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::flllHll/JI/J= flll!/'1- LIGHTING DESK This 4-channel lighting desk is intended for theatre, disco and music group applications. It has heavy duty circuitry and is able to cope with spotlights rated up to 1000 watts or more. By JOHN CLARKE & LEO SIMPSON This 4-channel lighting controll er h as been designed and built for the rigours of commercial use. It has been designed in consultation w ith a professional lighting engineer and has the features normally expected by professional users . It is not intended for domestic use. If you want a lighting controller which can be h ooked up to your stereo system, check out the articles on the Discolight published in th e July 1988, August 1988 and October 1990 issues of SILICON CHIP. These days , no theatrical or musical group worth its salt would dream of putting on a show without a properly set up lighting system. Instead, the lighting is usually fully chorea- Main Features Operating features • Four channels • 60mm slider pots • Click action flash switches with integral LEDs • Separate dimming for each channel • Master dimmer control • Flash on each channel • Master dimmer for flash • 4-channel chaser • Master dimmer for chaser • Adjustable chaser rate • Single step for chaser • Chaser on and off • Flash switch LEDs mimic lamp display 36 SILICON CHIP Electrical features • 2400W or 3600W maxi mum lamp load for 4-channels • 2400W maximum lamp load on each channel • 40W minimum lamp load on each channel • 15A fused mains supply • Isolated tab Triacs for safety • 40A 600V rated Triacs • 7.5kV optocoupler isolation between control circuitry and 240VAC mains for e xtra safety • RF interference suppression • Preheat for lamp filaments • Industry standard 0-1 0V dimming control graphed, with the number of lighting or "scene" changes running into the hundreds for a full night's show. Normally, most of the lighti ng gear required must be hired , along with the services of an operator to set it up , run it and then take it down after the show. This can cost heaps but by building your own professional 4-channel dimmer and then setting up your own lighting, you can save lots of dollars. Features The SILICON CHIP 4-Channel Dimmer is suitable for all types of stage work. It is powered from a standard single-phase 240VAC outlet and depending on wheth er it is a 10 or 15amp circuit , can control a total lamp load of 2400 or 3600 watts. A similar limitation applies to any of its four output channels - ie, each channel is rated for a maximum lamp load of 2400 watts. The unit is easy to use, with all controls well spaced and logically placed on a sloping front panel. Dominating the front panel are eight fader pots and th ese are all 60mm types to ensure smooth control and ease of setting. In addition, each channel can be operated individually by means of "flash" pushbutton swi tches. These switch es each incorporate a LED to show what is happening w ith the lamps and their relative brightness. In operation, you can fl ash each channel up to any brightness level, as set by the "Flash Master" fader. Similarly, the "Channel Master" control fades all lights up or down, to or from their individual fade settings. Chaser The two chaser faders control the rate and lamp brilliance when the The 4-Channel Dimmer is built into a metal case with a sloping front panel & features 60mm slider controls to set lamp brilliance & chaser rate. The chaser can be set to operate automatically, or you can flash each channel manually using separate pushbutton controls. unit is operated in chaser mode. Firstly, the lights can be flashed to full brilliance or to an intermediate setting as set by the "Chaser Master" fader. In this mode, the four channel faders set the minimum brightness level. The speed of the chaser is controlled by the "Chaser Rate" slider which can be set anywhere between one flash per second and six flashes per second. Pushing the "Chaser Rate" slider down to its minimum setting turns the chaser off and you can then flash the lights sequentially under manual control using the "Single Step" button. The role of the "Chaser On/Off" button is self-explanatory. As a matter of interest, you can have all the above functions operating together if you wish, so the unit is very flexible. For example, you can set each channel to a different brightness and then you can superimpose chaser and flash operations together. Sound too far-fetched? Not at all. Professional operators use various combinations to obtain special visual effects. Big Triacs The Triacs specified in the output stages are rated at 40 amps each and have surge ratings as high as 600 amps. These ratings are far greater than will ever be needed during normal dimmer operation, even taking into account the surge currents through cold filaments (up to 10 times normal current). The reason why such rugged Triacs have been specified is to cope with the high fault currents that flow when the filaments of large (say 1000 watt) lamps blow. When the filament of a large lamp blows, the loose filament ends usually flail around and establish an arc between the main filament stems. This arc current can be very high - 100 amps or more - until the stem fuses in the lamp blow. Typical 8-amp Triacs , such as those used in ·our Discolight, are quite adequate for normal dimming applications but they can't withstand the heavy fault currents just described they blow before the stem fuses go. So 40-amp Triacs are mandatory. Lamp preheat We've just mentioned cold filament JUNE 1991 37 SLIDERS FLASH BUTTONS MAINS OUTLETS COMPARATORS (IC3) S1 each mains half-cycle to give high or low lamp brightness. This switching action can cause considerable interference to sensitive audio, radio and even video equipment unless it is adequately suppressed. Naturally, our 4-Channel Dimmer incorporates suppression circuitry. Block diagram +10V VR5 CHASER (IC2b ,IC5) RAMP GENERATOR (IC2a,IC1b) Fig.1: the 4-Channel Dimmer has four mains output sockets which are controlled by separate Triacs. The Triacs in turn are controlled by separate comparators. In operation, each comparator compares a ramp waveform with the DC level set by an associated slider control (VR1-VR4) & switches its Triac to control lamp brilliance. The Flash buttons override the DC levels from the channel faders, while the Chaser circuit can also override the DC level from the channel master control (VR5). surge currents and this is a really big problem with lamps rated up to 1000 watts or more. When full power is applied to the cold filament of a 1000W lamp, the initial surge current can be as much as 10 or even 15 times the normal value - up to 60 amps or more. While these huge currents don't last long (less than one cycle or 20 milliseconds), they cause very high stresses on the lamp filament, for two reasons. First, because a lamp filament may not have the same diameter throughout or it may be kinked (it is coiled anyway), it will inevitably have hot spots. As time goes on, these hot spots get worse and become weaker, as the filament material is boiled off. Second, a current of around 50 or 60 amps in a coiled filament naturally causes a high magnetic field and this can literally shake a weakened filament to pieces - a double whammy, if you like. To overcome this problem of surge current which is particularly bad during chaser operation, all lamp filaments are maintained in a preheated 38 SILICON CHIP state, just enough to make the filaments glow a dull red but not give off any appreciable light. This greatly increases the life of these very expensive high power lamps. As with any mains dimming circuit, the 4-Channel Dimmer uses phase control of the Triacs to vary the lamp brightness. In other words, the Triac is switched on early or late in Warning! This 4-Channel Dimmer is not a project for beginners. Unless you have already 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 and therefore must be regarded as potentially dangerous. The wiring conforms to SAA regulations and if constructed correctly it is a safe appliance to use. Although the 4-Channel Dimmer has quite a few control features, the basic circuit principle is fairly simple. The block diagram of Fig. l shows the general circuit arrangement. It depicts four output sockets which are controlled by four separate Triacs. Each Triac is controlled by a comparator and each of these is controlled by DC voltage from a slider. The inverting (-) input of each comparator is connected to a decaying ramp (sawtooth) signal which is synchronised to the 50Hz mains waveform (see Fig.2). Meanwhile, the noninverting (+) input of each comparator is connected to its own respective slider pot, VR1 -VR4. The DC level from the slider sets the brilliance of the lamps connected to the output. The comparator compares the DC level from the slider with the decaying ramp waveform at its non-inverting input. When the DC level from the pot is higher than the ramp waveform voltage, the comparator output goes high and turns on the Triac. Fig.2 shows this comparison process in a little more detail. It shows the mains 50Hz waveform at (a) and the decaying ramp waveform immediately below it at (b). Notice that the ramp waveform starts at a high level at the beginning of each mains half cycle and then decays to a low level by the end of the half cycle. Also shown on the ramp waveform plot (b) is the DC level from one of the sliders (applied to the non-inverting input of the comparator). Each time the ramp waveform drops below the DC level, the comparator output goes high and turns on the Triac. This is reflecfed in waveform (c) which shows the voltage applied to the lamp. Waveforms (b) and (c) are for a relatively high brightness setting so almost the full mains waveform is applied to the lamp. Waveforms (d) and (e) show the conditions for a low level of brightness. Here the DC level from the slider is lower and consequently the Triac is turned on later in each mains half cycle. Now refer back to the block diagram of Fig.1 and you will see that the · flash buttons can override the DC levels from the channel faders (or sliders). Each flash button is connected to the wiper of the "Flash Master" and also to the non-inverting input of its respective comparator. Chaser operation can also override the DC level from the channel masters. Accordingly, each of the four outputs from the chaser circuit connect to the non-inverting inputs of their respective comparators. Main circuit Now take a look at the full circuit diagram of Fig.3. The similarities to the block diagram of Fig.1 may not be readily apparent but we'll point them out. The four comparators in Fig.1 are contained in IC3, an LM339 quad comparator, on the main circuit. The four channel sliders (or faders) are shown as VR1-VR4 on the main circuit, while LEDs 1-4 on Fig.1 have the same numbers and function on Fig.3 but are associated with a bunch of transistors. Let's first talk about the ramp generator, which is one of the core parts of the circuit. It is based on comparator IC2a and associated components, at the top right of Fig.3. In reality, IC2a acts as a "zero crossing" detector and gives a pulse whenever the mains voltage sinewave passes through zero volts, once every 1 0 milliseconds. The zero crossing pulses are then used as markers for the beginning of each mains halfcycle. Diodes D5 and D6 rectify the AC voltage from the power transformer secondary and apply the resulting pulsating voltage via a voltage divider to the inverting input (pin 6) of IC2a. Pin 7, the non-inverting input, is connected to a voltage divider which gives a DC reference of about +60mV. Fig.6 shows the action. At the beginning (or end) of each mains half cycle, the voltage at pin 6 will drop below pin 7 and so the output of the comparator at pin 1 will deliver a brief +12V pulse. So you get a string of these pulses coinciding with the zero crossing points of the 50Hz mains waveform. These positive pulses are fed via diode D7 to a .039µF capacitor which (a) MAINS WAVEFORM 10V (c) WAVEFORM TO CH1-CH4 ;--•~..__ _ _ _ , _ _ _ _ __ LAMPS (d) FILAMENT BRIGHTNESS LEVEL IC1b RAMP FILAMENT BRIGHTNESS IC2c J I I (e) / .,, ,,.- BRIGHTNESS LEVEL I I I I MINIMUM LAMP t--,,,___ __. WAVEFORM ~ / I I Fig.2: this diagram shows the 50Hz mains waveform at (a), while immediately below it is the decaying ramp waveform (b).Also shown on the ramp waveform is the DC level from one of the channel sliders. Each time the ramp waveform drops below the DC level, the · associated comparator output goes high & turns on its Triac to produce the waveform shown at (c). Note that waveform (c) is for a relatively high brightness setting. Waveforms (d) & (e) show the waveforms for a low brightness setting. is shunted by a 100H2 resistor and 10V zener diode. The resultant waveform across the .039µF capacitor is the ramp voltage waveform depicted in Fig.2. It is clamped to a maximum of +10V by the zener diode and its rate of decay is set by the 100kQ resistor. The ramp waveform is then fed to pin 10 of IC1b which functions as a unity gain buffer to drive the inverting inputs (pins 4, 6, 8 & 10) of IC3, pin 11 of IC2c and pin 9 of IC2d. 10V slider supply In the specification panel, we make mention of the fact that this 4-channel Dimmer uses the industry stand- ard 0-10V dimming control voltage. What's so important about this? Well, it could be important if you want to use this lighting desk to control a commercial lighting rack or "slave" another lighting desk to it. So all the fader controls in the circuit am fed from a common +10V supply which is derived from the +12V rail via 5kQ trimpot VR9 (near IC2d). This feeds master faders, VR5, VR6 & VR7. The output from the wiper of each of these master faders is buffere d by an op amp - IC1d for VR5, IC1a for VR6 and !Clc for VR7. Actually, IC1c isn't really necessary but we've used JUNE 1991 39 it anyway otherwise we would have had an op amp left over. These buffer op amps prevent loading effects by the following circuitry. ICl, an LM324 quad op amp (ie, four op amps in a single package), is an important choice for these buffer amplifiers. Not only is it cheap but it can operate from a single rail supply and its output can go right down to OV which is desirable for this circuit. <at>+ Channel faders ~~ The four channel faders , VR1 -VR4 , all connect to the output of the master level buffer, ICld. In turn, the wiper of each channel fader connects to the normally closed contact of its associated Flash switch, S1-S4. The other side of each Flash switch connects via a lOkQ resistor to the non-inverting inputs of IC3d, IC3c, IC3b and IC3a respectively. IC3 is an LM339 quad comparator. Its inverting inputs (pins 4, 6, 8 & 10) are fed with the ramp signal from pin 8 of IC1 b, as noted above. And as described previously, in relation to block diagram Fig.1 and waveform diagram Fig.2, the outputs of these comparators drive the Triacs to deliver high or low power to the lamps. In practice, the IC3 comparators don't drive the Triacs directly, as there are gates, transistors and optocouplers in the signal path to the Triac gates. However, the principle is true. ;~~r-::i cz:>u>CC:.:.: c.., :iE ~ :::c~ci::>o z U>~ I· a:ff3~ wz« ~~i a:o + ~ ;:: ~H•· :,:a:.., u:>~ ~ C, I· I· z ~~::i u>~ C, :,, c,~ ~+a, "' ,- a: IC = w:,, ..... - "' I· z :,:a:-' u>~ ;:: r- ---- - SILICON CHJP NN~ C:Cc.:, + Each channel can be flashed to a level set by the Flash Master fader, by pressing the relevant Flash switch (S1 , S2, S3 or S4). These switches disconnect th e signal from the relevant channel fader pots and connect the inverting inputs of the comparators to· the Flash Master fader output at pin 1 of !Cla. To prevent any contact bounce 40 ,. <n,- a: i:: Flash buttons Fig.3: all of the circuit elements shown on the block diagram can be directly related to the main circuit diagram. IC2a & IClb form the ramp generator, while IC2b & IC5 form the core of the chaser circuit. IC3a-lC3d are the comparators & these drive the Triacs via NOR gates IC4a-1Cd, transistors Qt-Q4 & four MOC3021 optocouplers (IC9-IC12). IC2c forms the filament preheat circuitry, which limits the inrush current to the lamps. I· :c;;~ I I I I I , ~_ _ i _ _ _JI C, C, 3: +~i;; ·~•- ~ C, .,. ~ + z ~~ f-1•· ~I· --«-"' I a:~ w ~ <no~ :,:Z c..:,O ~ I• +12V ◄ • -◄ '\ RAMP r-- 16 SINGLE 1, STEP S5 r-- °+ +12V 3'7 04 BC557 1N4148 .i. 47kf , + 01:J: 12 13~ 6BO!l .,. IC71 14 •cK 1k-> 1 CH4 001:; ,- 21 . ~ FLASH 3 S3 _J,_ 1 +12V +12V IC12 MOC3021 ·5ao1 1 1kt 1t IC5 4017 SINGLE~ STEP }s LEDS i CH3 LED3 +12V +12V 1N4148 .i. ~ 01J CHAS.ER COUNTER .... u.i.... n,. ) ~ B 2'. CH2 LED2 _A2 ~ ~ +12V 5.6k )o3 Wit ) A ( Jrt A2 A~K 3 3k t t CH1 LED1 I 4-CHANNEL DIMMER A~~~~T 500~1 470kl. 05 BC547 l t TRIAC2 , 06040J7 ; ~ 0.22 .:., 250VAC~ · AL~- ~'Y'• CH2 )// .,. 01 BC557 !C 680i2 A2 TRIAC1 06040J7 G 4v Iii 10k~06 FILAMENT HEATER ~ +12V 2.2kt 13 B lkf +12V +12V GNO (!) (!) A K· ,._ ..."" CASE 680!.l 1t FLASH 1 Sl u·3 )rtUT 02 BC557 A CE 13 Ee~·\ .,. 'C 10kf H }s 10k 10,4 VIEWED FROM BELOW CH3 K' 09 .____ A2 A FLASH 2 S2 15,R B 680ll s?r) ac~c B j: '1.:. .J,. LEDHED6 WITHIN S1-S6 LH4 18T . 0.8mm ENAMELLED COPPER WIRE ON NEDSIO 17·730-22 TOROID CASE problems, a .OlµF capacitor is connected to the non-inverting input of each comparator in IC3. Chaser operation The 4-Channel Dimmer is easy to build, with most of the parts mounted on two PC.boards. Because of the efficient switching technique employed to control the lamps, the case provides adequate heatsinking for the four Triacs. Fig.4: this CRO photograph shows the uncalibrated mains waveform at top & the ramp voltage from pin 8 of IClb immediately below (5ms/div & 5V/div). The ramp voltage decays from a peak to almost 0V over each half cycle. 42 SILICON CHI!' Essentially, only IC2b and IC5 provide the core chaser function but quite a bit of gating and logic is required to make it all work properly with the rest of the circuit. So IC6 , IC7, IC8 and IC2d also get into the act. Let's look first at IC2b which provides the chaser clock function. IC2b is one section of an LM339 quad comparator and it is connected as a Schmitt trigger oscillator. The 4. 7µF capacitor at the inverting input (pin 4) is charged and discharged via a 10kQ resistor which connects to a voltage divider comprising a 22kQ resistor, the lOkQ Chaser Rate pot (VR8) and a 2.2kQ resistor. When the wiper of VR8 is closest to the 2.2kQ resistor, the 4. 7µF capacitor charges and discharges quickly and when set near the 22kQ resistor, it charges and discharges slowly. But when the wiper of VR8 is set to the minimum setting, the 4.7µF capacitor cannot charge to the upper 8V threshold of the Schmitt trigger (set by the resistors at pin 5) and oscillation stops. The output of IC2b then latches high. LED 5, driven by IC7e, indicates when IC2b is oscillating. When IC2b's output is high, IC7e's output is low and the LED is off. When IC2b's output is low, the LED is on. Therefore, the LED is off when the oscillator is Fig.5: the top trace in this photo shows the waveform that's applied to the lamp when the dimmer is set for a high brightness level. Note that the mains in not switched to the load until it is almost half-way through each cycle. stopped and flashing when the oscillator is operating. IC2b also drives inverter IC7fwhich is used to clock IC5, a 4017 decade counter. Alternatively, IC5 can be clocked with single step switch S5 which is intended to be used when the Chaser Rate control is set to zero. The "0", "l", "2" and "3" outputs ofIC5 are used for the chaser sequence, while the "4" output at pin 10 resets IC5 at the count of five. Thus, IC5 's outputs go high at 0, 1, 2, 3, 0, 1, etc. These outputs are fed to AND gates IC6a-IC6d (4081) and these in turn drive comparators IC3a-IC3d via diodes D8-D11. The AND gates of IC6 work as follows. When the commoned inputs at pin 5, 2, g and 12 are low, the AND gate outputs are low and so chaser operation is not enabled. Conversely, when the commoned AND gate inputs are high, the chaser signals are fed through. Chaser brightness & enable When the Chaser mode is operating, the lamps can be made to flash at full brightness or at any level, depending on the setting of the Chaser Master fader (VR7) which is buffered by IClc. The voltage from this fader (and buffer IClc) is compared by comparator IC2d with the ramp voltage from IClb. IC2d's output is a pulse train which goes high either earlier or later in each mains half cycle, depending on the setting of VR7. . IC2d's output controls the reset mput offlipflop IC8a. This JK flipflop is connected so that when its Set input is high , the Q-bar output at pin 2 follows the Reset input. The Q-bar output is inverted by IC7c and drives the commoned inputs of AND gates IC6a-IC6d. When these inputs are high, the chaser signals from IC5 are gated through to stages IC3a-I_C3d. . When IC8a's Set input (pm 7) 1s low, its Q-bar output is high and the resulting low output from IC7c forces the IC6 AND gate outputs low. This turns the chaser function off. The control signal for the Set input of IC8a is derived from IC8b, the second JK flipflop in IC8. This flipflop is connected so that its Q output changes state on each clock pulse to pin 13. The Q output controls the Set input of IC8a while the Q-bar output drives LED 6 via IC7d to indicate when chaser operation is enabled. The clock sig- PARTS LIST 1 metal case, 274 (W) x 209 (D) x 55 (H front) x 100mm (H rear) 1 Dynamark front panel label, 271 x 206mm; or screen printed front panel 1 PC board, code SC 10106911, 201 x 158mm 1 PC board, code SC10106912, 160 x 67mm 4 10A panel mount mains sockets (Jaycar PS-4012) 1 15A 3AG panel mount mains fuseholder 1 15A 3AG fuse 1 SPOT 8A illuminated mains switch (Jaycar SK-0985) 1 2851 12.6V mains transformer 3 metres 10A mains cord 1 10A mains plug 1 3-way mains terminal block 1 mains cordgrip grommet 3 earth solder lugs 12 5mm ID rubber grommets 4 Neosid 17-730-22 iron powder toroid cores 2 8-way vertical pin headers (Jaycar HM-3210) 2 8-way right angle pin headers (Jaycar HM-3215) 4 8-way connector sockets (Jaycar HM-3220) 1 2-metre length of 12-way rainbow cable 8 10kQ 60mm travel linear slider pots (SV602NP 20A 1-B1 Ok) 8 knobs to suit sliders 6 PC-mount SPOT momentary action switches with LED indicator & 17.5 x 17.5mm black key 1 5kQ miniature horizontal trimpot 1 500Q miniature horizontal trimpot 20 3mm countersunk black dress screws 16 3mm grub screws 4 3mm cheesehead screws 20 9mm x 3mm tapped spacers 4 6mm x 3mm spacers 8 screws and nuts for mounting mains sockets 4 screws and nuts to secure main PC board 8 screws and nuts for Triacs , transformer and earth connections 2 screws and nuts for terminal strip 4 screws and nuts for securing toroids 4 plastic insulating bushes for mounting toroids 25 1mm PC stakes 6 cable ties 1 4-metre length 0.8mm enamelled copper wire 1 1-metre length 0.8mm tinned copper wire 4 rubber feet Semiconductors 4 Teccor O6040J7 40A 600V insulated tab Triacs (from NSD) 1 LM324 quad op amp (IC1) 2 LM339 quad comparators (IC2, IC3) 1 4001 quad NOR gate (IC4) 1 4017 decade counter (IC5) 1 4081 quad AND gate (IC6) 1 74C14, 40106 hex Schmitt trigger (IC?) 1 4027 dual JK flipflop (IC8) 4 Motorola MOC3021 7.5kV isolated Triac drivers (IC9-12) 1 7812 3-terminal + 12V regulator 6 1N4002 1A rectifier diodes (D1-06) 5 1N4148, 1N914 signal diodes (D7-012) 1 10V 1W zener diode 4 BC557 transistors (01-04) 2 BC547 transistors (05,06) Capacitors 1 1000µF 25VW PC electrolytic 4 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 4 0.22µF 250VAC mains rated 1 0.1 µF monolithic 1 .039µF metallised polyester 4 .01 µF metallised polyester 2 .001 µF metallised po lyester Resistors (0.25W, 5%) 4 470kQ 1 3.3kQ 3 100kQ 4 2.2kQ 2 47kQ 7 1kQ 1 22kQ 8 680Q 11 10kQ 1 220Q 4 5.6kQ Miscellaneous Solder, heatshrink tubing, heatsink compound , hookup wire. JUNE 1991 43 Each of the four output channels is capable of controlling loads up to 2400 watts, while the maximum total load for all four channels is 2400 watts for a 10-amp circuit or 3600 watts for a 15-amp circuit. nals for IC8b are derived via switch S6 and IC7a which is connected as a one-shot. Each time S6 is pressed, it either enables or disables the Chaser mode. At power up, the RC delay at the input of IC7b produces a reset pulse for flipflop IC8b, to reset it to "chaser off" mode. This means that the 4Channel Dimmer always powers up in Dim only mode, with the chaser function off. Filament preheat We 1ve already discussed the need for th e filaments to be preheated and you might think that this could be easily arranged by having a fixed resistor in the earthy side of the dimmer faders. However, it isn't quite as easy as that since LEDs 1-4 are supposed to indicate the lamp brightness and if the simple approach were used , the LEDs would also respond to the preheat Triac trigger pulses. Hence, the preheat facility works in a more complicated way. lt's ·based on comparator IC2c and the NOR gates ofIC4 (4001). IC2c works in similar fashion to the comparators of IC3. It compares the ramp voltage to a preset level from trimpot VRl0 and produces a pulse towards the end of each mains half cycle. IC2c's output pulses are then fed through to the Triac trigger circuits via the NOR gates. Each NOR gate drives a BC557 transistor (Ql-Q4) via a 5.6kQ resistor and the transistors each drive a MOC3021 optically isolated Triac driver (IC9IC12) via a 680Q resistor. The transis- +12V PULSE OUTPUT PIN1 , IC2a tars also drive the LEDs (associated with Flash switches S1-S4) which mimic the lamp display. To isolate LEDs 1-4 from the Triac filament preheat pulse, Q5 and Q6 come into play. Instead of the LEDs merely being connected to the 0V line, they all go to the collector of transistor Q6. Q5 is switched on each time the preheat signal from IC2c is high. This switches offQ6 and prevefits the LEDs from being turned on. At other times, when the preheat signal from IC2c is low, Q5 is switched off, Q6 is switched on and the LEDs respond to the signals from comparators IC3a-lC3d. Thus, the indicating LEDs are driven only when the brightness of a channel is set above the preheat level. As a result, the LED display provides a good indication of the brightness of the lamp setting. Each of the four Triacs in the circuit has an interference suppression circuit involving a 0.22µF capacitor and an inductor (Ll-L4) wound on an iron dust toroid. Note that our specifications show a minimum loading on each channel of 40 watts . This is necessary because the "holding current" specification for Q6040J7 Triacs is such that they won't turn on predictably for lamps oflower rating. So if you try testing this Dimmer with 25W lamps, it probably won't work correctly. Power supply Power for the dimmer circuit is derived from the mains and is applied via a 15 amp fuse and switch S7 to power transformer Tl. Note that the mains supply to the Triacs does not pass through S7 as the current would greatly exceed the switch rating. The 12.6V transformer secondary drives bridge rectifier Dl-D4 and this then feeds a lO00µF filter capacitor. The resulting DC voltage is then applied to a 7812 +12V regulator. Next month , we will complete the description of the 4-Channel Dimmer by giving the construction details. Acknowledgements Fig.6: this diagram shows how IC2a produces square wave pulses which are synchronised to the zero voltage crossing points of the mains. Each time the rectified 50Hz sinewave on pin 6 swings below the reference at pin 7, IC2a's output switches high. 44 SILICON CHIP Our thanks to the following companies for their assistance with this project: Jaycar Electronics, for the metalwork and other components; NSD Australia, for the Q60407 Triacs; and VSI Electronics Australia, for various Motorola !Cs. SC