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Add dimming controls to the Discolight Did you build our very popular Discolight project as described in the July and August 1988 issues of SILICON CHIP? If so, you can now add manual fade to the four channels. By JOHN CLARKE The Discolight is a very successful design which has been built by the thousands. As testimony to that continuing popularity, it is still available as a kit from a number of suppliers. The overriding reason for its popularity, apart from the 72 SILICON CHIP fact that it has proven to be a reliable design, is that it included a lot of operating features, enabling anyone to stage a small-scale light show. As well as being able to use your stereo system to control four chan- nels of lights, it could also be used as a chaser, with or without modulation by the music. We won 't dwell on all the features of the Discolight here - if you want to read the articles in full, we still have limited numbers of the July and August 1988 issues in stock. As with all good things though, someone can always see ways of making them better. In this case, one of our readers, Greg Suttle, came up with the idea of adding dimmer control to the Discolight. He made up a circuit and sent it into us and we liked the idea. The results appear in this article. We've taken his circuit concept, tidied it up a bit and put it all on a PC board to make it easy to build. Added features The new circuitry enables the Discolight to be used simply as a four channel dimmer or with all Discolight functions available while each channel operates from a preset brightness level. By using this latter mode to set the minimum brightness of the filaments, there is an extra benefit because the lamps will last longer. As well as the small printed board, the modifications include adding four sliders for the dimmer controls, four pushbuttons as "flash" switches, and a toggle switch to select Discolight/dimmer operation or dimmer operation only. These extra controls are mounted on a panel on the lid of the Discolight case. 1J: CJ ::::i 0 ~ c a: 0 u.. ...I 0 a: 1- z 0 (.) a: w ~ ~ c Circuitry Apart from the hardware items just mentioned, the additional circuitry comprises two quad op amps, 14 diodes and little else. All of this new circuitry operates at low voltage ( ± 12V} but note that the Triac part of the Discolight circuitry operates at full mains voltage and is therefore potentially lethal. If you intend to add these modifications, you will have to refer to the original articles on the Discolight for the full circuit details. As described on page 48 of the August 1988 issue, two mains wires can be removed from the circuit to make it safer to work on. Now let ' s look at the new circuitry. The four dimmer (slider} controls are shown as VR101, VR102, VR103 and VR104. They are each connected to the + 12V rail via a 200k0 trimpot. The " cold" end of each slider pot connects to a 1.2V voltage reference provided by two .;,-. ...... ~s..,,..,.+-.➔.:"4 Fig.1: the circuit uses two quad op amps. The voltage on the wiper of each slider control (VRlOl-104) is · buffered by a voltage follower (ICla-d) and fed to a comparator stage (IC2a-2d) where it is compared with a ramp voltage from the Discolight circuit. The comparators then drive the Triac stages. OCT0BER1990 73 / 79;!1~u~~~!uT 6 TRANSFORMER I 1 1 VR101 VR102 VR103 PIN 14, IC4 . .--•I---. !.i;;---;il,,,;;=:-==f'-.~JI .,,,.__.,...:s..:. : NYLON SCREW PCB VR104 1 ,~, (~ <at> 113 \._~01 101 * INDICATES CHANGED WIRING, THE NEW DIMMER CIRCUITRY IS IN SERIES BETWEEN THESE WIRE NUMBERS. (SEE P45 , AUGUST 1988) LID OF CASE Fig.2: the PCB & its associated slider controls are mounted on the lid of the Discolight case. The wiring points marked with stars are connected in series with existing Discolight wiring leads. diodes, D113 and D114. Buffered sliders The wiper of each slider control is fed via a diode (D101 for channel 1, etc) to an op amp (IC101a, b, c or d) connected as a voltage follower. Since the op amp provides a high in- put impedance and a low output impedance, it "buffers" the 50kfl slider control. Following each of the buffer op amp stages is a comparator (IC102a, b ,c d) based on an LM324 quad op amp. Each comparator compares the voltage of its respec- Fig.3: this is the full-size artwork for the PC board. 74 SILICON CHIP tive buffer op amp with the ramp voltage from the Discolight circuit (from pin 14, IC4). The ramp voltage is the key to the dimmer operation and is synchronised to the 50Hz AC mains. The ramp starts at its maximum value at the beginning of each AC mains half cycle and then decays away to a low value at the end of each half cycle. The ramp voltage is connected to the inverting input of each of the four comparators . When the voltage from the slider controls is low, the output from the comparators can only go high towards the end of each AC half cycle. This means that the associated Triac (in the Discolight) will turn on late in each AC half cycle and so the lamp brightness will be low. This is just what we want. Similarly, if the slider is pushed up to its maximum setting, the output of its buffer amplifier will be high and the comparator's output will go high early in each AC half cycle. Thus, the lamp will be bright. The output of each comparator drives the front panel LED of the Discolight and thus the Triac driver of its respective channel via a series diode (D105 for channel 1, D107 for channel 2, etc). PARTS LIST 1 PC board, code SC10111902, 79 x 90mm 1 Dynamark label, 122 x 108mm 1 aluminium panel, 122 x 108mm 4 knobs to suit slider pots 4 6mm pillars for slider pots 1 SPOT toggle switch (S105) 4 momentary contact pushbutton switches (S101, 102, 103, 104) 4 68A nylon screws and 1 2 nuts 4 screws and nuts for aluminium plate 4 50k0 linear slider pots 4 200kn miniature vertical trimpots 1 solder lug 1 600mm-length 8-way rainbow cable 1 200mm-length green/yellow mains earth wire An.extra diode, D106 for channel 1, connects between the front panel LED (LED 1 for channel 1) and foe output of IC8 of the Discolight circuit. This isolates the operation of the Discolight from the dimming circuit so that the outputs of the corn- 1 180mm-length 0.5mm tinned copper wire (for on-board links) 28 PC stakes Semiconductors 1 LF34 7, TL07 4 quad JFET op amp (IC101) 1 LM324 quad bipolar op amp (IC102) 14 1N4148, 1N914 signal diodes (D101-D114) Capacitors 2 4 7µF 1 6VW PC electrolytic 4 .047µF 1 OOVW metallised polyester (greencaps) Resistors (0 .25W, 5%) 4 1MO 1 3.3k0 4 1k0 Miscellaneous Solder, cable ties. parators do not attempt to drive the outputs of IC8. In effect, the diodes (D105, D106} form an OR gate so that the dimmer operation is combined with the Discolight operation. If each dimmer control is set to The leads running hack to the Discolight circuit should he hound together using cable ties to keep them away from internal mains wiring. Be sure to use a mains-rated green/yellow lead for the earth connection. minimum brightness, then the Discolight will operate normally. When the dimmer controls are advanced, the Discolight lamps will have a background brightness as set by the dimmers but Discolight operation will otherwise be normal. Toggle switch S 105 allows Discolight/dimmer operation or dimming alone. When S 105 is set to position 1, the Discolight is set to normal operation since the INHibit input of IC7 is connected to the wiper of switch S4a. When S 105 is set to position 2, the INH input of IC7 is tied to the + 12V rail and thus IC7 is prevented from operating and all its outputs are held low. IC8 therefore provides no Discolight functions. Flash switches Asssociated with each slider control is a pushbutton which functions as a "flash" switch. When you push the button it overrides the slider control and the lamp goes to full OCT0BER1990 75 r- 0 I I ffio C er ·~I er --' LL LL 0 :il: > 0 LL LL >< er ...J I z 0 a: w :il: :il: 0 c C!J (.) • !{l :::.'1 :::-i- l i M1~ ~ I >< er • I 0 C'-1 C'-1 a: w :c en C er LL 0 er ...J LL 0 I LL LL (') (') a: 0 Cw :c Cl) er 0 er ...J LL LL 0 :il: C ~I~:=: : I I LL LL :il: I- ::i 0 I >< er 0 :c I I >< er :il: I LL LL 0 -:f' -:f' a: :c er LL LL • ~o ~, 0 Fig.4: this full-size artwork can be used to mark out the front panel. brilliance. As soon as you take your finger off the button, normal dimmer operation resumes. Consider, for example, switch S101. This bypasses diode DlOl and feeds pin 12 of ICla directly, so that full lamp brilliance is obtained. Power for the added circuitry is taken from the regulated ± 12V supply rails in the Discolight. Construction While the additonal circuitry for the dimmer function is fairly simple, as just described, adding it to the Discolight is a bit fiddly. The most straightforward part is assembling the PC board. This done, you need to cut slots for the sliders in the top of the Discolight case, make up a control panel and then wire it all up. Before assembling any components onto the new board, carefully check it out for any faults such as shorted or open copper tracks or undrilled holes. Fix these 76 SILICON CHIP before you go any further. 28 PC stakes are required on the board. Put these in first and then the low profile components such as the ICs, diodes, links and resistors. Make sure that each IC and diode is oriented correctly as shown on the overlay diagram. Next, install the electrolytic capacitors, making sure that they go in the right way around. Finally, the trimpots can be mounted. Metalwork The next part of construction is the metalwork for the dimmer control panel. We made ours from a sheet of 1mm thick aluminium. The front panel artwork shows all the needed dimensions, as well as the holes and slots. The slots required for the sliders are best made by drilling rows of holes and finishing to size using needlepoint files. Once the panel is complete the Dynamark label can be secured and the holes cut out with a sharp utility knife. With the panel done, it needs to be mounted on the lid of the Discolight case. Again, use the panel as a template to drill holes. A large cutout can be made to accommodate the sliders, rather than cutting separate slots and so on . The panel should be located on the case top so that the sliders line up with their respective LEDs on the front panel. The dimmer board is then located next to the sliders, as shown in the photos. The PC board is supported on nylon screws and secured by brass or nylon nuts. We have specified nylon screws here because we don't want exposed screws on top of the case which need to be separately earthed. The dimmer control panel, for its part, is earthed back to the mains earth, as shown in the wiring diagram. Why the emphasis on earthing exposed screws, etc? It is to avoid the possibility of any metalwork becoming live to mains if one of the internal mains wires comes adrift. The four sliders are mounted on 6mm pillars. Mount these first, then install the four pushbuttons and the toggle switch. Now for the wiring. To keep this as neat as possible, use miniature rainbow cable between the dimmer board and sliders and between the dimmer board to the Discolight board. Panel wiring To make the wiring connections to the Discolight, you will need to follow the original wiring diagram as published in the August 1988 issue of SILICON CHIP. In order to wire to the front panel channel LEDs, you first have to disconnect the exisiting anode lead to each LED. You then connect the new wiring between the anodes of the LED and the four wires from IC8. Similarly, switch S105 is connected in series with the wire to the PC stake near pin 8 of IC9. Connect the pole of S105 to the PC stake and the other two wires from the switch as shown in the circuit and wiring diagrams. The ramp signal wire attaches directly to pin 14 of IC4. Just solder the mains wmng, either at the power switch or at the rear of the case. When complete, check your wiring thoroughly. Testing To ensure safety, nylon screws are used to mount the PC board on the lid of the case. Don't use metal screws - they could become live if one of the internal mains wires comes adrift. it directly to the IC pin. Similarly, the + 12V and - 12V supplies are obtained from the output pins of the 7805 and 7905 regulators respectively. Connect the 0V (GND) wire to the PC stake marked "6" on the Discolight wiring diagram. Finally, there must be a mains earth connection from the dimmer control panel on the ca se lid to the Discolight earth on the rear panel. Use the correct green/yellow mains earth wire for this and solder each end to the solder lugs on the dimmer control panel and rear panel of the Discolight. Use cable ties to secure all the wires tidily together. Tie them so that when the lid of the case is fixed in position, they do not come near Initial testing can be done without mains lights. All operations can checked by observing the front panel LEDs. Put the lid on the case and then power up the unit. Check that the LEDs can be lit by moving the slider controls for each channel up and down. Check that all Discolight functions are disabled when the toggle switch is set to the Dimmer position. Similarly, check that Discolight and dimmer functions can be run together when the toggle switch is set the other way. For a full test, connect lights to the Discolight power outlets. Now test that the slider controls effectively control the lamps from full brightness to off. Trimpots VR101, VR102, VR103 and VR104 can be adjusted to give maximum lamp brightness at maximum slider setting. ~ Check your wiring carefully before switching on to test the circuit. Take care - the wiring adjacent to the rear panel and the power switch is at mains potential. OCT0BER1990 77