023C 29 0F 0033 023E F0 0D 0100 0230 AA 0101 0231 A3 03 0102 0293 85 D2 0103 0235 A0 20 0104 0237 20 9E 02 0105 02 9 A CA 0106 023B D0 F8 0107 029D 60 0103 029E FS 0103 023F 18 01 10 02A0 A5 D2 0111 02A2 65 D1 0112 02A4 35 D1 0113 02A6 A3 00 0114 02A8 65 D0 0115 02AA 35 D0 0116 02AC 38 0117 02AD D0 F0 0113 02 A F D8 0119 02B0 60 0120 02B1 , CONVERT BINARY TO BCD HEXBCD LDA #400 STA ACUM STA ACUM+1 lDX LB BEQ HIGH L00P1 LDV TYA #401 STA SUM JSR BEX ADD ENE L00P1 HIGH LDA HB AND #*0F BEQ TAX LAST LDA #403 STA SUM LOOP2 lBY #420 JSR DEX ADD ENE LOOP2 LAST RTS ADD SED LOOPS ClC lDA SUM ADC ACUM+1 STA ACUM+1 lEA #400 ADC ACUM STA DEV ACUM BI-IE ClB RTS LOOPS • END CLEAR ACCUMULATOR , CONVERT LOW BYTE / CONVERT HI BYTE REMOVE FLAGS , ADD 1 OR 256 TO ACUM ERRORS OOOO SYMBOL TABLE 1 A7E 80 1 A7F 03 Finally, the initialisation routine which is needed whether or not the interrupt routine is used: 0000 8D 86 1 A STA 1 A 86 0003 58 CLI 0004 4C 00 02 JMP-MAIN Now a reference voltage is connected to the input of the meter circuit (4 V, for instance) and PI is adjusted so that the display indicates the value of the refer- ence voltage. In the absence of a known voltage, any DC voltage of about 4 V can be used instead and the display can be compared to that of another, accu- rate meter. These instructions make sure the PI A produces a negative-going edge at PA7 and that the interrupt-disable bit is restored once the processor is reset. When this has been done and the circuit in figure 1 is connected to the Junior Computer, the program can be started at address 0000. H Zhe Junior Computet... ACUM 00D0 ADD 023E DDRB 1 Hco DELAY 00D3 DLY2 025D ENDD 0267 HIGH 0233 LAST 023D L00P1 02 7E LOOP 2 0235 MOL 0212 NOT 02 IE PRA 1A80 PRB 1 A82 TMPX O0D4 END OF ASSEMBLY CHART 026D DDRA lASl DISTAT 0249 DISVLT 0224 HB 1800 AEXBCD 0273 LB 1801 LOOP 022D L00P3 029F MAIM 0200 NXT 0244 UL 020B SCAND1 1DCC SUM O0D2 Table 2. IRQ service routine: 9380 48 INTS: PHA ; save A 9381 8A TXA ; save X 0382 48 PHA 9383 98 TYA ; save Y 9384 48 PHA 0385 AO 85 1 A LOA 1 A 85 ; reset IRQ 9388 AD 00 18 LDA 18 00 ; read high byte 938B 85 00 STA D0 ; store it 038D AO 01 18 LDA 18 01 ; read low byte 0399 85 D1 STA D1 ; store it 0392 68 PLA ; restore all registers 0393 A8 TAY 0394 68 PLA 0395 AA TAX 0396 68 PLA 0397 40 RTI ; return to main program J. van Laren CLOCK IJIJTiTJlJlT^rLJTJTrLriJ I I edge t * t * t I ) * i » t t I ' 1 , fmax fmax fmax fmin = — - — Figure 1 . The Manchester code is generated from a data and a clock signal. The data determines whether a negative or a positive-going edge should be produced. If the data does not change in logic level, additional edges will have to be inserted. 2 A B C A = 1 A = 0 0 0 1 0 1 0 B c B c 1 0 0 ~0l 0 0 1 1 1 1 1 1 1 0 81161 2 T o a fi h ff tt h V a' fi IS a! k t! w h c a li o b t< q u C' Figure 2. The truth table of an ordinary exclusive NOR gate, which is highly suitable for use as an encoder. *■ V li The Manchester code has a number of advantages: it is reasonable efficient, a clock signal can easily be retrieved from it and the circuit used does not nave to be complicated. The code features 50% efficiency, this means that the data baud rate may be as high as the highest frequency. More recent codes manage to operate at twice the baud rate using the same frequency level, but the system involved is much more complex. The nice thing about the Manchester II is that the owest frequency level is exactly half the maximum frequency. In other ■vords, the recorder does not have to -ave an absolutely linear frequency curve which is a great advantage to the amateur. In practice this means that, in order to obtain a data transfer speed of 4800 raud, the cassette recorder must be able to reproduce 4800 Hz well. If the fre- ouency range of the recorder extends -P to 10 kHz, a baud rate of 9600 Hz could even be achieved. Manchester II encoder '(1131 does the Manchester code look like? There are different ways of look- ing at it and we start here with the most complex point of view. Supposing we have a clock signal of 4800 Hz. The Manchester signal must feature an edge at every negative going edge of the clock. If the data at that par- ticular moment is logic zero, this will have to be a positive-going edge. If, on the other hand, the data concerned is logic one, the edge will have to be nega- tive-going (see figure 1). But what happens if the data is not constantly changing? After all, it is impossible to transmit only negative-going or only positive-going edges. The remedy is have to be included halfway between each data transmission. The information frequency will now be equal to the clock frequency. Judging from above, most readers should now be able to imagine a com- plex circuit in which the required pulse edges are generated using logic gates and flipflops as memory devices. As it happens this is not necessary and all it requires is a single exclusive NOR gate! It can be seen from the truth table in figure 2 that a "1" at input A will allow the output C to follow the B input. With a "0" at input A however, the C output will now invert the B input. In short, if the clock frequency is con- nected to input B and the data is sup- plied at input A, the clock signal will be inverted or not, depending on the logic level of the data. This is like "clock- work" for figure 1, where a data low inverts the clock and a data high does not. Readers are aware that inversion is often described as a 180° phase-shift. It can be said then that the phase of the clock signal is rotated either 180° (= inverted) or 0° (not inverted). Thus the Manchester II system belongs to a group of codes which use phase modu- lation since the clock is phase-modulated by the data. This is known as a "bi phase code", in other words, two phases are used, in this case 0° and 1 80°. A further aspect makes decoding the recorded information very easy in practice. A long pulse period indicates that the decoder output has to change. It must become logic 1 , when the level of the period is logic one, and logic zero, when the level is logic zero. The decoder From the latter definition it is a small 10-36 — elektor October 1981 high-speed cassette interface Figure 4. The Manchester code decoder. The cause and effect situation is clarified by the enlarged view of part of the signals; the arrows indicate the chronological order. Note that information may be lost straight after the power supply is switched on (at A), which is why initial data is repeated. step to the Manchester II decoder. The code consists of a combination of the data and the clock signal and the encoder has already been dealt with — this was the exclusive NOR gate. The decoder is however a little more extensive. During each of the longer pulse period the data must change. A monostable multivibrator can quite easily detect a longer period and, when one arrives, - generate a pulse. With this pulse the logic level of the Manchester II code at that time is stored in a memory — another flipflop. The output of this flipflop is then retrieved data. After all, if a longer period was logic one, it stands to reason that the data must also be logic one, and vice versa. It's as simple as that. The circuit Now that all the principles behind the circuit have been explained we can be very brief about the actual encoder and decoder circuits. Figure 3 shows the cir- cuit diagram of the encoder. The data is usually produced by a UART. In any case, the clock frequency will have to be a whole multiplication factor of the baud rate. In other words, in a 16x clock the clock frequency will be 16 x 4800 = 76.8 kHz, in a 64x clock this will be 307.2 kHz, etc. The 4040 is a binary divider which divides by 16 (or by 64 if output 2 is taken instead of output 5). Flipflop FF1 ensures that data changes occur exactly during the positive-going tran- sition of the divider clock frequency. This is necessary to prevent the final data from containing extra pulses which would confuse the issue. The Manchester code is derived from the clock signal and the synchronised data in the usual manner. Since an exclusive OR is used instead of an exclusive NOR the code will be inverted, but that does not affect the principle. With the aid of R1 and Cl the square wave signals are tapered off a bit. Two outputs are provided: preferably output A should be used, as output B produces an attenuated signal that is meant for recorders which only have a microphone input. Figure 4 shows the circuit diagram of the decoder. The recorder signal is fed to a comparator which turns it into a square wave, N3 polishes up the edges The Manchester code is now fed to the data input of flipflop FF2 and also to monoflop 1 which must detect the longer periods. Gate N4 is included in the signal path to monoflop 1 and assuming that N4 does not invert, monoflop 1 will be triggered by the first positive-going edge to arrive. If the pulse duration is short, the two monoflops high-speed cassette interface elektor October 1981 — 10-37 :e >R >e$ of ec > a es he to :he in ic irt rs: Ise >P* 81161 5 Figure 5. An auxiliary circuit for readers without an oscilloscope. This correctly calibrates the monostable period of monoflop 1 . If the baud rate is altered, the calibration procedure will have to be repeated. 6 Figure 6. A block diagram which shows how to connect the cassette interface to a micro- processor system. are reset as soon as the input signal goes low again and nothing else happens. If on the other hand the pulse duration was long, monoflop 2 is activated at the end of the monoflop period. Monoflop 2 again generates the clock pulse for flipflop 2 which then takes over the data when it arrives at its D input. As a result, the outputs of FF2 will (always!) change. The decoded data can be derived from one of these outputs. Both of them are available, so that even the inverted data may be used if necessary. Which output must be selected also depends on the type of cassette recorder in use. If it inverts the data, the other output will have to be used. So far the operation has been de- scribed with reference to positive- going edges. However, there are also long pulse durations which start with a negative-going edge. What's more, any Photo 1 . The lower trace constitues the signal as produced by the cassette recorder. The output data should change after every long period. The lower left hand corner shows a long positive-going pulse duration followed by a long negative-going counterpart. The data changes accordingly. long pulse duration that starts with a positive-going edge is bound to be followed by one starting with a negative- going edge. The point is, monoflop 1 does not react to negative-going edges and in this case gate N4 provides the answer. As soon as the data at the out- put Of FF2 changes, N4 inverts the phase. Thus, the negative-going edges are converted into positive-going ones, and vice-versa. That solves the problem, as the monoflop can now react to any type of edge. Only the very first time are things likely to go wrong, for instance after the power supply is initially switched on. By the end of two long pulse durations however the circuit should work correctly. The hesitant beginning had best taken into account by starting the program with a synchronisation byte! Calibration Monoflop 1 is provided with an ad- justment to enable the correct time value to be set. The monoflop period must last for not more than three quarters of the long pulse duration. With an oscilloscope this can easily be arranged, otherwise the auxiliary circuit in figure 5 must be used. This produces a logic 1 at the output within the range of PI. It only remains for PI to be adjusted to the point where the meter indicates a maximum level and the calibration procedure is complete. During calibration all the points 1 to 1, 2 to 2 and 3 to 3 should be connected to each other. In addition 4 and 5 should be linked. These points apply even if the calibration is carried out using an oscillo- scope. The signal will then not have to pass via the recorder but can be derived directly from the encoder section. Finally To avoid any misunderstandings, please note the following: The Manchester code is usually implemented for syn- chronous data transmission. This enables a whole data block of, say, 256 bytes to be transmitted. The circuit des- cribed here, however, is designed for asynchronous rather than synchronous data transmission. Slight differences (small percentages= between the re- cording and the playback speed are permitted, since during asynchronous transnission a break occurs to re- synchronise the UART after every byte. During synchronous transmission, on the other hand, a different decoder (one including PLL, for instance) has to be used to retrieve the clock signal and so prevent the differences in speed from becoming too noticeable. As an 'extra', the block diagram of the cassette interface connections to the author's microprocessor sys- tem is shown. In principle, the cir- cuit is suitable for any speed, both higher and lower than 4800 baud. Only Cl and C4 will have to be adapted accordingly. M 10-38 - elektor October 1981 RF-test generator At one time or another. Ham radio operators who built their own sets are going to need a generator for receiver alignment. A commercially available test transmitter would, of course, be ideal, but they tend to be rather expensive and rather over-sophisticated. In nine cases out of ten a much simpler device will do the job, provided it produces a reliable, stable test signal within the required frequency range. There is, however, one snag: an absol- utely stable generator with an output frequency that is continuously adjust- able is almost impossible to obtain. This measured frequency. The required fre- quency (here: 9005.000 kHz) must therefore be tuned precisely with coil LI . With the aid of the varicap diode D1 the oscillator can be frequency modu- lated. The useable modulation level (presettable with PI) is not particularly high, but high enough to test narrow band FM amateur and other special band receivers. SSB receivers can be 'whistled through' with the generator. In order to obtain intelligible modulation level for such receivers, the frequency modulation (FM) should merely be converted into a RF-test generator a 'mini test generator' for the 2 metre, 70 cm and 23 cm wave bands This straightforward little circuit will be an extremely useful tool for high frequency enthusiasts. It is a kind of 'harmonic generator' that can be modulated and which will produce test signals in 9 MHz steps up to the giga hertz range. It can be used both for FM and SSB receivers and is a fairly inexpensive circuit to build. plus the fact that we are looking for an economic alternative meant that another solution had to be found. A crystal generator was therefore designed that was capable of producing a wide frequency range without having to be tuned. The secret is for it not to be a 'clean' oscillator, but one with an output signal that contains many har- monics. Even though it includes an ordinary transistor, the oscillator pro- duces powerful harmonics of gigahertz proportions in addition to its 9 MHz fundamental frequency! This means that the test generator could also be used for reception and trans- mission on VHF and UHF. The gener- ator's third harmonics are on the 27 MHz wave band (CB), its 16th har- monics are at 144.08 MHz (2 metre wave band), its 48th harmonics are at 432.24 MHz (70 cm wave band) and its 144th harmonics are at 1296.72 MHz (on the 23 cm wave band). The circuit is also ideal for testing speech processors. j The circuit The remarkably simple circuit diagram is shown in figure 1. Around T1 there is a colpitt-like oscillator using a 27 MHz crystal. This does not make use of the third overtone of the crystal but rather the fundamental frequency, this being 9 MHz. This happens to be a very favourable frequency for our present purpose, since its harmonics extend over a range that is very practical for radio amateurs. When a crystal is used at the fundamen- tal there is always a considerable differ- ence between the theoretical and the phase modulation (PM). This can be done quite simply by connecting a small capacitor (Cl) in series with the modu- lation input — thus, SI can now switch between FM and SSB. In most test generators a separate at- tenuator is used to measure a receiver's behaviour at very low signal levels. In this particular case this was found to be superfluous since the oscillator con- tinued to be reliable even when barely operating. It is therefore quite a straight- forward matter to built an attenuator by making the emitter resistor belonging to T1 adjustable. Pots P2 and P3 have a fairly wide range: at a frequency of 144.08 MHz (2 m wave band) the maxi- mum output signal is around 1 mV and a minimum of around 30 nV (or 0.03 pV)! Construction Obviously, building the board (figure 2) is a simple matter. Even the coil LI should be no problem; just 22 turns of enamelled 0.2 mm copper wire wound around a Kaschke core, type K3/70/10. If readers happen to dislike this chore, an adjustable 4.7 pH inductor coil obtainable from Toko will also be suit- able. With the exception of the mains trans- former, the simple power supply shown in figure 1 is included on the printed circuit board. Since the circuit con- sumes very little current (and therefore the transformer can be quite small) the test generator and its power supply can be a highly compact instrument. When putting it into a case, be sure to provide a metal screen between the mains trans- former and coil LI , otherwise there will be a lot of hum — a type of modulation that is not always to be desired! M Parts list Resistors: R1.R2.R4 = 220 k R3 = 5k6 R5 = 220 n R6 = 68 n R7.R8 = 3k3 PI = 10 k linear P2 = 100 k linear P3 = 1 00 k preset Capacitors: Cl = 3n9 C2 = 560 n C3 = 1 20 p C4 = 68 p C5 = 1 n (cer.) C6 = 10 p/16 V (tantalum) C7,C8 = 100 m/35 V C9.C10 = 47 n Semiconductors: T1 = BC547B D1 = BB 105 D2 - LED IC1 = 78L12 B1 » B40L500 round version Miscellaneous: XI » 27.005 MHz crystal LI = 4.7 pH coil (see text) Trl =24 V/25 mA transformer 51 = SPOT switch 52 = DPDT switch Figure 2. The printed circuit board for the 'mini test generator' is very compact. 10-40 — elektor October 1981 wide range dark room timer Although dark room timers are widely available from photographic stores, there aren't many economical do-it- yourself alternatives. The nice thing about 'building your own' is that you can modify it to fit in with your own particular requirements. The circuit wide range dark room fimer electronic developments in the dark room Photography is probably the fastest growing hobby at the present time and together with all other aspects of life, electronics is involved. For the electronics/photography enthusiast the scope for combination of the two hobbies is very wide indeed. Those of our readers who are fortunate enough to own a dark room do not need reminding that electronics can almost control the whole process. Although many applications spring to mind, this is where the project featured in this article will really be at home. This dark room timer is fully automatic and its range is wide enough to cater for all but the most exceptional requirement of photography. As an added refinement, it even controls the safety light. featured here tries to do just that by offering a variety of possibilities. The range of 0.1 to 99 seconds with preset 0.1 second steps is switchable to a 1 second to 999 seconds range with preset steps of 1 second. In addition, two LEDs are used to indicate which of the start and stop buttons has been operated. Once an exposure process has been started it can easily be inter- rupted if necessary. Both the enlarger and the dark room lights are controlled by the circuit. On occasion, the enlarger has to be switched on independently 1 Figure 1 . The basis of the dark room timer is a counter chain. This provides a series of outputs from which a range of preset exposure times from 0.1 second to 999 seconds can be selected. wide range dark room timer elektor October 1981 — 10-41 from the timer and this too has been taken into account. The circuit IC4 (4566) produces the reference for the timer. The 1C incorporates divide- by-five and divide-by-ten counters together with a puls generator, so that a 50 Hz sine wave signal that is derived from the secondary of the transformer can be directly used as a clock signal. The clock signal reaches the clock input of IC4 (pin 15) via the filter R1/C4. The output signal of the divide-by-five is available at pin 14 and the frequency at this point is 10 Hz, which corresponds to a 0.1 s period. This signal is fed to the input of the divide-by-ten, pin 1, to provide an output signal at pin 6 with a frequency of only 1 Hz. The pulse duration will be exactly 1 second. According to the position of the switch S8 either 10 Hz or 1 Hz clock signals will reach the counter chain IC1 . . . IC3. At each positive edge of the clock pulse the count is incremented by one. When the start switch S6 is operated, a logic 1 is produced at the output of the flipflop N1/N2 (built with NOR gates). The differentiator network C6/ R10 converts the level change into the positive pulse that is used to reset the counter chain. This pulse, at the same time, sets the flipflop N3/N4. The logic 1 appearing at the Q output will now switch on T3 and activate the relay Rel. One of the relay contacts is used to switch the dark room lights on and off. In short, when the start button is pushed the dark room lights go out and the enlarger lamp goes on. The relay should be a 12 V/35 mA type although the circuit caters for a maximum of 100 mA passing through the relay coil. In the latter case the current rating of the transformer must be uprated to cater if a relay requiring more than 35 mA is used. The counters IC1 ... IC3 will start to count up from 0 upon the arrival of the start pulse. Once the preset time has been reached, a positive pulse reaches the reset input of the N3/N4 flipflop causing the output of N3 to become logic 0. Transistor T3 will now stop conducting — the enlarger lamp will go out and the dark room lights go on again. The exposure process will start again from scratch when the start button S6 is pressed. Together with the resistor R9, diodes D8, D10 and Dll form an AND gate, the inputs of which are formed by the cathodes of D8, D10 and Dll. The common anode junction acts as the out- put. If the preset time does not corre- spond to the output of the counter chain IC1 . . . IC3, the output of the AND gate will be logic 0. A high logic level will not reach the reset input of N3 via R9 and D10 until all the cath- odes of D8, D10 and D1 1 are connected to a positive potential, that is, when the count equals the preset time. When this happens of course, the exposure period ends. That covers the main principle of the dark room timer but the system also has a few additional features. Switch S5 allows the exposure to be interrupted at any given time. When operated, a positive pulse is fed to the reset input of the N3/N4 flipflop. Tran- sistor T3 then stops conducting and the relay switches the enlarger lamp off and the dark room lights on. Occasionally the photographer may need to extend the exposure time. Again, this has been taken into account by S7. When this switch is operated transistor T3 will continue to conduct regardless of the logic state at the out- put of gate N3 and the enlarger lamp can be neld on tor as long as required. The rotary switches S2 . . . S4 have knobs with transparent 'skirts' marked from 0 to 9. The LEDs D15...D17 are then mounted beneath the front panel and behind the skirt so that they illuminate the preset time period. This allows the controls to be used with ease in the poor light conditions that exist (hopefully) in your dark room. The numbers can be put on to plain skirts with letteraset if necessary. LEDs D13 and D14 indicate which button is ready to operate (START or STOP). As soon as an exposure pro- cedure has started, transistor T2 con- ducts and the STOP LED D14 lights. This means: the exposure can be inter- rupted with the STOP key. When this happens, or when an exposure has ended, transistor T2 will stop con- Figure 2. The front panel for the tinier can follow the suggested layout illustrated here although any layout will be suitable providing the start and stop buttons are placed in prominent positions. Figure 3. Except for the switches, the mains transformer and a few other components, the timer is contained on the single printed circuit board shown here. Parts list Resistors: R1,R10,R13» 100 k R2,R3,R4,R9 = 10 k R5.R7 = 47 k R6.R8 = 1 k R11 = 680 n R1 2 = 4k7 Capacitors: Cl ,C2,C5 = lOOn C3 = 470 p/35 V C4 = 33 n C6 = 1 0 n Semiconductors: D1 . . . D4,D7 = 1N4001 D5.D6.D8 . . . D12 = 1N4148 D13 . . . D17 = LED T1 . . ,T3= BC547 IC1 .. . IC3 “ 4017 IC4 = 4566 IC5 = 4001 IC6= 7812 Miscellaneous: SI = DP mains switch SI . . . S4 = 10 way wafer switch S5,S6 = Digitast switch 57 = SPST switch 58 = SPOT switch FI = 100 mA slow blow fuse Trl = 15 . . . 18 V/80 mA transformer Rel = 12 V/35 mA Siemens pcb relay ducting and LED D14 will go out. Now T1 will conduct and the START LED D13 will light. This shows that the timer is ready for another exposure. Digitast switches with built-in LEDs can be used for S5 and S6. Operation For the first timing range (up to 99 seconds) switch S8 remains in the 0.1 s position; for the 1 ... 999 seconds range this switch is set at 1 second. For example, a time interval of 9 seconds requires the following switch positions: S8 = 0,1 ; S2 = 0; S3 = 9 and S4 = 0. For an interval of 153 seconds the switches are positioned as follows: S8= 1; S2 = 1 ; S3 = 5 and S4 = 3. Construction The illustration in figure 2 is a suggested front panel layout. It must be remem- bered that the timer will almost always be in use during poor light conditions (a dark room should be dark after all) and it is therefore advisable to place the start and stop buttons in prominent positions. If a metal case is used it must of course be properly earthed. A plastic case, such as the Vero type 202-21 033A, will be far safer in view of the fact that liquids tend to be in abundance in dark rooms — liquid + 220 V + darkness do not add up to an ideal mixture. H teletext decoder elektor October 1981 — 10-43 Teletext is information that is provided in the form of picture pages on a TV screen which the viewer is free to select. The pages consist of text blocks con- taining 24 lines of 40 characters, which may be partly replaced by illustrative graphic information (such as weather charts, etc.). In principle, the entire information package could be trans- mitted 24 hours a day by broadcasting stations along with the daily programs. This is possible, because use is made of two lines during field-blanking interval, so that the usual program material is unaffected. Up to 800 pages can be broadcast during an ordinary program and these are distributed as 8 'news- teletext decode* . . . that does not require modifications to the TV set Teletext is no longer a new word especially to our readers. It is also well known that a TV set requires a decoder in order to make use of the service. However, all the decoders available for home construction suffer from one major disadvantage in that they require modifications to the TV set itself. It is understandable that this fact is never greeted with very much enthusiasm. The good news is that the Elektor Teletext decoder system does not require any modifications to the TV at all . . . unless you really want to fit it internally. As well as the decoder itself, an additional "black box" enables the Elektor Teletext decoder to be placed between the aerial and the TV set. A lot simpler than persuading the lady of the house that you really must "have the tele in pieces all over the carpet!" papers' of 100 pages each. Each page requires 0.24 seconds broadcasting time (12 rasters). Since all the pages are transmitted one after the other, the average delay will be about 12 seconds for every 100 pages, which therefore limits the number of pages. If on the other hand an entire TV channel was devoted exclusively to teletext broad- casts, about 30,000 pages could be transmitted simultaneously without any long delays. The computer plays a key role in this communication process. An editorial team collates the information and it is stored in the computer memory. When- ever a page needs to be updated, new data is entered and the computer en- sures that it appears on the correct page. In addition, it is the computer's job to convert the entered data into an accept- able form for broadcasting purposes. The stored pages are combined into an infinite sequence of pages, so that they can be broadcast continuously together with the TV programs. At the moment, up to 1 00 pages are being broadcast. How it all came about The expression 'great minds think alike' is very appropriate here, as the develop- ment Of a teletext system was started in various countries at the same time. The leading system was developed in Great Britain in the research labora- tories of the BBC and the IBA. The first practical results were realised in 1970. The two companies used a field-blanking system (see figure 1 ) to transmit inter- nal program information. The field- blanking (shown as a black rectangle at both the top and the bottom of the picture) contains 25 lines which exist outside the normal picture. A number of lines lead to excellent data trans- missions (see figure 1). The program identification/information systems used in 1970 enabled up to 15 characters to be broadcast per line, which is very little. New possibilities were created to- wards the beginning of 1972 when Teledata (BBC) was developed. This allowed for up to 32 characters to be transmitted per line. In September 1972 the IBA introduced the Oracle system. Around the same period the BBC changed the Teledata name to Ceefax (= See facts). The two systems are based on the same prin- ciples, but use different modulation methods and data transmission speeds. This means the two systems cannot be decoded with one and the same decoder. Obviously, a standard solution has to be found. BBC, IBA and BREMA (British Radio Equipment Manufacturers Association) and later the GPO sent representatives to set up a team which in September 1974 published a report with the dubi- ous title of 'Specification of Standards for Information Transmission by Digi- tally-coded Signals in the Field-blanking interval of 625-line Television Systems'. This describes a teletext system that combines the best characteristics of both Oracle and Ceefax. In addition, the new system enables graphics and colour to be used. In the same year, albeit on a small scale to start with, test broadcasts were carried out. These tests resulted in many new ideas and in September 1976 the definitive teletext specifications were established. The 'Broadcast Teletext Specification' gives a description, among other things, of more extensive graphic facilities and the decoder described in this article meets the requirements stipulated in that specification sheet. Decoding teletext is by no means easy. If the decoder were built up with ordi- nary TTL ICs, a whole bag of chips would be needed. Thus, Large Scale Integration (LSI) is the only method to produce a compact decoder that can be included inside a television set. Various manufacturers are currently offering package deals which allow a complete 10-44 — elektor October 1981 teletext decoder 1 82001-1 Figure 1 . The coded teletext data is transmitted during two row times belonging to the field -blanking interval. The diagram shows what happens on British television. 100%_ JStilTi . LEVEL 0 % Binary '0' level: 0(±2)% Binary T level: 66(*6)% Figure 2. A simplified version of a row filled with teletext data. The modulation levels in the figure correspond to a logic zero and logic one, respectively. 12-0 !%ps Figure 3. The decoder is synchronised to the teletext data signal during the clock run-in period. With the aid of the framing code the decoder detects the beginning of a data row. decoder to be constructed from a few RAMs and perhaps a single TTL chip. After careful consideration Elektor chose the four chip option provided by Mullard. Before going into detail about these four ICs, it might be as well to look at a few technical aspects involved in teletext. Signal formation As mentioned earlier, data is trans- mitted during the field-blanking inter- val. All the television standards in Europe are based on a rate of 50 frames per second. Each frame consists of 312 '/j lines (see figure 1). Two frames together constitute one complete 625- line picture. To suppress any frame fly- back takes 25 line periods. The flyback stroke in modern television is so short however that usually 6 or 7 lines are enough. The remaining lines in the field- blanking interval are therefore available for other purposes. As early as 1970 the BBC and the I BA were already making use of this facility to transmit program information and for test transmissions. About 15 lines can be used in this manner. At the moment however only two lines are being used for teletext transmission. The lines are not yet subjected to rules and regulations and, as far as we know, a different set of lines is being used in every country. The Netherlands uses lines 15, 16, 328 and 329, the U.K. lines 17, 18, 330 and 331, Belgium lines 19, 20, 332 and 333 and W. Germany lines 20, 21, 333 and 334. Which lines are used makes no differ- ence to the decoder, as, provided the information is situated inside the 'data entry window' (see figure 5), the data will be recognised and stored in mem- ory. Sometimes the teletext signal may appear at the top of the picture as an irritating flashing bar. This phenomenon is most likely to occur on German sets, since the teletext lines are very close to the picture. The reason for this inter- ference is occasionally due to the picture height being somewhat too restricted. This is easily remedied. In other cases it is due to the way i n which the CRT is built into the set. The two teletext lines contain digital information known as data. Here the logic 0 level corresponds to 0 ± 2% brightness modulation and logic 1 corresponds to 66 ± 6% (see figure 2). A whole row of teletext data is trans- mitted per line for 53 p seconds of the line period (64 p seconds). The data on a single line is made up of 45 bytes (1 byte = 8 bits). The first three bytes teletext decoder elektor October 1981 — 10-45 4 Eight Hamming Code* peculiar to Page Header i 1 . PMC NumCSl , . Time coat- Wmulei Time Code- Hours’ 1 ' ' 1 1 1 First character Figure 4. Every teletext row is preceded by a number of clock run-in periods, a framing code and a combination of the magazine and row address. In addition, the page header contains the page address, the time code and a number of control bits for the decoder. For increased reliability the data is transmitted with a Hamming code. are used to synchronise the decoder and to determine the data's starting point. This information is structured around two 'clock run-in' bytes for synchron- isation purposes and a 'framing code' byte to indicate the data's starting (joint. The remaining 42 bytes contain the magazine number and the row address (2 bytes) and the codes (40 bytes) for the line contents. Both the row address and the magazine number are trans- mitted on every row (see figure 4). This vital information for the teletext decoder is transferred with the aid of a special code to reduce to a minimum the likelihood of errors during recep- tion. Every bit in this Hamming (error detection) code is accompanied by a protection bit. To describe the Hamming code in detail would take too long, but, in a nutshell, what it does is add extra bits to facilitate a much more extensive parity check. This is so efficient that even mistakes that only appear once can be corrected, the multiple ones leading to a rejection of the character received. The row structure described above is relevant for rows 1 ... 23. These con- tain the actual page text. Every page is preceded by a page header which has row number 0. This row contains, in addition to the magazine and row num- ber, 8 additional information bytes in Hamming code. These bytes take the place of 8 text bytes, so that the page header may never exceed 32 characters. 5 Blank .ng V R G 8 Figure 5. The block diagram of the teletext decoder on its own. The decoder in this form is only suitable for fitting inside a TV set. 10-46 — elektor October 1981 teletext decoder vV *JLr Figure 6. By extending the block diagram in figure 5 with sections A and C shown here, a tele- text converter is obtained which can be installed without difficulty between the aerial and the TV set. To U C SAAS0O0 Figure 7. The block diagram of the Video Input Processor. The information bytes contain a reliable time code and eleven link bits to let the decoder know what kind of teletext page is being received. C6 for instance (see figure 4) serves to switch on the sub-title facility. The block diagrams All in all, the teletext signal has a rather complex structure, and is therefore not very easy to decode. Thanks to LSI technology, however, most of the necessary electronics is included in only four chips. Figure 5 shows the functions that each chip fulfils in the form of a block diagram. The page memory and the keyboard interface are the only other components which the decoder chips need in order to be able to operate properly. With just a few 'extra's', a decoder can be built into any TV with the aid of this block diagram. In most cases however this will result in quite an operation, how- ever, especially if the teletext pages are to be shown in colour. If the user restricts his requirements to a modest system and decides against colour with only essential switching on the key- board, the circuit can be built fairly easily into the television set. The sim- plest way to do this is indicated in the instructions provided later on in this article. Figure 6 shows the various connections in the form of a single block diagram. The simplest solution uses only block B and the keyboard. The video signal is then derived directly after the video detector in the TV set. After being decoded, the teletext signal (Y + sync.) is fed to the TV set at the same point. Obviously, the connection between the video detector and the video amplifier will then have to be broken. Block C can be connected in roughly the same way and provides a few more facilities. It will still be necessary to modify the TV set, but the teletext pages will be shown in colour of a reasonable quality. The above alternatives mean having to open up the TV set. Even though this is a relatively easy task as only one signal wire has to be cut, it is obviously best not to fiddle around with a television unless readers really know what they are doing! The point is, every type of television will require a different oper- ation so that it is impossible to give a standard recipe. Nonetheless, an ex- perienced hobbyist should not have too many problems provided the instructions given below are followed with care. A teletext converter can also be fitted into the set. This can be done by adding block A to the decoder. The aerial is connected to the teletext converter and the VHF output of the converter is linked to the aerial input of the TV set. The result is a highly acceptable teletext picture and saves having to 'operate' on the TV. LSI chips Although the block diagram gives a fairly simplified view of the teletext decoder, the latter will be seen to be far more complex in reality. In fact it is so complicated that a block diagram has to be provided for each chip to be able to clarify the circuit's function. The teletext decoder is split up into an analogue and a digital section. Before the digital teletext information can be 'gleaned' from the video signal, this signal has to undergo a number of ana- logue processes. This is taken care of by the video processor. SAA 5030 VIP (Very Important Processor) This Video Input Processor fulfils two main tasks: the video signal is split up into a data and a synchronisation sec- tion (see figure 7). The synchronisation divider produces a substitute synchron- isation signal for the TV set (only used if the decoder is included inside the set). At the same time this signal is used as a reference for a 6 MHz Phase Locked Loop. The VCO in this PLL is a 6 MHz crystal oscillator which can therefore only be mistuned by a small degree. This is necessary, since this oscillator must be able to produce a fairly stable signal even in the absence of the refer- ence signal. 8 D«w Date Entry Window From remote control SAAS0I0 Figure 8. The block diagram of the Teletext data Acquisition and Control chip. teletext decoder elektor October 1981 — 10-47 9 Tinting Signal* to TROW 82001 9 82001-10 Figure 9. A block diagram showing the interior of the Timing Chain. Figure 10. The Teletext Read Only Memory. 11 „ 12 1 — 1 — L. - | B1 1 1 1 1 i I i ‘-"Hi: i i i 1 :-Tj: i l i i 4-T- -[“i- 1 i :i“L; i i ! 1 -4-j— i »’ i i i S x 7 -dot matrix mmmam L-B — Si — i ■ mm Mi 10 lines ~sLj- ~dsl E E* Figure 11. The graphic characters are directly linked to the bits Figure 12. Character rounding improves teletext readability and belonging to the code words. The drawing shows the significance each makes it a lot better than other similar text display systems, bit has within a graphic sign. Interlacing is used to double the 5x7 matrix. A phase detector provides the oscillator with a control voltage and compares a frequency derived from the 6 MHz (15,625 Hz derived from the SAA5020 see figure 9) to the incoming synchron- isation signal. This has the effect that all the clock signals inside the decoder can be latched to the incoming video signal, so that the program and the teletext page may be transmitted simultaneously, one on top of the other. A signal quality detector determines whether the latch function really took place. If the video signal is poor or totally nonexistent, all the clock signals are derived from the (unlatched) 6 MHz signal. In this particular case the TV will be fed with the AHS signal (After Hours Sync, derived from the SAA5020) so that a (previously received) teletext page can be displayed. The data section in the SAA5030 serves to separate the teletext data from the video signal and also to generate a clock signal to process this data. The 'clock run-in' section of the teletext signal is used to synchronise a loop circuit to the data clock frequency of 6.9375 MHz = 444 x 15625 Hz). An internal delay fine ensures that the positive-going edge of every clock period occurs exactly in the middle of the received data bit. SAA 5041 TAC The 'brain' of the teletext decoder con- sists of the SAA 5041. This chip deals with the control and data processing tasks. TAC stands for 'Teletext data Acquisition and Control' (see figure 8). The teletext data is only received in a certain area of the field-blanking inter- val. It is only during this period that the SAA 5041 may take action to prevent the wrong data from being pro- cessed. This is done by the DEW signal (Data Entry Window) derived from the SAA 5020. Data received during this DEW is split up into the sections shown in figure 4. Depending on the nature of the data, either Hamming or parity checks will be carried out. Any charac- ters that do not have a suitable parity are written as a space in memory to prevent them from affecting the final teletext page. If the page is received again, the 'gaps' are filled with the correct characters, if at all possible. The row numbers are passed on to the row address latch as part of the page memory addressing. If the row number is zero (0) the page numbers are com- pared. If the page number corresponds to the number (DATA + DLIM) selected by way of the keyboard, the new information is stored in the page mem- ory. For this the serial data is first con- verted into 7 bit words. In addition to page numbers the key- board can give a few other commands which are decoded and carried out by the SAA 5041. Examples are: enlarged displays of half a page, a time indication at the top right-hand corner of the picture (during ordinary programs) and a mixture of program and teletext picture. SAA 5020 TIC The Timing Chain SAA 5020 constitutes the clock in the teletext decoder. The divider stages of the SAA 5020 produce all the control signals which are necess- ary to display a teletext picture. This includes the DEW and AHS signals which were described earlier. In ad- dition, the TIC provides several control signals for the character generator SAA 5051. Page memory addressing is also taken care of by the TIC. For this the 1C generates a 5 bit row address and a clock signal (RACK) for the external address counter which must address the 40 columns on a single row (see figure 5). The counter configuration consists of a divide-by-ten followed by a divide-by-24 and partly reveals the 10-48 — elektor October 1981 teletext decoder Bits mmmi °°o °°1 °i 0 \ 1q O D % 0 2 2a 3 j 3o 4 5 6 r 6o 7 ! 70 NUL® 0L£® □ □ ©!□ 0 0 0j □ 0 a Alpha" Red Graphics Red □ □ 0 D 0 0 0 ! □ 0 □ Alpha" Green Graphics Green □ □ 0 □ 1 0j □ □ □ Alpha" Yellow Graphics Yellow 0 n 0;B 0 0 0 n 0 □ Alpha" Blue Graphics Blue m h 0 E 0 0 0j H 0 B Alpha" Magenta Graphics Magenta E 0|E 1 0 @j E 0 L Alpha" Cyan Graphics Cyan h r 0 0 h a si B <2> Alpha White Graphics White □ b 0 n i 0 a n 0 1 C Flash Conceal Display □ h 1;H 1 0 hi a 0| a ~ <2> Steady Contiguous Graphics □ H 0!b 1 □ B □i 9 0 a © End Box Separated Graphics 0 0 □ H 0 0 □I a mj a Start Box © ESC a a □In 0 0 h a s a <2> Normal Height Black® Background □ B @ B □ E 0 B m\ H Double Height New Background □ S Hi B 0 0 0 B a: ■ so® Hold Graphics □ a Sjl M 0 h B h| a SI® Releas^ Graphics 0 H 0 1 0 i @1 H B ! ■ ©. These control characters are reserved for compatability with other data codes 2 ) These control characters are presumed before eoch row begins □ Character rectangle Black represents display colour White represents background Table 1 . The transmission code used largely corresponds to the ASCII code. The only difference is that the 'non-printable', or rather control, characters do not have the same significance here, as they are adapted especially for teletext purposes. ary, a more detailed explanation will be given in the section describing the use of the teletext decoder to be published in a forthcoming issue. Whenever a control code is used some- where in the middle of the page text, the corresponding place on the screen is filled with a space. Although this means that every transition from text to graphics and any change in colour will result in a space, this is not notice- able in practice. This decoder control method has the advantage that no additional memory space is required. The page memory of a teletext decoder can therefore be restricted to 1 k byte. In any case only seven bits of every byte are used. The 64 graphic characters shown in columns 2a, 3a, 6a and 7a in table 1 are derived directly from the seven bit code word. Figure 11 illus- trates how a graphic character is made up of various bits. Bit 6 is not used, because this is always logic 1 (see table 1 ). Characters are formed according to the well-known 5x7 matrix. Usually text is written on the screen in the non-inter- lace mode. This means that the two fields which make up the picture are written on top of each other instead of being interlaced (see figure 12). In the case of teletext, however, the interspace mode is used for improved readability. As figure 12 shows, the diagonally op- posed matrix dots in a character undergo a sharp transition which is detrimental to the character's legibility. The TROM detects such transitions and adds half a dot wherever necessary in the even raster. This doubles the matrix density, resulting in a 10 x 14 matrix. The final product is an easy-to-read, well-rounded character. Preview The complete decoder is drawn in fig- ure 13 and, as readers will see, is a rather complex project and impossible to describe in full detail here. The main circuit diagram is made up of two sec- tions, the actual decoder and the con- trol unit. Further details will be saved for a following issue. M page structure: 24 rows of 10 picture lines each. SAA 5051 TROM After the SAA 5041 TAC, this Teletext Read Only Memory chip is by far the most important 1C in the entire decoder. The TROM not only acts as a character generator but also fulfils a number of other significant tasks. As can be seen from figure 10, the chip decodes control characters, switches colour outputs, generates both alpha- numeric and graphic characters and converts parallel bit patterns received from the character ROM into a serial signal. Finally, it processes each charac- ter that is to be transmitted until the matrix format fades (characters rounding) and each character is clearly legible. The page memory controls nearly all these functions. All that the signals produced by the SAA 5020 TIC do is make sure everything happens at the correct time. The code in which the teletext information is transferred is practically a perfect copy of the ASCII code. The codes 'unprintable' (or illegal) characters are often used in a computer as control characters. This is the same case in the teletext decoder, only here the code words have a different meaning. Table 1 shows the relation- ship between the code and the charac- ter set + control codes. Wherever necess- teletext decoder elektor October 1981 — 10-49 1C 13 * FF1 + FF2 74LS74 market elektor October 1981 — 10-51 Flat-screen display units Two flat screen electro-luminescent display panels, one intended mainly for messages and the other for general graphics, have been launched in the U K. by Impectron Limited. The units, both manufactured by Sharp Corporation, are only 39 mm thick and of extremely lightweight construction. Each unit inforporates all necessary decoding and drive circuitry, and both make use of the well established electro-luminescent principles in which light is emitted when an luminescent ayer is excided by an applied electric field. The Message Display Unit, model S-1050, pro- vides a screen area of 1 86 x 50 mm, contain- ing 65,536 pixels (picture elements). It is con- structed using 51 2 lines of vertical transparent electrodes on a glass substrate, upon which a layer of luminescent material is sandwiched oetween two insulating layers. On top of these layers is a stratum of 128 lines of hori- zontal electrodes. When an appropriate drive voltage is applied to one vertical and one horizontal electrode, one pixel at the 'crossing point' emits a bright orange-yellow spot of ight measuring approximately 1 00 *im square. The Graphics Display Unit, model S-1021A, operates on exactly the same principles, but has 320 lines of vertical electrodes with 240 lines of horizontal electrodes — providing a total screen of 76,800 pixels. Both t/pes of display incorporate logic and driver circuits which may be controlled from externally applied signals, and both types may be used to display moving or stationary graphic patterns, symbols or characters as squired. Four input signal lines are required, e. data signals, data transfer clock, horizon- tal synchro signal and vertical synchro signal — as well as power supply lines. The desired display position of any image is specified by selecting the appropriate vertical and hori- zontal electrodes in an X-Y matrix. Because each pixel is generated at a fixed point, the -nage is sharp, stable and without either dis- tortion or glare. The orange-yellow colouring and uniform distribution of luminous inten- sity also combine to minimise eye strain. Each unit is offered with two control-board options, for simplified interfacing with micro- computer systems. The S-1026S Unit Inter- face Board is designed for character and coded graphic displays, while the S-1026F board is used for full graphics display appli- cations. A separate power source unit (desig- nated S-1040) is also available. Impectron Limited, Foundry Lane, Horsham, West Sussex RH 13 5PX. Telephone: 0403.501 1 1 (2111 M) Low-cost speech module The Tinytalker is claimed as the simplest introduction yet to low-cost speech synthesis. Just announced by Texas Instruments, it is a self-contained unit which speaks any of eight phrases at the touch of a button. Add a 9 V supply and loudspeaker, says Tl, and it is fully operational. Priced at £ 39.50, it can be interfaced with microprocessor control for such applications as warning systems and video games. Tinytalker uses the TMS5100 voice synthesis processor, linked with three TTL parts and the TMS2532 EPROM to make it a stand- alone module. A single EPROM holds eight phrases, each of which — through the high data compression of Tl's Linear Predictive Coding technique — can be more than four seconds long. The module's phrase selection can be simply increased by the addition of TMS 2532s (or TMS2516s for shorter phrases). If microprocessor control of the module is required, the addition of a TTL part buffers the Tinytalker for interface to any parallel output port. An evaluation EPROM and full instructions are included with the Tinytalker. Other phrases can be built from Tl's extensive speech 'library' or — for production system requirements — can be custom-manufactured. The Tinytalker and speech library are avail- able through all Tl's franchised distributors. Texas Instruments Limited, Man ton Lane, Bedford, HK41 7PA. Telephone: 0234.67466 (2106 M) World's first 64K bipolar PROM Harris Semiconductor has introduced the world's first 64K bipolar PROM (Pro- grammable Read Only Memory). The HM-76641 is a monolithic device, utilising the same basic process technology as the Harris HM-761 61 16K PROM. The 64K PROM is in an 8K word by 8 bit/word format, currently available in a 24-pin DIP and guaranteed over the commercial temperature and voltage ranges. The new device has a TAA (Address Access Time) of 85 NSec maximum. The HM-76641 offers a four-fold increase in memory size over currently available 16K PROM's in the same 24-pin package and with approximately one fourth of the power dissipation per bit. The device is programmed with the Harris generic PROM programming specification. Therefore, presently available commercial PROM programming equipment will program the HM-76641. Harris Semiconductor, Harris Systems Limited, 145 F am ham Road, Slough, Berkshire. Telephone: 0753 34666 (2104 M) 10-52 - elektor October 1981 market Light weight wire wrapping tool OK's EW-7D electrically-powered wire wrapping gun has been designed for pro- duction use with 22 - 32 AWG (0.8 - 0.2 mm) wire, having a rugged motor designed for extended periods of use with long life and low maintenance. The tool's body is made of a tough reinforced plastic, for lightness. Versions are available incorporating a backforce device which prevents over wrapping, which could occur with inexperienced operators, and a reversible model can also be supplied for both wrapping and un wrapping. A wide range of wire- wrapping bits and sleeves, to handle the various wires, complements the EW-7D. OK Machine & Tool (UK) Ltd.. Dutton Lane. Eastleigh, Hants S05 4AA. Telephone: 0703 610944 (2069 M) New multi-LED arrays Recently introduced by ZAERIX Electronics Ltd. a new range of multi-LED arrays provide a choice of 2. 3, 4 or 5 segment LED lamp units, all housed in black bezels. Being fully end stackable and thereby enabling multiple arrays of any number of segments to be assembled, the housings are designed to be push-fit into a correctly dimensioned panel cut-out. Available in any combination, the diffused lens range is red, green and yellow, behind which are housed red (GaP), green (GaP) and yellow (Ga AsP/GaP) LED's, giving up to 1 .5 MCD <* 20 mA luminous intensity. Capable of operating over a temperature range Of -40°C to +80°C, typical power dissipation figures are 120 mW and wave- lengths at peak emission are 695, 565 and 585 nm for red, green and yellow respect- ively. Zaerix Electronics Limited, 46 Westbourne Grove, London W2 5SF, Telephone: 01 .221 .3642 (1997 M) The Heat Beaters! A recent addition to the thermostat range from Cetronic Dynamics, the dual-purpose Titherm thermo switch will provide protec- tion against over current or dramatic heat rise in ambient temperature. If required, both these functions can be combined, using the same thermo switch, by choosing the correct rating Two types are available - 5 amperes in a glass sealed bulb and 9 amperes in a metal case, both measuring no more than 35 mm in length Typical applications include trans- formers, chokes, heaters, motors, battery chargers, soldering machines or any type of electrical coil. Cetronic Limited, Hoddesdon Road, Stanstead Abbotts, Ware. Herts SG 12 8EJ, England. Telephone: 0920 871077 (2074 Ml Selective level meter The first instrument to be manufactured in the UK by W & G Instruments Ltd. is a selec- tive level meter, designed and manufactured at the Greenford-based company's Plymouth Factory, recently acquired in their takeover of Hatfield Instruments. Designated SPM-30, this selective level meter is compact and accurate with a wide fre- quency range of 200 Hz to 1.6 MHz. The instrument is also capable of wideband level measurement and has been specifically de- signed for the surveillance and maintenance of modern FDM systems. Features include semi-automatic calibration and a synthesised local oscillator which greatly increase the accuracy and stability of frequency control enabling 1 Hz resolution in tuning. Frequency setting is shown on a seven segment liquid crystal display. The meter is equally suitable for measurements in the audio frequency range and on balanced or unbalanced FDM systems up to 300 chan- nels. Another feature of the SPM-30 is the fast signal detector. While tuning through a range of frequencies a signal may appear so momen- tarily that the meter is unable to respond. However, if the signal is higher in level than a certain defined threshold then the signal detector LED will illuminate for half a second. Location of discrete frequency signals is therefore very much simplified. The measurement bandwidth is switchable between 24 Hz and 1.7 kHz. Wideband is selected for rough or end-to end measure- ments, to speed level measurements, and to enable noise measurements on voice channels, effective noise bandwidth being equal to that of the CCITT psophometrically weighted filter. The SPM-30 can be used in conjunction with the built-in Tracking Generator PSE-30 which supplies both a balanced output signal in the range 200 Hz to 620 kHz and an unbalanced output signal from 200 Hz to 1.62 MHz. Levels are continuously adjustabled from +10 dB down to -60 dB. The combination of SPM-30 and PSE-30 forms a complete level measuring set-up for making end to end measurements on FDM systems. The instruments are housed in a compact die-cast metal case fitted with a convenient carrying handle, and operates from AC mains or internally fitted re-chargeable batteries. OOn behalf of: W8i G Instruments L td.. Progress House, 142 Greenford Road, Greenford , Middlesex. Telephone: 01-575 3020 (2103 M) UK 14 — elektor October 1981 advertisement EIEKTOR BOOK SERVICE sc/mputer w ^^fhusic syrppeiser sc/mputer ® \ buid your own microprocessor system circuits I I I JUNIOR COMPUTER BOOK 1 - for anyone wishing to become familiar with (micro)computers, this book gives the opportunity to build and program a personal computer at a very reasonable cost. Price — UK £4.25 Overseas £4.50 JUNIOR COMPUTER BOOK 2 — follows in a logical continuation of Bookl, and contains a detailed app- raisal of the software. Three major programming tools, the monitor, an assembler and an editor, are dis- cussed together with practical proposals for input output and peripherals. Price — UK £4.75 Overseas £5.00 300 CIRCUITS for the home constructor - 300 projects ranging from the basic to the very sophisticated. Price - UK £3.75 Overseas £4.00 DIGIBOOK - provides a simple step-by-step introduction to the basic theory and application of digital electronics and gives clear explanations of the fundamentals of digital circuitry, backed up by experiments designed to reinforce this newly acquired knowledge. Supplied with an experimenter's PCB. Price - UK £5.00 Overseas £5.25 FORMANT - complete constructional details of the Elektor Formant Synthesiser - comes with a FREE cassette of sounds that the Formant is capable of producing together with advice on how to achieve them. Price — UK £4.75 Overseas £5.00 SC/MPUTER (1) — describes how to build and operate your own microprocessor system - the first book of a series - further books will show how the system may be extended to meet various requirements. Price - UK £3.95 Overseas £4.20 SC/MPUTER (2) — the second book in the series. An updated version of the monitor program (Elbug II) is introduced together with a number of expansion possibilities. By adding the Elekterminal to the system described in Book 1 the microcomputer becomes even more versatile. Price - UK £4.25 Overseas £4.50 BOOK 75 - a selection of some of the most interesting and popular construction projects that were originally published in Elektor issues 1 to 8. Price - UK £3.75 Overseas £4.00 When ordering please use the Elektor Readers' Order Card in this issue (the above prices include p. & p.) %&*** ,»n* 'lftV '' 3 \ \\° v „ (\\ ' ’Sfe^*" \t> r- \ ***%»**? \?5“:S : ' 'i# • fe&r V*«* ftvlW • •M VtuW S '>" e \* vC *& I IS*: JZZ 4809 * • I dMo '>d*n) ft ■»' .1 For personal service visit one of our stores. Our new store at Hammersmith is conveniently situated near the end of the M4 and the North and South Circular Roads. There is excellent street parking on meters a few steps away and Hammersmith Underground Station is nearby. Call in and see us soon. > in our I CATALOGUE 320 big pages packed with data and pictures of fc over 5.500 items < Over 100.000 copies sold already! Don't miss out on your copy. On sale now in all branches WH Smith &t price £1. In case of difficulty check the coupon below. < 5 S i make it easy... •* w ^ 1% With impure •ft . • Easy to build, • • w superb specification. Comparable with organs selling for UP to £1,000. Send for our new book giving full construction details, order as XH55K price £ 2.50 inclusive. mnpun ELECTRONIC SUPPLIES LTD. All mail to: P.0. Box 3, Rayleigh, Essex SS6 8LR. =x Same day service on in stock lines * Very large percentage of our stock lines in stock x= All prices include VAT * Large range of all the most useful components * First class reply paid envelope with every order * Quality components-no rejects— no re marks =* Competitive prices ■f Your money is safe with a reputable company On price, service, stock, quality and security it makes sense now more than ever to make IrtilrBptlirii your first choice for components every time! Post this coupon now. E 1 08 1 Please send me a copy of your 320 page catalogue. I enclose £1 25 (mcl 25p p&pi. If l am not completely satisfied I may return the catalogue to you and have my money refunded. If you live outside the U K. send £1 68 or 12 International Reply Coupons. Name Address L... ........... Tel: Southend (0702) 554155 Sales: (0702) 552911