darkroom timer remote control audio compact LCD thermometer AIR-MAIL COPY low -power digital thermometer E. Schmidt The thermometer described in this article can be operated continuously for more than six months without changing the battery. That is quite an achievement and already makes our last thermometer circuit appear a little obsolete! MC/MM phono preamp The phono preamp is an important part of the Prelude, or indeed of any audio system. Most really good cartridges are the moving coil type, and these require a step-up transformer or, as is more common nowadays, a so-called pre-preamp. The phono preamp and the moving coil pre-preamp described in this article are designed to form a single module. This can be incorporated in any audio system, although it is intended as part of the Prelude. technical answers National filter with application fault; marking of digitast switches; slave flashgun powered by the mains; Crescendo coils and S/N ratio. membrane switches The membrane switch — or foil switch as it is becoming popularly known — is almost too good to be true. The switch is both reliable and inexpens- ive. These two factors coupled with its 'science fiction’ appearance can provide the basis for a very elegant and economical keyboard that the more conventional switch cannot hope to match. interlude The convenience of remote-control for television sets is fairly well-known. The Interlude brings the same armchair operation to the world of hi-fi audio: volume, balance, tone, even input select — all controlled from the 'ideal listening position'. Gontenila mm i- &*. . The front panel for the 7-day timer/controller includes a membrane-switch keypad and red trans lucent windows for the LEDs and display. Anyone who happens to notice something out of the ordinary in the time display is referred to the top right- hand corner of this month's 7-day timer/controller The last time we published a single-chip programmable timer was way back in May 1979 and the circuit became very popular (and still is). However, the 1C used at the time is now a little 'thin on the ground' and it's about time the subject was brought up to date. The circuit here uses the TMS1601 from Texas Instruments, a single-chip microcomputer specifically designed for this purpose. Prelude (Part 3) The end is in sight! We have now reached the final constructional article in the Prelude series: the printed circuit board for the tone control. programmable darkroom timer Many amateur photographers will be interested in this circuit: a program- mable timer for seven preselectable times. The time can be adjusted in two ranges (up to 99.9 s and up to 999 s). Fourteen keys are used for program- ming. When the start key is pressed, the time countdown can be observed on the display. talking-clock extension 4 * advertisers index 4-80 4-82 4-03 £39.99 DM2350/C MINIATURE AUTORANGING D.M.M. £52.00 Super compact autoranging digital multmeter featuring continuity buzzer and large (10mm) clear 3'/>-digit display. Full fuse overload protection. Carrying case, leads, battery, spare fuse and instruction manual supplied. — d.c. Amps:- 0 - 200mA ± 1 .2%; 20A with ar. Volts:- 2 - 20 - 200 - 600V ± 1.0% shunt (included) Impedance 10M d.c. Volts:- 0.2 - 2 - 20 - 200 - 1000V Resistance:- 200 ohms - 2k - 20k - 200k ± 0.8% B.K. ELECTRONICS UNIT 5 .COMET WAY. SOUTHEND ESSEX . SS2 6TR . INCLUSIVE OF POST & PACKING AND VAT. advertisement elektorapril 1983 METERTECH DIGITAL MULTIMETER MODEL DMM 3T £49.95 BOTH UNITS FEATURE 13 mm (0.5") LCD DISPLAY SIZE 180 x 90 x 35 mm BATTERY LOW INDICATION FUSE CIRCUIT PROTECTED OVER RANGE INDICATION POWER SUPPLY 9 V DC BATTERY BATTERY LIFE 200 HOURS WEIGHT 300 g. INC. BATTERY LOW POWER CONSUMPTION PUSH BUTTON CONTROLS ACCESSORIES INCLUDED TEST LEADS BATTERY INSTRUCTION MANUAL. METERTECH DIGITAL CAPACITANCE METER MODEL DCM MT301 DMM 3T lOMft INPUT IMPEDANCE AUTO ZERO ADJUSTMENT AUTO POLARITY SWITCHING hFE MEASUREMENT FACILITY DIODE/CONTINUITY CHECK SAMPLING TIME 0.25 SEC. DC VOLTAGE (5 RANGES) 1 00 pV to 1 000 V AC VOLTAGE (2 RANGES) 100 mV to 1000 V DC CURRENT (5 RANGES) 100 nA to 10 A RESISTANCE (4 RANGES) 1 £2 to 2 Mft OPTIONAL ACCESSORIES PROTECTIVE CASE £6.95 10 mA-10 A AC CURRENT SHUNT £6.95 THERMOCOUPLE INTERFACE £39.95 (—50 to 1 100°C x 1°C 1% ACCURACY) EXPOSED JUNCTION THERMOCOUPLE £3.95 DCM MT 301 FRONT PANEL ZERO ADJUSTMENT CRYSTAL TIME BASE PROTECTED FROM CHARGED CAPACITORS SAMPLING TIME 0.5 SEC. CROCODILE CLIPS INCLUDED 8 RANGES 0.1 pF-2000pF 05% ACCURACY RANGE RESOLUTION TEST FREQUENCY 200 pF 0.1 pF 800 Hz 2 nF 1 pF 800 Hz 20 nF 10 pF 800 Hz 200 nF 100 pF 800 Hz 2pF 1000 pF 800 Hz 20 pF 0.01 pF 80 Hz 200pF 0.1 pF 8 Hz 2000 pF 1/iF 8 Hz OPTIONAL ACCESSORIES PROTECTIVE CASE £6.95 4-13 lektor april 1983 advertisement eiejotor eiekrtor Back numbers of Elektor currently available are detailed above, with a brief description of their contents. Send for your copies now, using the pre-paid Order Card inside the back cover of this issue. Prices are as follows: any one issue (except July/August) £1.20 additional issues, each £ 1 .00 July/August (Summer Circuits) £ 2.40 Prices include postage and packing. Overseas orders requiring airmail postage add £ 1.50 per issue (£ 2.00 for July/August 4-14 advertisement slektor april 1983 Volume purchase from Acorn brings massive savings for you! Cash in on our misfortune! Over £50 off an Atom Microcomputer We recently made a bulk purchase of over 800 Acorn Atoms for sale overseas. The deal fell through! We are now offering those Atoms to you at the price we paid for them. The Atom normally retails at £174.50 inc. VAT we are offering it to you at a mail order price of only £ 1 1 5 inc. VAT - an incredible saving of £59.50 plus a free power supply and software worth over £20. The Computer The Atom has 2K of RAM and 8K of ROM but of course this can be boosted enormously. The computer has a full sized keyboard laid out in a conventional way. To use it just connect the power supply and a cable into the aerial socket of a TV set. As well as integral sound output and direct cassette and TV interface, a wide range of additional interface boards are available to fit inside the casing. Extra 64K RAM. Colour. Printer. Laboratory, Cassette, 6522, 80x25 VDU, Analogue, Econet etc etc allowing the user to build a very sophisticated application machine. Full details of all accessories, disc pack, software etc are supplied with each machine. The language used by the ATOM is BASIC, the language used by most personal computers. The Atom’s version is very fast, making it ideal for real time applications. It has all the normal functions you would expect plus many powerful extensions making it very easy for you to operate and write your own programs. The Atom is fully guaranteed. There are 80 nationwide authorised service centres. Just clip the coupon below or ring 01-930 1612 with your credit card number. Computer Marketplace Ltd, 20 Orange Street, London WC2H 7ED The potential of the bucket brigade Charge-coupled devices, or CCDs as they are usually called, have evolved through semiconductor technology. Electronics engineers and technicians often think of them as 'bucket- brigade' elements in a circuit, from the way they work. But CCDs are making a remarkable contribution to such highly sophisticated fields as astronomy and particle physics. Because they are semiconductor devices, CCDs are robust, consume little power, have a long life and are compact. The unit of length used in describing them is the micrometre, and the extremely small dimensions must be remembered when looking at explanatory diagrams which have to be drawn large. Discussions range round charges of, say, only a thousand or so electrons, whereas a normal n-type doped silicon has some 1 0 12 electrons per cubic millimetre. With CCDs we are in a minute world of microscopic size. Indeed, devices can have hundreds of thousands of unit cells on a chip hardly more than a square centimetre Furthermore, the manufacturing technology is simpler than that for integrated circuits, because it involves very few successive stages. It is based largely on well-known techniques used in making field- effect transistors. The basic, simplified CCD is outlined in figure 1 , where (a) represents two CCD units, A and B. There is a charge of electrons in A. If the voltage on A is reduced to nothing and that on B is raised, the electrons travel through the n-type channel •towards B. In this way the charge is 'coupled' or transferred between A and B. Part (b) is a somewhat more realistic side-elevation, showing the silicon p-type substrate (which has a deficit of electrons) on the top of which is a very thin n-type layer. Over this is an insulation layer of silicon dioxide. Above them all there are the conducting electrodes, which overlap but are insulated from one another. Depending on the purpose of the device, the thickness of the substrate may be from 35 to 300 pm and the n-type layer may be only a few micrometres thick. The first, essential point to be understood is that the doped layers are depleted for use in CCDs. That is to say, they are deprived of mobile carriers, which are electrons in the n-type and 'holes', or electron deficits, in the p-type. This is done by applying a reverse bias, or Figure 1. Elementary idee of two charge elements coupled together via the n-type channel and controlled by voltages at A and B in sequence (see text). opposing potential, of about 20 V. It ensures that the n-type layer has plenty of space for electrons, often described as a conducting layer or channel. The depleted p-type layer is left negatively charged to a small depth, keeping electrons up into the channel. The arrangement shown is not of any practical use, for alternation of the voltages on A and B would merely keep the charges going backwards and forwards between them. However, a linear arrangement of a number of elements could be useful if we can have a way of getting the voltages on the electrodes in the correct sequence. This is done by what is called a three-phase system. If we assume a charge of electrons under A (figure 2) and that the potential sPi is reduced to zero while i 02 is given a voltage, the.', 'he charge moves from A to B, also being zero. Next, ^ is reduced to zero and ipj given a voltage, and the charge moves to C. Next, y> 3 goes to zero and y>i is given a voltage. The charge moves to D. So the correct sequence of voltages in time makes the charge move along, eventually reaching J, where it goes to an amplifier and so forth. Obviously, there has to be a clocked sequence of voltages in the right order. It is obvious, too, that there is never a condition where there are two or three charges contiguous. The array so far considered is a linear one. It can be used in analogue mode, with each charge differing, or digital mode with individual charges representing 1 or 0. It can operate as a shift register, a serial-access store in computers, a delay line in telecommunications and television circuits and even, suitably modified, as a band-pass filter. It has been used for aerial surveillance and in other practical gear. But what has put the CCD into the frontier of several fields of scientific research and development in the last few years is the idea of making it into a two-dimensional array, an area rather than a line. The reason lies in the fact that charges can be created in several ways. In the linear device, the first charge usually comes from preceding circuits and at exit goes into another circuit. However, when light enters silicon it creates charges by photoelectric effect, and the spectral range of sensitivity is quite wide. Also, if a high-energy particle enters an element of a CCD array it creates a charge by ionization. For these kinds of charge- creation the electrodes have to be transparent, which is arranged by making them of very highly doped silicon or of polycrystalline silicon instead of metal; they conduct well enough. If we imagine the linear array of figure 2 to be up-ended, with A at the top, and turned to show the electrode faces, and then add similar arrays side by side, we get the idea of an area array (except that, of course, the whole area is formed on one silicon chip). In addition, in order to restrain the electrons from moving sideways (that is, in the direction of the conventional x-axis in our imagined picture), so-called channel stops are formed from top to bottom after every three elements. Each stop is an extremely shallow channel and consists of very heavily doped p-type silicon; when depleted, it becomes a strong, negatively- etektor april charged barrier. A picture-element, or pixel as it is known, then consists of an area covering three CCD elements horizontally and vertically. Obviously, the charges in each pixel cannot be seen. They are transferred, one row at a time, (in a so-called frame-transfer technique, which is not the only way) to a linear shift register with its own three-phase clock-pulse supply, so that each pixel charge is transferred individually into circuitry that includes micro- processors: the Cartesian co-ordinates and the size of the charge in each pixel are stored and transferred to a television screen or visual display unit (VDU), on which we can see the picture of how the original light or ionization is disposed, just as if the area CCD array were a photo- graphic plate. The advantage of such a CCD image sensor for astronomy has been described by one scientist for the UK Science and Engineering Research Council in quite unscientific terms: he says the sensitivity is "staggering". One sensor having 576 x 385 pixels in a surface of about 1 5 mm x 9 mm is in use at the Royal Observatory, Edinburgh. A CCD sensor has been commissioned for the Anglo- Austral ian telescope and yet another for the South African Astronomical Observatory. Stars and galaxies extremely difficult or impossible to photograph even with hours of exposure can now be recorded in minutes. The need in high-energy physics is quite different. As everyone now knows, many particles have been identified and given strange names, with properties such as 'beauty' and 'charm'. Most of them have lifetimes expressible only with the help of large negative indices, for example 10'“ seconds, so measuring them and their interactions has become more and more exacting. To do the job, highly-sophisticated pieces of apparatus such as the bubble chamber have been devised. Now, with the high sensitivity and resolution of CCD image sensors, together with the easy processing of the results by computers, physicists have yet another refined tool to use. One experiment with four sensors electrons, i CCD elements horizontally and is shown in figure 4. An energetic particle hits a target, T. At one collision it causes the emission of particles with tracks P! and P 2 . Then, perhaps a couple of millimetres further on, it has another collision that gives rise to particle tracks Q 1 and Q2 . Ordinarily it would be impossible to differentiate between the Ps and Qs but with a CCD technique it should be possible to measure what happens with a precision of a few micrometres. C.L. Boltz, Spectrum 182. 4-21 E. Schmidt power^^ digital thermometer A large number of digital thermometers are available at present. Elektor has also kept up with the trend and published a standard digital thermometer with liquid crystal display in the October 1982 issue. Why is it that only a few months later we are coming out with a successor to the circuit already published? The thermometer described in this article can be operated continuously for more than six months without changing the battery. That is quite an achievement and already makes our last thermometer circuit appear a little obsolete! continuous operation for more than 6 months In spite of the wide choice of devices avail- able, digital thermometers have not yet become fully accepted. The advantages are: the temperature can be clearly read off at a distance; the response time is very short and the temperature sensor can be installed at some distance from the unit. In contrast to the usual mercury and 'pointer' thermometers, however, the elec- tronic version requires its own power source. Of course, it is also much more expensive. It seems, therefore, that digital thermometers will only become more popular when the problem involving price and energy con- sumption has been at least partially solved. Special ICs are needed for this type of I circuit, but the investment is worthwhile. The circuit In principle, this thermometer consists of two main circuit components (see figure 1). These are the temperature sensor IC2, whose output voltage is proportional to the tem- perature, and the analogue-to-digital con- verter IC1, which converts this voltage value to the corresponding binary number and also has the task of driving the liquid crystal display. The analogue-to-digital converter ICL7136 (IC1) is a chip with an extremely low current consumption, approximately 50 jtA, which operates according to the dual-slope principle. With a reference voltage of 100 mV which must be applied to it 4-22 between REFlo and REF HI. its measuring range is then ± 199.9 mV. Thus the converter directly indicates with proper polarity the voltage applied to its inputs INlo and INhi. The sign is positive if the potential applied to INhi is higher than that at INlo; in the inverse case, a appears on the display. The frequency of the internal oscillator is adjusted with external components Cl and Rl. At the specified values, a clock frequency of approximately 16 kHz is obtained; this means that the ICL7136 executes approximately one con- version per second. Polycarbonate or poly- ester capacitors should be chosen for Cl, C3 and C4 to keep the measuring error of the A/D converter less than 0.1°C (0.1°C corre- sponds to 1 LSB). The backplane signal must be inverted to generate the decimal point. This is performed with transistor T1 and resistor R6. An alternative is to utilise a BC 549C with a 4M7 base resistor. The temperature sensor ICL8073 (IC2) low-power digital ther- operates with a supply voltage of approxi- n ’ om * ,er mainly consists mately 5V which is presented to it by ° "° “ the A/D converter at the +Ub and TEST operates as a temperature terminals. The 1 00 mV reference voltage for sensor, ici evaluates the the A/D converter is presented by the sensor voltages of IC2 and drives between Uref and - Ur. The 1 mV/K volt- the LCD. The operating age which is proportional to the temperature current drawn by the (with respect to -Ub) is generated at entire circuit does not UpTAT- This signal is applied to the exceed 1 50 «A. measuring input of the A/D converter via a lowpass network consisting of R3 and C5. At 0°C (corresponding to 273 K), therefore, an output voltage of 273 mV is present. As already mentioned, the difference voltage between INhi and INlo is indicated by the A/D converter. Thus a voltage of 273 mV must be presented to the INlo input in order for the LCD to display the value ‘0’ at 0°C. This is achieved by dividing down the reference voltage Ubg (approximately 1.23 V) via potential divider R4, PI, R5 and by adjusting preset potentiometer PI to obtain exactly 273 mV at INlO- The tem- perature sensor ICL 8073 draws a current of approximately 50 /jA. Energy consumption A typical current consumption of 120fiA can be expected for the entire circuit. To calculate the life of a 9 V battery, the data sheet of the VARTA 4022 alkali-manganese battery was taken as a basis; this includes the typical discharge characteristics under continuous load using resistances of 100 £2 ... 5 k£2. These curves are shown in figure 2. The entire thermometer circuit operates quite happily with supply voltages of 12 V to approximately 6.2 V. Taking 6.2 V as the lower limit of battery voltage and an average load resistance of 75 k£2, the group of curves can be extrapolated to obtain an operating time of approximately 5400 hours. This corresponds to continuous operation for more than 7 months. It is quite possible, however, that longer or shorter operating times will be achieved because the calculations relate to typical data for the discharge characteristic and current consumption of the temperature sensor and of the A/D converter. In general though, the typical data are met by approxi- mately 50% of the components. Precision Linearity of the thermometer depends almost exclusively on the temperature sensor, because the specifications of the ICL7136 A/D converter are so good that its error contribution can be discounted. The ICL8073 temperature sensor can be pro- cured in different classes of precision and specified temperature ranges. Its transfer characteristic has the form shown in figure 3a. The thermometer can be aligned at any temperature with preset potentiometer PI. The slope of the characteristic is needed to be able to calculate the measuring error in the region of this alignment point. In this case it is practical to replace the curve by a straight line as an approximation, as shown in figure 3a. After the 25°C alignment the ideal characteristic is shifted as shown in 3a b figure 3b. If one calculates the error of the lowcost ICL8073 JIUT sensor, whose speci- fied maximum linearity error is ±L5°C over the temperature range 0 . . . 70°C, the resultant maximum error is ±0.2 C over a range of ±5°C around the alignment point. The highgrade ICL8073 KIUT sensor would have a maximum error of ±0.026 C under the same conditions. A slight, additional measuring error is caused by the tolerance of the reference voltage presented to the A/D Components are mounted layers. The capacitors and 4-24 ... &T"lo ii *fo oLo 1 otfio i o\\ fo si < Tjlj* I'sS D “■ 5 J \ ■ a l l t c a_£ Lo.oJ l L • J Resistors (1/8 W): R1,R3,R6 = 560 k R2 = 180 k R4 = 82 k R5 - 22 k PI = 10 k preset (multiturn) converter by the sensor; its value should be exactly 100 mV. However, this error is mainly noticeable at the limits of the measuring range and is negligible in ambient temperature measurements. Construction and alignment Construction of the digital thermometer using the printed circuit board of figure 4 requires some care. The thermometer is constructed in three levels. Fit all passive components first. The capacitors within the socket for IC1 (IC contact strips) must be bent over to prevent them from protruding beyond the contacts. Incidentally, these capacitors must be fully encapsulated in plastic to prevent short-circuits. The contacts used for the liquid crystal display can be of the same type as those for IC1. It is better, however, to use the more solid, plastic-base type. The temperature sensor IC2 can be directly inserted into the p.c.b. or solder pins can be inserted in its place, to allow its connecting leads to be soldered on later. IC1 forms the second level of the ther- mometer module. The liquid crystal display is mounted on it in piggyback fashion. Care should be taken to produce a strong mechan- ical design, i.e. so that IC1 and the display are firmly seated in their sockets. Since the thermometer is particularly suitable for measuring room temperature, on account of the sensor used, the 'comparison method' is utilised for simple alignment. In the Elektor laboratory a standard alcohol thermometer was used as a reference. PI is adjusted to obtain its value on the LCD. An occasional comparison between the two readings indicates whether the low-power digital thermometer is accurate. It may be necessary to readjust PI. Incidentally, there is no on/off switch in our circuit. With a current consumption of 150 /rA such a switch seemed an unnecessary luxury! ^ IC1 - ICL7136 (Intersil) IC2 ■ ICL 8073 JIUT (Intersil) 9 V compact battery wi terminals and leads 314 -digit liquid crystal display: Hitachi LS007CC, H 1331C-C LXD 43D5R03 Hamlin 3901.3902 Norsem NDP 530-035A S-RF-P1 4-25 The phono preamp is an important part of the Prelude, or indeed of any audio system. Gramophone records are still the recording medium that offers the highest quality, provided a good cartridge and preamp are used. Most of the really good cartridges are the moving coil type, and these require a step-up transformer or, as is more common nowadays, a so-called pre-preamp. The phono preamp and the moving coil pre-preamp described in this article are designed to form a single module. This can be incorporated in any audio system, although it is intended as part of the Prelude. MC/MM phono preamp 100 mV in, 100 mV out A phono preamp must do two jobs. It must boost the output from a moving magnet (or ‘dynamic’) cartridge to a sufficient level, and it must modify the frequency charac- teristic in a precisely-defined way. Figure 1 shows the theoretical recording charac- teristic as a set of bold straight lines, running from lower left to upper right; the thin line running through these is the actual charac- teristic. To achieve a flat overall response, the inverse characteristic must be applied during playback; this is the thin line running from upper left to lower right in figure 1. Achieving this type of characteristic may seem quite a feat, but in fact it is mainly a question of ensuring that a set of normalised RC time constants are included in the preamp. For once, the Americans agree with us: their RIAA response corresponds to the European IEC specification. Reading from left to right in figure 1, the first time con- stant is 3180 /js: this causes the response to fall at 6 dB/octave from about 50 Hz. Then we come to 318 ps - this tends to flatten out the response above 500 Hz - almost immediately followed by 75 ps, which leads to a further 6 dB/octave slope above 2120 Hz. Knowing what time constants are required is one thing; knowing where to insert them in the circuit is another. There are two basic possibilities: you can either use passive RC networks, or you can include them in a feedback loop to obtain active filters. Obviously, since there are three time con- stants to be included, you can also include one or two as passive networks and the rest as an active network. Some of the possi- 4-26 preamp 1 MC/MM phono elektorapril 1983 bilities are illustrated in figure 2. A passive filter can be mounted at the preamp input, as shown in figure 2a. This has the disadvantage that the signal level is drastically reduced at high frequencies (-30 dB or so!) before it reaches the input stage. This is asking for a poor signal-to-noise So, try it the other way: mount the RC net- works at the output, as shown in figure 2b. Now we run into a new problem: the input signal is at such a level that it will not only mask the noise, but there is a good chance that it will drive the preamp into distortion! The most common approach is shown in figure 2c. The RC networks are included in the feedback loop. Provided the circuit is properly designed, this can give quite good results.* However, the system shown in figure 2d is even better: the two lower time constants are included in the feedback loop, so that there is little risk of over- loading the preamp. The third RC network, however, is included as a passive filter at the output. This means that the higher frequencies are passed through the preamp at relatively higher levels, thus improving the overall signal-to-noise ratio. As an added bonus, this system makes it slightly easier to design a good preamp: rolling off the frequency response of an amplifier towards higher frequencies tends to lead to insta- bility! Reading between the lines in the last para- graph, it will be obvious that the circuit described here corresponds to figure 2d. C reproduction (upper left Figure 2. The official time The moving-magnet preamp The right-hand section of figure 3 should by now be familiar: basically, it is the ‘discrete opamp’ that is used throughout the Prelude. The circuit was discussed at length in the articles on the headphone amplifier and the line amplifier, so there is little point in repeating the whole story here. In a nutshell: the input stage is a differential amplifier (Tl, T2) with a current source in the commoned emitter line (T3). The collector output currents are combined by constants can be included in the preamp in several ways. Purely passive networks can be used, either preceding (2a) or following (2b) the pre- amp; or the networks can be included in the feed- back loop (2c). The solution shown in figure 2d (two time constants in the feedback loop and best of both worlds. 4-27 Figure 5. The p.c. board for the moving magnet preemp end that for the cinch input sockets. These a current source as collector load impedance gain of 50 set by the ratio of R8//R9 to R7. (T8) to the class-A output stage (T9, T10). The feedback loop (C5, R7 . . . RIO and C3) includes the first two time constants, as From 500 Hz down to 50 Hz, the response rises at 6 dB/octave; below 50 Hz, the gain is again constant (500, set by the ratio of 4-30 moving magnet inputs. These are shown at the left in figure 3. There are actually three sockets for each of these inputs: one for the signal input, and two for impedance matching (R x and C x ). This is explained in detail in a separate article ('RC equalizer'), elsewhere R8//R9 + RIO to R7). The third time con- stant, causing the response to roll off above 2120 Hz, is the passive output network (R19, C6 and CIO). There are three inputs, selected by means of SI: the moving coil pre-amp and two 4-31 MC/MM phono preamp elektorapril 1983 in this issue. Note that the basic input impedance of the preamp is 107 k (R1//R2), so that four 82 k resistors must be inserted in the R x positions to obtain the ‘standard’ 47 k input impedance. The moving-coil preamp Moving coil cartridges give a beautifully ‘clean’ signal, but at a very low level (100 . . . 500 pV). This means that a ‘pre- preamp’ must be included between the cartridge and the ‘normal' moving magnet preamp. The pre-preamp is the left-hand section in figure 3. It is a fairly simple-looking circuit, but it is designed for extremely high per- formance - in particular, the signal-to-noise ratio must be exceptional in this application. A fully-complementaiy class-A design is used. T1 . . . T4 give high gain, and T5 and T6 are the output drivers. The gain is set by R14 and R13 in the feedback loop; very low values are used to obtain an extremely low input noise figure. The gain of this pre- preamp is 20; overall, from MC input to phono preamp output, this means that the gain is 1000. In other words, 100 pV in will give 100 mV out. The DC setting is determined by R2, R3, R6 and R7 ; the current through the input devices is determined by R4 and R5. This means that anyone who feels like experimenting with different input transistors can easily set the optimum collector current by mod- ifying these two resistors. The positive and negative supplies are derived from the main +15 and -15 V rails. Integrated 12 V regulators are included, mainly to ensure that no hum, noise, inter- ference spikes or whatever can possibly reach the pre-preamp. The input impedance is approximately 100 fi: suitable for practically all moving coil cartridges. If an even lower impedance is desired, the value of R1 (and R1 ’) should be reduced accordingly. Construction Although the moving-magnet preamp and the moving-coil pre-preamp are both com- plete units that can be used separately, we will only discuss the construction of the complete phono input module - using both, in other words. Even if the pre-preamp is not (yet) needed, it is best to use both boards to obtain a reliable electrical and mechan- ical construction. No components need be mounted on the second board in that case. As can be seen in figures 4 and 5, both boards consist of two sections. Before doing anything else, these must be separated. The small piece from the moving-magnet board is intended for cinch-type input sockets; the piece that is cut off the other board is used to mount the input selector switch. As usual, good quality components should be used; R7 . . . R10, C5 and C6 should be 5% types or better. When all four boards are complete, the one with the input sockets is mounted at right-angles at one end of the moving-coil board (on the component side near the electrolytics) with the connections MCL, 1, MCR and 1 mating correctly. These four connections run from the track side of the cinch board to the component side of the moving-coil board. The next step is to mount a set of connec- tion wires. The four sets of three cinch plugs must be interconnected, if this was not done earlier, and a longish lead is soldered to the 1 connection on the free edge of the cinch board (on the track side). Four wires (4 or 5 cm long) are soldered to connections MM1L, MM1R, MM2L and MM2R on the selector switch board, again leading back from the copper track side. Two shorter sections (2 cm or so) are connected to MCL and MCR on one long edge of this board, and another pair to points MML and MMR on the opposite edge. Having done all this groundwork, it is a good idea to compare all the bits with the photo of our prototype. When the MCL and MCR leads from the selector switch board are mated to the corresponding points on the moving-coil board, the four long leads from the switch board should reach easily to the cinch connectors. Note that the indications near the leads correspond to the four sockets that they must connect to. These leads can now be shortened to length and soldered to the sockets. Next, the moving-magnet board can be mounted (component side facing in); this involves two leads from the selector switch board and one from the input sockets. Finally, five interconnections are made be- tween the two preamp boards - note that they all run straight across. The completed module can now be mounted on the Prelude bus board. Unless you are very lucky, the switch shaft will have to be extended. If the rest of the modules are also in position, including the tone control unit described this month, you can proceed to the first test. With either of the MM inputs selected, no noise should be audible; for the MC input, a faint hiss may be audible with the volume control turned fully up. Not to worry: you’ll have to turn the volume down again before putting on a record! Input impedance matching To get the best out of a dynamic (moving magnet) cartridge, the input impedance of the preamp must be properly matched. This is dealt with in greater detail elsewhere in this issue, but a few points are worth noting. The input impedance of this preamp is approximately 107 k, parallel to 25 pF. As a first approximation, as mentioned earlier, four 82 k resistors can be mounted in the Rx sockets. This brings the impedance down to 47 k. In practice, most cartridges tend to give best results when loaded with approxi- mately 300 . . . 500 pF; bearing in mind that the cable capacitance can be anything from 50 pF up to a few hundred pF, it is worth experimenting with several values for C x (from zero up to about 470 p), to see which gives the best results. So much for the analogue section of the Prelude. All that remains for the next few issues are some final comments, practical tips, and the remote control unit. M upper pan. If this inclined surface is marked (character height 2.1 mm) the lettering is not subjected to wear. In order to mark the fronts of digitast switches that have already been soldered in, there are two possibilities: the switch must either be unsoldered or the moving lower soldered pan of the digitast switch. I Thesecond possibility can be implemented quite simply by pressing the upper half of | the switch first backwards and then gently upwards. The upper pan then jumps out. To insen it again, only a slight pressure from above is required to engage the upper well-p rover i trigger- Circuits iss n transforr order and Crescendo coils and S/N ratio National filter with application fault We are grateful to an observant reader fo pointing out that the 'Applikator' in thi September 1982 issue, concerning th Slave flashgun powered by the mains Marking of digitast switches The membrane switch - or foil switch as it is becoming popularly known — is almost too good to be true. The switch is both reliable and inexpensive. These two factors coupled with its 'science fiction' appearance can provide the basis for a very elegant and economical keyboard that the more conventional switch cannot hope te match. membrane switches the super-switch of tomorrow? If Asimov had been a hardware designer instead of a science fiction writer we would have had the membrane switch about twenty years ago! They really do have the aura of space-age technology. The most striking point about them is their appearance, they are, quite literally, wafer thin. This fact, paradoxically, is a disadvantage from the psychological point of view, they just don’t look as if they can possibly be very reliable - but they are! From the aesthetic point of view, a key- board consisting of membrane switches has no equal. They can be any colour you like (including black), are dust proof, rust proof and would probably suffer no harm if they went through the washing machine a couple of times! How is this list remarkable of advantages achieved? The illustration in figure 1 will help to answer many of the questions. The ■exploded diagram’ shows the total absence of 'working parts’. No springs, rockers, gold plated sliding contacts, in fact, not even a terminal! Each switch consists of four layers of plastic foil that are common to all the switches in the keyboard. The top layer of plastic is the actual key- board panel, the part that is touched when the switch is operated. This can be coloured by a special printing process and can take any design or pattern that is desired. Further, the keyboard itself can be any shape that comes to mind, including, in some cases, bending round comers. We would like to see the toggle switch that does that! The switch contacts are carried on two layers of flexible plastic and these are separated by a third layer of polythene foil. This ‘spacer’ has holes or cutouts in it that coincide with the switch contact areas in the upper and lower foils. When the front panel, or ‘keyboard’, is pressed, the upper contact layer us locally distorted allowing the two contact areas to ‘make’. The contact areas are made from silver, graphite or a mixture of both. The insulating layer or spacer is only a few thousandths of an inch thick and therefore operating pressure is very low. This creates the misconception that the membrane switch is a true ‘touch’ switch, that is, a capacitively coupled solid state switch. As we have seen, this is not the case. The complete keyboard is connected to the outside world via a flexible connecting strip. This normally pushes into an edge connector on a printed circuit board but it can also be soldered directly onto a board if needed. However, soldering such a thin (plastic!) strip is fraught with much difficulty and is not encouraged. Advantages The manufacture of a keyboard consisting of membrane switches is very easy. It is also economical due to that fact that a number of keyboards can be printed on a single sheet and then cut out. The operation of a switch is simple, a soft touch is all that is required. If necessary a 4-34 1 light can be fitted behind the switch to show that it has been operated as anybody who has ordered a ‘Big Mac’ can recall. The complete keyboard is entirely water and dust proof. Corrosion is also not a problem as very few chemical substances can attack the plastic foil used. Mounting the keyboard is simplicity itself - it is usally backed with a self stick material and the whole keyboard is just placed on a sheet of aluminium and that is that! As mentioned previously, shape can take many forms for any number of applications. This keyboard has no fear of that horror of horrors - the office coffee. Good key- boards have been known to die instantly from a liberal dose of this but the foil keyboard carries on quite happily. A good scrub down with a mixture of nitric acid and liquid methane will obviously be required at a later date to remove the stuff but operation is unimpaired. Cost always rears its ugly head when any- thing good comes along but in this case there is a happy ending. The complete keyboard can be produced at a fraction of the cost of conventional keyboards. What about long term reliability? With no moving parts what can go wrong? A persist- ently strong finger may make some im- pression after a hundred years or so but it is unlikely! So we now have what appears to be the super switch of the century. Does it have no Achilles heel? Disadvantages If one is ill-advised enough to attempt to switch 10 amp loads with our switch it reveals a marked tendency to get somewhat ‘soggy’ - in a very few microseconds! (It also creates severe bluntness of the fingers.) In all fairness, the restriction to 100 mA loads can not really be classed as a disadvan- tage as that is more than enough to carry out the purpose for which it was intended. Whether high current versions will eventually be produced is yet to be seen. Switch configuration looms large on the disadvantage side of the fence. We have a choice - press to make - or nothing! Not even a single pole changeover. Here again, for their alloted task in life, that of a key- board switch, this is sufficient. Where to use membrane switches As we have seen, the membrane switch will not make the conventional switch in its many varieties instantly redundant. It is also not really economical where just one switch is required. However, when keyboards are considered it leaves the competition gasping for breath, especially when cost is con- sidered. The Elektor 7-day multi-time switch that is published in this issue makes use of a membrane keypad. It is available from the Elektor ESS service. H Technical details operating pressure: contact separation: load current: contact resistance: switching time: service life: approximately 1 mm 1 - 2N approximately 0.2 mm 100 mA at 30 V < ioon operational temperature: -30° C to +65°C 4-35 The convenience of remote-control for television sets is fairly well-known. The Interlude brings the same armchair operation to the world of hi-fi audio: volume, balance, tone, even input select - all controlled from the 'ideal listening position'. Although this unit is intended as a plug-in module for the Prelude, it is almost a complete preamplifier in its own right. You only need a little ingenuity, a few hard wired potentiometers and a power supply! interlude a remote- control preamplifier To provide a preamplifier with a remote- control facility, the first step is to ensure that all its controls can be operated by means of a DC voltage. The input selector switch, for example, could be replaced by a box of relays or by so-called analogue switches. However, relays are expensive and require heavy-duty drive circuits; con- ventional analogue-switch ICs are not bad, but they are not really good enough for a top-notch audio system. The analogue controls (volume, balance and tone) are even more difficult. An ideal solution would be to use some kind of motor-drive for the potentiometers: just think of the gimmick-value of a knob that is rotated by a ghostly hand! Yes, and just think of the (mechanical) problems. Another alternative would be to use OTAs (Operational Transconductance Amplifiers) as electronic potentiometers. Unfortunately, their performance tends to be below par. To cut a long story short: any type of remote-control system in an audio installation will have some disadvantage. Either it will be mechanically clumsy, or it will be ex- pensive, or both; and if it is not too ex- pensive, the quality will almost certainly be less than that offered by a more conven- tional preamplifier design. The solution chosen for the Prelude gets the best of both worlds, in a surprising way: use two preamplifiers! The Prelude itself is a top- quality design, using conventional controls. When switched from ‘manual’ to ‘remote’, its control section is cut out of circuit and replaced by a remote-controllable preamp: the Interlude. This circuit configuration made the name ‘Interlude’ an obvious choice. Similarly, the infra-red transmitter and receiver (to be described in forthcoming issues) are called the ‘Maestro’ and ‘Conductor’, for an obvious reason: how else would you call someone who controls a musical performance from a distance? Not ‘recording engineer', ‘tonmeister’ or ‘producer’, surely! 4-36 Second best? As mentioned above, activating the Interlude means that the quality must suffer. This is the price of convenience. However, the overall specifications are not at all bad, as shown in table 1. The Interlude may not be XLent, but it certainly is hi-fi. For just two ICs, as shown in figure 1, that’s not bad. These two, the National Semiconductor LM 1037 and LM 1035, are in a friendly environment of course: the input selector, IC1, only needs to select inputs of adequate level (approximately lOOmVRMS)- These are provided by the tuner, tape deck or whatever, or by the existing phono preamps in the Prelude. The control amplifier, IC2, needs a fairly high signal level (1 Vrms or so) and this is provided by the existing line amplifier. Typical technical data Distortion factor (at 1 kHz. 1 V rms output): < 0,15% Frequency range (+0. -1 dB): 20 Hz ... 20 kHz Signal-to-noise ratio tuner, aux, tape: >75dB MM1.MM2: > 65 dB MC: > 55 dB Tone controls: bass (40 Hz): t 15dB treble (16 kHz): ± 15 dB Cross-talk (20 Hz ... 20 kHz) : >40d8 Volume control, range: 80dB Balance control, attenuation of 1 channel: +1 dB . . . —26 dB Figure 1. The circuit of the Interlude merely consists of two ICs. The circuit IC1 is the equivalent of a double pole switch with four positions. Depending on the logic 4-37 interlude Figure 2. Cable instead of infrared. This figure shows testing the Interlude. It small preamplifier can be switch, four potentio- printed circuit board! Figure 3. Terminals +B (IB VI, A... O', E end E\ F and F', G and G' and 1 are wired so that can be 'plugged' into the bus board of the Prelude. levels at control inputs D1 . . . D4, the cor- responding signal inputs A . . . D (A’ . . . D’) are switched to output E (E'). In order to select an input, a voltage of 2.5 V ... 50 V must be applied to the corresponding con- trol input. At a control voltage of less than 1.0 V the corresponding input is inhibited. Resistors R1 . . . R4’ provide the bias voltage for the IC inputs and also determine the input impedance (in conjunction with the potentiometers on the connecting board). Outputs E and E' are low-impedance. The gain of IC1 is 0 dB (unity gain). IC2 contains six electronic potentiometers; volume, treble and bass require one poten- tiometer each, times two for stereo. Balance control is achieved by varying the volume potentiometer. The latter also has the widest adjustment range: more than 80 dB. The treble and bass controls give symmetrical cut and boost: ± 15 dB at 16 kHz and 40 Hz, respectively. The input signal for IC2 comes from the 2 i line amplifier of the Prelude, which delivers approximately 2 Vnns- This signal (at points F and F’) is reduced to the optimum input level of 1 Vnns f° r the IC by means of R17/R18 and R177R18'. The control voltage for the potentiometers at terminals H ... M can vary between 0 V and the voltage at point TP (5.4 V). A load of up to 5 mA can be connected to point TP (potentiometers, for example). The IC offers an additional feature which is not exploited in the Prelude: a loudness facility. In order to make use of this optional function, pin 7 must be connected to pin 12 instead of pin 17. Construction The printed circuit board is mounted on the bus board in much the same way as all the other boards in the Prelude. The connec- ting wires are fitted to points +B (15 V), A . . . D', E and E’, F and F', G and G' and i. As usual, the short lengths or wire (about 2 cms) are soldered in place and then bent parallel to the board. This very economical 'edge connector’ is fitted and soldered to the bus board in the position shown in figure 3 on page 2-20 of the February 1983 issue. The printed circuit board must be fit with the component side facing to the left when looking at the front of the Prelude. Sockets for the remote control connections can be mounted on the rear panel of the case just behind the printed circuit board. Either one ten pole socket can be used or, perhaps more economically, two five pole sockets. These latter can be ordinary DIN sockets but, to avoid confusion, they should be of different configurations (for instance, one 180° and the other 270°). The Prelude is now ready for remote control. The circuits for the infrared transmitter and receiver will be published next month. Meanwhile, the circuit shown in figure 2 can be used for testing. K R1 . . . R4,R1’ . . . R4’ = 100k R5 . . . R8,R17,R17’ = 10 k R9...R12 = 47k R13 . . . R16,R25,R25’ = 100 k R18.R18’ = 18 k R19 . . . R22, R19' . . . R22’ = 1 M R23.R23’ = 330 k R24.R24' = 4k7 Capacitors: Cl . . .C4.C1’ . . .C4’, C12 . . . CIS = 220 n C5 - 100 p/10 V C6,C7,C7’,C9,C9’,C18, C18’ ■ 10p/16 V C8.C8' - 470 n CIO- lOOn Cl 1 -47 p/10 V C16.C16'- 10 n C17.C17' - 390 n Semiconductors: IC1 - LM 1037 (National Semiconductor) IC2 - LM 1035 (National Semiconductor) frequency response equalisation for moving magnet cartridges RC equalizer Moving-magnet cartridges must be terminated with the proper impedance in order to achieve optimum sound. This article attempts to clarify the electro-acoustic factors involved and presents a simple and inexpensive method of obtaining a great improvement in sound response. Anyone who has purchased a record player or amplifier after extensive listening tests in a hi-fi studio will have noticed two things: 1. Each record player and each amplifier sounds different. 2. The new piece of equipment sounds different at home than in the shop. One should not be tempted into resignation in the face of these idiosyncracies of audio- electronics. These differences in sound have a perfectly rational explanation and can be measured. The only question is how. Cause and effect The causes of the differences in sound can be found in the construction of a moving magnet system, and are explained by the equivalent circuit of this type of pick-up in figure 1 . The stylus is connected to a small permanent magnet. Coils are arranged in its magnetic field and the field variations caused by stylus movement are converted to a varying voltage by these coils. This type of coil has many turns. Since there is very little space in the cartridge, the wire used for the windings is very thin. In addition to the coil inductance (L: 200 mH . . . 1 H), this leads to a considerable internal resistance (Rj: 200 Si . . . 1000 Si) and capacitance (Cj: coil and cable, up to 100 pF). Cartridges are designed to exhibit a flat fre- quency response when terminated with a particular impedance. In conjunction with the terminating impedance, the influence of L, Rj and Cj is neutralised. The DIN standard specifies a terminating impedance of 47 k/400 p. Everything would therefore seem to be in order as long as Cl + C x + C p result in 400 pF, and R x and R p in parallel have a value of 47 kSi. 4-40 But things are seldom so clear-cut. Manu- facturers of cartridges and hi-fi equipment tend to interpret this standard as ‘merely a guide’. Cartridges are actually designed for terminating impedances of 33 kS2 . . . 100 k£2 and for terminating capacitances of 80 pF . . . . 1 nF; and readers who follow the tests in hi-fi magazines will have noticed that the in- put impedances of preamplifiers for magnetic pick-ups often deviate considerably from the values specified. Only so-called high- end equipment is provided with a means of selecting the input impedance. It is therefore no wonder that different performance is encountered when record players and amplifiers are combined in a system. Phono-equalizer Since we cannot change the cable capaci- tance Cl anymore than we can change the construction of the pick-up system, the only solution is to match the preamplifier. Within certain limits, this is possible without modifying the unit, using a so-called phono- equalizer. This accessory is a box which is inserted in the line to the pick-up (or ‘phono’) input of the amplifier. Various capacitors and resistors can be switched in parallel with the input by means of push- buttons or other switches. In view of the very low cost of the components used in this accessory, it is clear that one is mainly paying for the technically sounding name 'phono -equalizer’. Furthermore, there is no quarantee that this accessory will result in any improvement. According to the usual operating instruc- tions, a record with plenty of overtones should be played and different settings should be selected on the phono-equalizer, until the sound is ‘right’. In many cases, however, the result is merely a difference in sound and not a technical improvement. At any rate, it is not a solution worthy of the name ‘hi-fi’. RC equalizer A better solution is to use an RC equalizer. As shown in figure 2, this accessory for home-construction simply consists of a metal box with two input sockets, two out- put sockets and four extra sockets. The input and output sockets are intercon- nected; the box is therefore inserted in the line from the record player to the amplifier like a phono-equalizer. The extra sockets are intended to accept Cinch plugs con- taining a small capacitor or resistor. In this way we have the equivalent of a phono-equalizer, i.e. a facility to connect capacitors and resistors in parallel with the input of the amplifier, but at much lower cost and with more flexibility. The sockets must be insulated from the metal case; this is important to avoid hum. The easiest way to achieve this is to use plastic washers of a suitable diameter. The metal case then has an earth from the turn- table, which is usually separate on most record players, and another terminal for the earth conductor to the amplifier. Incidentally, an RC equalizer of this type is already provided in the latest Elektor pre- amplifier - the Prelude. Equalising We now have a box with which we can con- | nect capacitors and resistors as desired. So far so good. But what values do we need? We could consult the manufacturer's literature to establish what load impedance is required by the cartridge and what input impedance is provided by the preamplifier. Alterna- tively, we can use a test record. This should include a sinewave sweep from 50 Hz to 20 kHz or so. Measurements can be done by ear, with the aid of the test circuit of figure 3 and a con- ventional, analogue multimeter. With the RC equalizer in circuit, we can now set to work. 1. Set the tone controls to their midpoints or, even better, switch them off if poss- ible; cancel all filters (subsonic, loudness, etc.). 2. Play the record with the sweep, and adjust the volume record to obtain a readable deflection on the multimeter (test circuit connected to the loudspeaker output). 3. If the deflection remains constant at high frequencies (± 15% of the value for low frequencies is acceptable), the system is in order and no RC equalizer is needed. If, however, the deflection clearly changes as the frequency rises, continue with the actual alignment: starting with small values for C (10 pF . . . ) and high values for R (1 M . . . ), solder capacitors and resistors into the Cinch plugs. With the capacitor plugs and resistor plugs inserted in the auxiliary sockets, play the record with the sweep again and observe the deflection on the multimeter. This is repeated with differ- ent values of C and R until the flattest fre- quency response is obtained. It should be noted that C and R are mutually inter- active. If the frequency response can only be worsened with the RC equalizer, the input impedance of the preamplifier must be too low already. This means modifying the input circuit, but this is best not attempted without a circuit diagram. M Figure 2.TheRC i Since it only cons a box with six Cin sockets and four a plugs, it is considerably less expensive than a commercially available phono-equalizer. Figure 3. This test circuit allows even the highest frequencies to be measurei with accuracy. It is con- nected in parallel with the loudspeakers. 4-41 The last time we published a single-chip programmable timer was way back in May 1979 and the circuit became very popular (and still is). However, the 1C used at the time is now a little 'thin on the ground' and it's about time the subject was brought up to date. The circuit here uses the TMS 1601 from Texas Instruments, a single-chip microcomputer specifically designed for this purpose. It doesn't, as the subtitle may suggest, actually stop and start time itself (that article will be published in an earlier issue) but it will do almost anything else. Sophistication doesn't end there either. The front panel that is available from the EPS service also contains the keyboard, consisting of built-in membrane switches. This represents another first for Elektor and gives the finished project a very professional appearance. 7- day timer/controller O 6 ® ® ® <§> 10- 30 Rfc® ® ® ® 2 — 3 — 4— J » # ® ® 00000 ! 000©0 00000 ELEKTOR TMS 1601 a micro computer controlled time switch The TMS 1601 timer/controller IC from Texas Instruments forms the basis of this circuit and, as can be expected, carries out most of the work. So what does it actually do? Briefly, it is a single-chip pre-pro- grammed microprocessor dedicated to timer applications. It forms a 24 hour clock using seven-segment LED displays and provides four outputs that can be programmed for daily or weekly cycles (or both). It will also display the days of the week. With the aid of an external RAM (one of the few extra ICs) 28 different times can be pro- grammed for each output per week. Alterna- tively, four switching times for each output can be set to repeat daily. In total, this means enough switching times per week to take care of Coronation Street and Dallas and egg for breakfast every morning and still open and shut the garage door morning and night (weekdays only) without even struggling! Oh, and don’t forget the porch light! Contrary to what you might expect, pro- gramming this unit is a fairly simple matter. For example, say that the porch light needs to be switched on every Saturday evening for a certain period of time. Using the 4-42 7-day timer/controller Elektor April 1983 Missing link? For complete flexibility, it program a mixture of 'weekly' and 'daily' times in one switching sequence. We have been informed keyboard, key in the day of the week followed by the switch-on time and then the switch-off time. That's all there is to it! Alternatively, if the porch light needs to be switched on every evening, as it probably will, simply press the 'DAILY' key. All switching times that are stored in memory can be displayed whenever required. Each or all of them can be deleted or modified easily. There is also a ‘reset’ facility that may be of interest to people who have an application involving repeated 24 hour cycles, but more of that later. One other mode of operation exists that can be very useful. It is possible to switch any output manually at any time without affect- ing the program stored in the memory. This has the added advantage that the timer can also be used as a central control point for all the appliances that are connected to it. Appliances can be connected to the timer via relays or solid state switches. These in turn can be mounted in the appliance itself or in the same housing as the timer. This allows the possibility of driving equipment using only low voltage wiring, a useful asset in a number of situations. A final detail that is not the least in import- ance. The circuit has been equipped with emergency battery backup. This ensures that in the event of a mains failure the clock continues to run and the memory remains intact. The circuit diagram About a dozen or so years ago, the detailed description of the circuit diagram for this project would probably have required a smil book! Fortunately this is not the case today as a glance at the circuit dia- gram of figure 1 will show. The heart of the circuit is IC7, the TMS 1601. This IC is a dedicated timer/controller microprocessor and we would not get anywhere without it (in this circuit at least). It contains an internal clock oscillator, 4 K byte ROM, 512 bit RAM and the four-digit seven- segment display decoder and multiplexer among other bits and pieces. Quite a tally for one 1C! The keyed in information is stored in an external RAM, IC6. Three of the address lines for this memory are driven directly by IC7 while the others are controlled by a shift register, IC5. In its turn IC5 gets its information from output R9 of IC7, clocked in by a signal derived from output Rll (IC7). The output control relays (or solid state switches if preferred) are switched via buffers (N8 ... Nil) from the R12 . . . R15 outputs of IC7. An important point to note here, the quiescent current of the output devices must not exceed 80 mA. The LEDs D19 . . . D22 indicate the state of each output. The membrane keyboard is connected between outputs R0 . . . R9 and K1 and K2 of IC7. The key functions will be de- scribed later. The four seven-segment dis- plays are multiplexed by IC7. The digit select outputs are R0 . . . R3 and these are buffered by display drivers N 1 . . . N4 in IC9. Segment control is from outputs 00 ... 07 and here transistors T1 to T8 are the buffers. The remaining display outputs R4 . . . R9 take care of the rest of the LED indicators (16 in all). It will be as well to list what these are. LD1 to LD4 are of course the seven-segment displays. These are accompanied by: a decimal point (D44), the days of the week (D32 . . . D38), the memory input LEDs (D24 . . . D27), an LED for the ‘reset’ (D28) and 'period' keys (D29) and finally the LEDs for the ‘on’ and ‘off’ keys (D31 and D30 respectively). The clock signal for IC7 is devired from the 50 Hz mains frequency and taken from the secondary winding of the trans- former. The waveform at this point is used to synchronise the 7555 (IC4) which is connected as an astable multivibrator with a frequency of - 50 Hz! This appar- ent ‘over -engineering' not only gives us a good square wave for the clock signal but it also serves as a clock oscillator if the mains supply fails. The frequency setting components in this case are resistor R2 and capacitor C5. This leads the story on to the power supply (emergency and otherwise). Two voltage levels are required for the circuit, -5 V and -9 V. Both are achieved by the use of voltage regulator ICs. The -9 V is the province of IC2; the -5 V supply consists of two sections, the supply for the display (IC1) and the rest of the circuit (IC3). In the event of a mains failure (when panic sets in round the freezer!) the 7 NiCad cells take over - but not quite everything. The -9 V for IC4 and IC5 together with the -5 V for ICs 5 and 6 are maintained. This ensures that the clock continues to run and the memory remains intact. The display, with its relatively heavy current consumption, must go and so we lose that. The correct time is of course retained, it just means that it is no longer visible. Also we lose the drive to the relays, another heavy current consumer. This is not such a disadvantage however. How will the switched appliances operate during a mains failure? Unless of course someone invents a NiCad capable of opening a garage door by the time this gets into print! In the emergency situation the current requirement for the whole circuit does not exceed 50 mA and the NiCads can cope with this for quite a while. If the NiCads are to be replaced by non- rechargeable batteries R1 and D5 must be removed. These two components provide a ‘trickle charge’ for the NiCads. Front panel controls This section can be taken as a small com- mercial for our front panels which are self adhesive, scratch proof and washable. They also have that deep-down colour that will not fade if . . . but enough of that! On to the nitty-gritty! The layout of the front panel is illustrated in figure 2. The control functions (an up- market term for keys and LEDs) can be decribed as follows: 4-44 0000 ( 1 ] 00000 00000 1. h- 7 n L O I O The four digit time display. It is used to indicate the switching Q \ H r\0 I U times during programming. (This is also true for the ‘day’ * ‘ — ‘I * M LEDs.) The centre LED flashes once a second. SFE Z T X Ami to make • 350 W Amp . 2X81 Sound on your TV .