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STAGE YOUR 0 LIGHT SHOW These days when you go to hear your favourite band or disco there is always a top light show. Now you can have many of these exciting light show effects - with the Discolight. By JOHN CLARKE & LEO SIMPSON The Discolight is a compact and affordable unit which drives four channels of coloured lights to produce an exciting and highly varied light show. It can be driven directly from your stereo system's loudspeakers or it can be triggered from live music picked up by an inbuilt microphone. It can also produce all sorts of light patterns on its own, with or without modulation by the music. With the Discolight you don't need to go to the trouble and expense of hiring a disco system and disc jockey - you can put on your own light show and do it better. In fact, we reckon that as soon as disco operators find out about SILICON CHIP's Discolight, they'll all be building them. The Discolight has a great many 56 SILICON CHIP features which are summarised in an accompanying panel. The Discolighf turns sound into light. Feed music from your stereo, Walkman or whatever into its rear terminals and it then drives four sets of mains-powered coloured lamps. The brightness of the lamps is directly proportional to the loudness of the music signal. In fact, the Discolight divides the audio signal from your stereo into four frequency bands and then uses each of those frequency bands to modulate the brightness of the lamps. Most people will have seen such set-ups on TV shows, in discos and in hotels. The lights may be coloured 100W floodlamps or banks of smaller coloured lights. Typically, four differently coloured floodlamps or sets of lamps would be connected, one set to each channel. You might use red for the lowest frequency band, then yellow, green and blue for the highest band. Not only does the Discolight turn music into fascinating light patterns but it also generates its own light patterns, for when the music stops. If you're operating a disco show and the band stops for a rest break or the music has to stop for some reason, the Discolight generates its own light patterns: chaser, strobe and alternate patterns (we'll describe these later). Understanding how to use the Discolight may be best explained by briefly describing its various front panel controls and rear panel features. On the rear panel are four 3-pin mains sockets to which you connect your banks of coloured lights. There is also a set of four spring-loaded terminals so that you can connect the audio outputs from your stereo amplifier. These are connected in parallel with your speakers and cause negligible loading of your amplifier's outputs. On the front panel are two knobs, a fuseholder, large power switch and five miniature toggle switches, three of which are 3-position types. There are also four LEDs (light emitting diodes), one for each of the light channel outputs. Let's look at the function of these miniature toggle switches first. Right next to the SENSITIVITY knob is the SOURCE (microphone/ speaker) switch. This selects the audio from your stereo system (via the 4-way terminals on the back panel) or the sounds picked up by the Discolight's inbuilt electret microphone. If you have a live band, you can simply place the Discolight nearby and switch to Microphone mode, thus eliminating the need for cable connections. As you might expect, the adjacent SENSITMTY knob adjusts the audio signal levels for the best light switch. The speed at which the display. lights chase each other is set by the In the centre of the five toggles is RATE control knob. the DISPLAY switch. This 3-position In the Strobe mode, all four sets switch is the key to the Discolight's of lights flash on simultaneously, as functions. In its top position, set by the RATE control. In the Alter"4-Band Modulated", you get the nate mode, two pair of channels basic Discolight function whereby flash on and off alternately, at a the audio signal is split into four speed set by the RATE control. separate frequency bands (low The DIRECTION switch controls bass, upper bass, mid-treble and the Chaser mode. You can have the upper treble) and each of these bands control their respective lamps chase in one direction or the other or change direction autolights. The brightness of the lamps at matically, every minute or so. any instant is directly proportional Finally, the BEAT switch gives to the sound level in the respective beat triggering from the music for audio frequency band. the Chaser, Strobe and Alternate In the Modulate (MOD) position pattern modes. In the Oscillator setof the DISPLAY switch, the audio ting of the BEAT switch, these funcsignal both modulates the lights and tions are controlled by the RATE . triggers the various modes selected knob. by the PATTERN switch. Finally, the The four LEDs on the front panel Unmodulated setting of the DISPLAY mimic the behaviour of the four switch allows the light display to be light channels, so that even if you set by the pattern switch. can't see the lamps directly (say The PATTERN switch gives three you are acting as disco operator), light displays: 4-light chaser, strobe .you can tell what they are d9ing by and alternate. The Chaser mode is looking at the LEDs. The LEDs will self explanatory; the four lights also come in handy during any chase each other in one direction or troubleshooting which may have to the other, as set by the DIRECTION be done. DISPLAY SELECTOR CHANNEL 1 CHANNEL 2 CHANNEL 3 CHANNEL 4 RAMP REFERENCE SIGNAL (C) MOOULATEO 1 INPUT MAINS (\ INPUTV 2 3 4 MAINS PAmRN GENERATOR PHASE ZERO DETECTOR t----1CROSSING INPUT Fig.1: the audio signal is split into four frequency bands, rectified and compared with a 100Hz ramp reference signal. The Triacs are then triggered either by the comparator outputs or by signals from the inbuilt pattern generator. ]ULY 1988 57 I / COMPARATOR OUTPUT (B) II "' TRIAC "ON" PERIOD T MAINS VOLTAGE LOW/ BRIGHTNESS Fig.2: when the rectified filter output is greater than the ramp voltage, the comparator output is high and the associated Triac turns on. If the Triac turns on early in the mains half cycle, the lamp will be bright; if it turns on late in the half-cycle, the lamp will be dim. How it works The circuitry for the Discolight consists of four quad op amp ICs, five CMOS ICs, four opto-isolated Triac triggers, four isolated tab Triacs, four LEDs, two 3-terminal regulators and 21 diodes. And that's just the semis. Add in the resistors, capacitors, inductors, switches, pots and all the other hardware bits and it comes to a stack of components. Our total component count comes to over 230, so depending on how you look at it, it is the most complicated circuit we have presented in SILICON CHIP to date. Fig.1 shows the block diagram of the circuit. Switch S1 selects ·the audio signal, either from the internal microphone or from the loudspeaker terminals (which connect to your stereo). The audio signal is then fed to four filters which split it into four distinct frequency bands: Low Bass, Upper Bass, Mid Treble and Upper Treble. The Low Bass frequency band is provided by a 200Hz low pass filter - this means that only signal frequencies below 200Hz are allowed to pass. Then there is the Upper Bass band which passes signals between about 200Hz and 700Hz. This is actually a bandpass filter centred on 440Hz. The Mid Treble band, from about 58 SILICON CHIP 700Hz to 2kHz, is another bandpass filter, centred on 1.5kHz. Finally, the Upper Treble band is from 2kHz to 20kHz and is provided by a 2kHz high pass filter (ie, everything a hove 2kHz passes). The audio signal from each of the four filters is rectified and smoothed to provide a varying DC level, which is then fed to one of four comparators. The comparators compare the varying DC signal to a 100Hz ramp reference signal which is derived from the 240VAC mains supply (via the power transformer). Phase control The next part is messy. We vary the brightness of the lamps, in Warning! The Discolight is not a project for beginners. Unless you have already successfully built a number of mains-powered projects we advise you to leave this one strictly alone. Part of the circuitry is powered directly from the 240VAC mains supply and therefore must be regarded as potentially dangerous. We have designed it to comply with SM regulations but if it is not wired correctly and tested out properly it could be lethal. response to the loudness of the audio signal, by using "phase control". This is a method whereby a Triac (a semiconductor switch) is turned on earlier or later in each mains half-cycle. If the Triac turns on early in each half-cycle, the power delivered to the lamp is high; if the Triac turns on later, the power delivered to the lamp is low. The four comparators shown in. Fig.1 each determine whether their associated Triac turns on early or late in each mains half-cycle. They therefore determine the brightness of the lamps at any instant, depending on the loudness of the audio signal, as represented by the varying DC levels referred to earlier. Fig.2 shows how each corn.parator controls its associated Triac. It looks as "clear as mud" but be patient and all will be revealed. First, look at Fig.2c. This shows the 50Hz mains supply waveform and also the time at which the Triac switches on in each each mains half-cycle. As before, when the Triac turns on early in the half-cycle, the lamp will be bright; when the Triac turns on late in each half-cycle, the lamp will be dim. Fig.2b shows the DC output from the comparator, corresponding to the Triac " on" times. The DC output from the comparator actually turns the Triac on (although not directly, as.we shall see later). Fig.2a shows the interaction of the varying DC, from one of the audio filters, with the 100Hz reference signal (note how this signal is synchronised to the mains waveform in Fig.2c). Whenever the slowly varying DC signal is above the 100Hz reference signal, the output of the comparator goes high and triggers the Triac. So that's the basic process of how the audio signal,is rectified and then controls the Triac to vary the respective lamp's- brightness. But, as you might have guessed, there's a lot more to it than that, otherwise the overall circuit of the Discolight (which you've probably looked at and shuddered) would be a lot simpler. Now refer back to Fig, 1. Instead of the four comparator outputs going directly to trigger the Triacs Main Features Operating features Four light channels controlled by four separate audio channels. Forward, reverse and autoreversing chaser patterns. Simultaneous strobe on all four channels. Alternate light mode. Music modulation available on chaser, strobe and alternate modes. Adjustable rate for chaser, strobe and alternate modes. Inbuilt microphone for beat triggering or audio modulation of lights. Direct inputs for beat triggering or audio modulation of lights. Sensitivity control. Internal presettable sensitivity levels for each channel. * low pass rectified output which uses a 2.2µF capacitor. The larger capacitor provides a somewhat slower response time for the low frequency band. The varying DC output from each filter stage is fed to a 50k0 preset potentiometer (VR1-VR4). Thus the sensitivity of each channel can be set to provide equal brightness of lamps for typical music signals. Following the presets, the DC signals are fed to comparators IC3a, IC3b, IC3c and IC3d. These compare the varying DC for each frequency with the ramp reference signal from IC4d. By the way, ICt, IC2, IC3 and IC4 are all standardised as LM324 quad operational amplifiers. These are cheap and readily available. * Front panel LEDs mimic light display. * * Electrical features 2400W maximum lamp load. 600W maximum lamp load in each channel. Fused mains supply to lamps. Isolated tab Triacs for extra safety. Opto-coupled Triac triggering for complete isolation of control circuitry from 240VAC mains supply. RF interference suppression components on each light channel. Zero voltage switching of Triacs used for unmodulated chaser, strobe and alternate modes for minimum radio interference. Ramp reference resistor. The electret's signal is coupled by a .047 µF capacitor to the non-inverting input of ICtb which boosts the signal by about 31 times. After SOURCE switch St, the signal is fed to the SENSITIVITY control (VR5) and then to op amp ICta (a stage identical to IClb) which again provides a gain of 31 times. ICla's output is then fed to the four filter stages to provide the four frequency bands mentioned previously when we described Fig.1. IC2a and associated components form the 2kHz high pass filter. This is a third order (three RC timeconstants) filter which means that signals below 2kHz are rolled off at 18dB/octave. IC2d and associated components form the 200Hz third order low pass filter for the Low Bass channel. IC2b and IC2c and their associated components form twin-T filters. These are the 440Hz and 1.5kHz bandpass filters for the Upper Bass and Mid Treble frequency bands (as shown on Fig.1). Each of these four filter stages has a gain of about unity. The output of each filter is rectified using diodes Dt, D2, D3 and D4 and smoothed with l µF capacitors except for the 200Hz As mentioned above, each of the four comparators in IC3 use a common ramp reference voltage derived from the 240VAC mains. A 50Hz signal is picked up from the secondary of the 2851 mains transformer via a 100k0 resistor, clamped to ± 0.6V by diodes D14 and D15 and fed to IC4b which functions as a Schmitt trigger to "square up" the waveform. IC4b's 50Hz square wave output is then fed to Schmitt triggers IC9a and IC9b to obtain complementary signals. These signals are then fed to two differentiating networks, each consisting of a .022µF capacitor and 470k0 resistor. The spiky outputs of these two networks are fed via diodes D16 and Dl 7 to the inverting input of IC4c. IC4c thereby delivers a 100Hz pulse train, with each pulse synchronised to the zero voltage points of the 50Hz AC mains waveform. The pulse output from IC4c then feeds a 0.15µF capacitor via D18 with the discharge path being via two 22k0 resistors in series and a 5.6V zener diode (Dl 9). The result is the ramp curve depicted in Fig.2a, delivered from the ouput of IC4d, to the inverting inputs of quad op amp IC3. The outputs of the four comparators connect to ICB which is the Display Selector depicted in Fig.1. ICB is a 4019 quad AND/OR gate which can be regarded as a ** * * * ** they go via a block labelled as the Display Selector. This can be regarded as a four pole switch which selects either the signals from the comparators or a pattern generator. Signals from the pattern generator drive Triacs and hence the lamps in the chaser, strobe or alternate modes. Well, that's probably as far as we can go with block diagrams in describing the basic operation of the Discolight. Now, we have to stop dithering about and get into the circuit description proper. Circuit description Let's start at the extreme top lefthand corner of circuit. This shows an input attenuator consisting of two 10k0 resistors, one for each speaker lead from your stereo amplifier. The 10k0 resistors connect via a common 1.8k0 resistor to ground. This network mixes the two stereo channels together as well as attenuating them. From there, the signal goes to the SOURCE switch S1. Op amp ICl b provides gain for the signal from the electret microphone . The electret is powered via a network consisting of a lkO resistor and lO0µF capacitor which provide decoupling from the main + 12V supply while bias current is fed via the 4.7k0 * * * * * * * JULY 1988 59 +12V SPEAKER INPUTS lDk LEFT0-'W',11,-, + 2kHz HiGH PASS FILTER .,. COM~ COM 220k 3300. +12V 1k r--~HNfl,....-+12v 100 + 16VWI .,. 220k +12V .,. 14 +12V 70k +12V 200Hz LOW PASS ALTER +12V 74C14,40106 9 12 13 15 CK f"\..J'-__ +12V 01 IC& 70k .,. 6 4029 +12V 60 SILICON CHIP J ... S2 10 U/D REVERSE--c>------'AUTO DIRECTION D19 4giJS.:w FORWARl Fig.3: the circuit diagram. IC2a, b, c & d are the audio filters; D1, D2, D3 & D4 the audio rectifiers; IC3a, b, c & d the comparators; and ICB the display selector. IC5, 6 & 7 make up the pattern generator. RAMP 5 8 .,. 1M DISCO LIGHT SC10·1·0688 +12V LED1 16 10 A 680ll K A2 7 11 3 IC8 4019 12 l +12V DISPLAY S4b 1 2 12 13 A 30 10k CHANNEL 4 S4 : 1 : 4-BAND MODULATED 2 : MODULATED 3 : UNMODULATED -:, +12V S3 : 1 : ALTERNATE 2 : STROBE 3: CHASER 10k L1-L4 : 29T, 0.63mm ENAMELLED COPPER WIRE ON NEDSID 17n32/22 IRON DUST TDRIIII S3b 2 3f PAmRN .,. IC7 4051 +12V ! 1 S3a 7812 +12V ,;ITT,.;~ .~ i 10 2 3 11 10k IIH 6 .,. 7912 . A2 GNU 22k +12V 470k S4a 2 '? 020 1N4002 JUUL OU +12V 470k POWER S6 2851 10A I A I I 10 -16VW 470k ..,. + - I I 240VAC I I CASE -12V N 4.7 + 1 16VW _ 16VW - ii JULY 1988 ,- E 61 PARTS LIST 1 plastic instrument case, 262 x 1 90 x 83mm (Altronics Cat. No. H-0482) 1 aluminium rear panel, 251 x 76mm (Altronics Cat. No. H-0488) 1 Scotchcal front panel label, 251 x 76mm 1 PCB, code 10106881, 218 x 172mm 4 1 OA panel-mount mains sockets 1 4-way spring-loaded speaker terminal panel 1 2851 12.6V mains transformer 1 panel mount 3AG fuse holder 1 10A 3AG fuse 1 electret microphone insert 4 Neosid 17 /732/22 toroids 1 DPDT 240VAC toggle switch 2 DPDT centre-off toggle switches 2 SPOT toggle switches 1 SPOT centre-off toggle switch 2 knobs 1 cord grip grommet 1 mains cord and moulded 3-pin plug assembly 3 solder lugs 34 PC stakes 4-pole 2-position switch. This is controlled by S4b, which is one half of the Display Selector switch. When pin 14 is high and pin 9 is low, the comparator outputs are switched through (when switch S4 is in the 4-band modulated position) and thereby control the Triacs. On the other hand, when pin 14 is low and pin 9 is high, the signals from the pattern generator circuitry are switched through (position 2 of switch S4) to control the Triacs in Chaser, Strobe and Alternate modes. Pattern generation The patterns are generated by IC5, IC6, IC7, three op amps and a bunch of diodes, D6 to D13. IC6 is a 4029 up/down counter which drives the one-of-eight decoder IC7. To make the Chaser run in the forward direction, IC6 is made to count up; to run in the Reverse direction, IC6 is made to count down, as set by DIRECTION switch S2 (via pin 10). 62 SILICON CHIP Semiconductors 4 LM324 quad op amps 1 4051 1-to-8 analog multiplexer/demultiplexer 1 4030 quad XOR gate 1 4029 4-bit up/down counter 1 4019 quad 2-input AND/OR selector 1 7 4C14, 40106 hex Schmitt trigger 4 MAC218A8FP or SC142-M or BT137F-600 isolated tab Triacs 4 MOC3021 optically isolated Triac drivers 1 7812 3-terminal +12V regulator 1 7912 3-terminal -12V regulator 2 1N4002 1A diodes 18 1N4148, 1N914 signal diodes 1 5.6V 400mW zener diode 4 5mm red LEDs 4 LED bezels Capacitors 2 470µF 25VW PC electrolytic 2 1 OOµF 1 6VW PC electrolytic 3 1 OµF 1 6VW PC electrolytic 1 4. 7µF 16VW PC electrolytic 1 2.2µF 16VW PC electrolytic The clock signal for IC6 is fed via IC5 from one of two sources, selected by the BEAT switch S5. With S5 in the Music setting, the clock signal is derived from the Low Bass filter via comparator ICtc, which converts the bass signal into a squared up waveform. With S5 in the Oscillator setting, the clock signal comes from ICtd which is wired as a Schmitt trigger oscillator. The oscillator speed is set by the RATE control, VR6. The clock signal selected by S5 is fed to diode D5 and then to exclusive-OR gate IC5. This is connected to give a short positive pulse output whenever the clock signal from S5 makes a positive transition. IC5 thus ensures that the pattern generator circuitry is clocked by every bass beat in the music signal. Back to the one-of-eight The Qt and Q2 outputs from IC6 are connected to IC7; Qt directly to the A input while Q2 goes via PAT- 6 1 1 4 1 2 4 5 4 1 1 1µF 1 6VW PC electrolytic 0.15µF metallised polyester 0.12µF metallised polyester 0.1µF 250VAC capacitors (Wima MP3 or Philips MKT-P 2222 330 40104) .056µF metallised polyester .047µF metallised polyester .033µF metallised polyester .022µF metallised polyester .01 µF metallised polyester .0068µF metallised polyester .001 µF metallised polyester Resistors (0.25W, 5%) 2 x 1MO, 10 X 470k0, 4 X 220k0, 2 x 180k0, 6 x 1 OOkO, 3 X 47k0, 6 X 22k0, 1 X 18k0, 8 X 11 kO, 15 x 1OkO, 2 x 4. 7k0, 2 x 3.3k0, 1 X 2 . 7k0, 1 X 1 .8k0, 2 x 1k0, 4 X 6800, 4 X 3300, 1 X 1 000, 4 x 50k0 miniature vertical trimpots, 1 x 1 MO linear pot, 1 x 1 OOkO log pot Miscellaneous Screws, nuts, self-tapping screws, rainbow cable, 0.63mm enamelled copper wire (ECW), tinned copper wire, mains rated cable, insulating sleeving, solder etc. switch S3a to the B input. The C input of IC7 is connected via the wiper of S3b. Depending on the code fed to the A, B and C inputs, the common input of IC7, pin 3, is connected through to one of the eight outputs, 0 to 7. One of the 0, 1, 2 and 3 outputs can be selected when the C input is low, and one of the 4, 5, 6 and 7 outputs selected when the C input is high. The B input when low can select the 0, 1, 4 or 5 outputs. When high the 2, 3, 6 and 7 outputs can be selected. The A input selects either the 0, 2, 4 or 6 output when low and the 1, 3, 5 or 7 outputs when high. When switch S3 is set for Chaser mode, IC6 counts (up or down) and connects IC7 outputs 0, 1, 2 and 3 to the common input (pin 3; + 12V) in sequence. These outputs connect to the four inputs of ICB (the Display Selector, described above and shown in Fig.1) to drive the Triacs. When S3 is set for Strobe mode, TERN Despite the circuit complexity, the Discolight is easy to build with most of the parts mounted on a single PCB. Note that part of the circuitry is powered directly from the 240VAC supply, so take care when working on the board. the B input of IC7 connects to OV (via S3a) while the C input is connected to + 12V via a 10k0 resistor. Thus outputs 4 and 5 of IC7 a:re selected in sequence. When output 4 is high, it is bridged to outputs 0, 1, 2 and 3 via diodes D6 to D9 so all four Triacs are driven. (Output 5 is connected to + 12V and plays no active part). So all four channels flash on and off in unison. Finally, when S3 is set to Alternate mode, both B and C inputs of IC7 are connected to + 12V. Thus, as IC6 counts, outputs 6 and 7 of IC7 go high alternately. Then, as output 6 goes high, diodes DlO and Dl 1 pull outputs O and 1 of IC7 high also. Similarly, as output 7 goes high, diodes D12 and D13 pull outputs 2 and 3 of IC7 high also. So two channels flash on alternately. To modulate or not to modulate Now for the messiest part; how to obtain the modulated Chaser, Strobe and Alternate modes. Normally, when DISPLAY switch S4 is set to the unmodulated mode, the input to inverter IC9e is pulled low via a 10k0 resistor. This feeds signal from the pattern generator circuitry through to the Triacs. Similarly, when S4 is set the Modulated mode, IC9e's input is still pulled low and the pattern generator signals are fed through to the Triacs but - and here is the tricky bit - have a look at S4a in position 2. Signal from the Low Bass audio comparator feeds via inverter IC9d and a 22k0 resistor, to the INHibit input of IC7. This allows the Low Bass channel to modulate the lamp brightness even though the normal mode would be for zero voltage switching of the Triacs and thus full brightness of the lamps. That is really tricky. Isolation IC8, the Display Selector (depicted on Fig.1), does not drive the four Triacs directly. It does so via four MOC3021 optically coupled Triac drivers. Between pins 1 and 2 of each MOC3021 is an internal LED and when this is driven, the emitted light turns on an optically triggered Triac. This, in turn, feeds the gate of the relevant power Triac and turns it on. continued on page 83 JULY 1988 63 input and output terminals are mounted on the sides. We used a 15V 1A transformer but since the current drain of the whole circuit is low, you could save a few dollars by going to a 12.6V 150mA transformer (such as the Altronics Cat. No. MM-2006). Asse_mbly of the PCB is a straightforward job. Install the links, resistors and diodes first, followed by the capacitors and integrated circuits. All the connections to the board are made via lengths of ribbon cable. The capacitors for the burst period switch are wired around the switch itself. Note that the wiring diagram shows most of the interconnecting wires with letter codes. For example, point K on the printed circuit board is joined to point K on VR3, the headphone level control. The case can be drilled for all the controls using the Scotchcal panel as a template. Then it is simply a matter of installing all the hardware and completing the wiring. Setting up You will need a frequency meter or oscilloscope to set up the instrument. After applying power and checking the 15V supply rail, adjust trimpot VR1 to obtain a frequency of 2kHz from pin 3 of IC1. This done, check that tkHz is obtained from pin 13 of IC3, then check the sinewave at pin 7 of IC4. Trimpot VR1 can be tweaked to obtain the cleanest sinewave but make sure the lfrequency is still close to tkHz. dheck that the continuous and burst modes can be obtained at the appropriate settings of switch S2. Check that the burst length varies in accordance with the setting of switch St. Finally, the DC output at pin 6 of IC5 can be adjusted to equal the continuous RMS AC voltage (with burst disabled) from the amplifier output. This can be done by trimming the resistor values associated with switch S4. To trim the xto setting, the 1.5MO resistor can be changed or shunted with higher values to obtain the correct reading. Similarly, the x3 setting can be trimmed by changing or shunting the 20k0 resistor with higher values. ~ T-2500MFP, Philips BT137F-600 or General Electric SC142M. The Discolight ctd from page 63 So the MOC3021s provide very high voltage isolation between all the low voltage control circuitry and the 240V AC circuitry to the Triacs. In series with each IC8 output to the MOC3021s is a LED and these are mounted on the front panel so that they give an indication of the behaviour of the Triac driven lamps. Associated with each Triac is an inductor (Lt to L4) and a 0.tµF 250VAC capacitor. These two components form an RF suppression network for each Triac so that radio interference is reduced when the Triacs are in the phase control mode (for the 4-Band Modulated and Modulated settings of S4). Note that we have specified Triacs with an isolated tab so that they can be mounted directly onto the rear panel without any need for mica washers, insulating bushes and so on. The types specified are Motorola MAC218A8FP, Motorola ~~ Power supply Power for the low voltage circuitry is provided by a 12.6V transformer feeding two half-wave rectifiers, D20 and D21, and two 470µF capacitors. The resulting unregulated positive and negative rails are fed to 3-terminal regulators to give ± 12V DC supply rails. The additional toµF and 1µF capacitors connected at the output of the regulators provide decoupling and bypassing of high frequency "hash". Note that the power switch (S6) connects mains power to the transformer only and not to the Triac output circuitry. The Active AC supply lead to the Triacs is fed directly to their anodes via the 10A front panel fuse. That completes the circuit description of the Discolight. 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So if you would rather put your money into better quality than in other people's pockets, VIFA 1 sp::~~:~sd~:1::~t~: ~:~~;o;~·the kits are supplied with all parts needed including fully built crossovers and pre-cut flatpack cabinets ready to assemble. No soldering or carpentry skills are needed, just a Phillips head screwdriver, some simple hand tools and a few hours of your leisure time. Are they as good as people say? Read the reviews, listen and compare with any other speakers twice the price or more. Need we say anymore? VIFA for the quality conscious audiophile. For full details please contact Sole Australian Distributor: SCAN AUDIO Pty. Ltd. P.O. Box 242, Hawthorn 3122. Fax (03)4299309 Phone: (03! 4292199 (Melbourne) (02 5225697 (Sydney) (07 3577433 (Brisbane) (09) 3224409 (Perth) Stocked by leading stores throughout Australia :-:-:-:❖:•:•:•:-. JULY 1988 83