Silicon Chip Online - Automatic Single-Channel Light Dimmer

The Automatic Light Dimmer is constructed on three PC boards: a Display board coded 10104021 and measuring 89 x 102mm; a Controller board coded 10104022 and measuring 84 x 96mm; and a filter PC board coded 10104023 and measuring 52 x 59mm. These are housed in a diecast box measuring 171 x 121 x 55mm.

You can begin by checking each PC board for any possible problems before they are assembled. In particular, check the pattern against the published artwork for any shorts or breaks in the tracks. Also check the hole sizes drilled to ensure they are suitable for each component. Note that switches S2-S4 and S6-S8 need slotted holes to suit their terminals. The component overlays for all three boards are shown in Fig.4.

Start assembly of the Display PC board by installing the links and diodes. Place the four DIP20 IC sockets in the LED1-40 positions on the PC board. The 10-LED bargraphs need to be inserted with the correct polarity. One of the corners of the LED bargraph package has a small chamfer down one edge. This is the cathode (K) side of bargraph and should be installed in the sockets as indicated on the overlay diagram in Fig.4.

WARNING!

Part of the circuitry used in this Automatic Light Dimmer operates at 240VAC (see Figs.1 & 6) and is potentially lethal. Do not touch any part of this circuit while the unit is plugged into the mains and do not operate the circuit outside its earthed metal case.

This project is for experienced constructors only. Do not build it unless you are entirely familiar with mains wiring practices and construction techniques.

Now install the 8-way right angle pin headers for connectors CONA, B, C and D. Switch S5 can be installed with the "flat" side oriented as shown. Do not install the remaining switches. LED41 is mounted so that the top of its convex lens is 10mm above the top of the LED bargraphs.

Work can now begin on the Controller PC board by installing the links and IC2, IC3 and IC4. IC1 uses a socket. Take care with orienting the ICs and socket and note that IC3 is oriented differently to IC2 and IC1.

When installing the resistors, use the resistor code table as a check when selecting each value. You can also use a multimeter to check each value. The eight 4.7kΩ resistors near IC3 are mounted vertically, ie, end-on.

Diodes D13 and D14 can be installed as well as Q1-Q5, taking care with their orientation. Install the 20MHz crystal and the capacitors, taking care to orient the electrolytic capacitors with the correct polarity, as shown. Note that the 1μF bipolar electrolytic can be installed either way.

Install the PC stakes near D13 and D14. Triac1 and REG1 are mounted under the PC board as shown in Fig.5. The leads are bent at right angles and soldered to the PC board so that the device tabs are 6mm below the bottom of the PC board. Bend the leads over on the top of the PC board to secure them in position before soldering.

Inductor L1 is wound on a Neosid 17-730-22 or Jaycar LO-1244 powdered-iron core using 1mm enamelled copper wire. Wind on 16 turns evenly spaced around the toroid and terminate the leads in the PC boards holes. You will need to scrape away the enamel insulation at each end of the wires and tin them with solder. After soldering the leads to the underside of the PC board, the inductor is secured with two cable ties, each wrapping around the toroid and through holes in the PC board.

Install the four 8-way vertical pin headers as shown.

Making the filter board

The filter PC board is assembled with the 0.1μF 250VAC capacitor soldered in place next to the 4.7MΩ resistor as shown. Inductor L2 has two windings on the one Philips 4330 0303 4220 toroid, using 1mm diameter enamelled copper wire.

Wind on 11 turns evenly spaced on one side of the toroid. The second winding needs to be wound as a mirror image of the first, so be careful with the winding direction. In other words, if the turns on the first winding go clockwise around the core, the turns on the second winding MUST go anticlockwise. Make sure you get this RIGHT!

Wind on 11 turns as before. Terminate the windings after scraping and tinning the wire ends. We secured the toroid using two 100mm long cable ties around it and the bottom of the PC board. This holds the toroid in position but also allows some movement, preventing it from shattering if the assembled Automatic Light Dimmer is dropped. Note that this ferrite toroid is rather brittle compared to the more robust powdered iron toroid used for L1.

Making the header cables

There are four cables to be made up using polarised 8-way pin header plugs and 8-way rainbow cable. The "A" and "B" cables are 90mm long, the "C" cable is 180mm long and the "D" cable is 135mm long.

The header cables are made up by first crimping the stripped wire ends into the pins. You may also wish to apply a small amount of solder to each crimp connection to ensure a good contact. The pins then clip into the header plug.

Note, however, that the wiring connections to the polarised header plugs are quite critical. For this reason, be sure to follow the wiring details set out in the next paragraph exactly.

Here's how to wire the four cables: on each of the "A" and "B" cables, connect pin 1 on one header plug to pin 8 on the header plug at the other end, pin 2 to pin 7 and so on. Conversely, for the "C" and "D" cables, connect pin 1 of one header plug to pin 1 of the header plug at the other end, pin 2 to pin 2 and so on.

Mark the ends of each cable with a felt-tipped pen as it is completed, so that you don't get them mixed up later on. That way, when the header sockets are plugged into the pin headers on the Controller PC board, all the connections will be correct, since the headers will only fit one way.

Once the cables have been completed, plug them into the Display PC board, so that it is ready for mounting onto the lid of the case. Take care not to transpose cables "B" and "D" - remember that Fig.4 shows the Display board from the component side.

Drilling the case

The lid of the case will require drilling for the switches, LED bargraph displays and the 10mm LED. Use the front panel artwork as a guide to the hole locations and use the display PC board to measure the exact positions. The rectangular holes for the switches and LED bargraphs can be made by drilling out a series of holes within the cutout area and then filing to shape.

Of course, if you buy a kit, all the drilling will have been done for you. We fitted clear red Perspex windows into the LED bargraph cutouts.

When the drilling and filing of the lid is complete, attach the front panel label and cut out the holes with a sharp hobby knife. Insert the switches into the holes allocated, making note of these important points: (1) Switches S1 and S4 need to be oriented so that the switch contacts are closed when set to the Flash and Rate B positions; (2) Switch S2 is oriented so that its contacts are closed when set to the Manual dimming position; and (3) Switches S3, S6, S7 & S8 can be mounted either way but it is best to orient them all with the same switch legend positions. We elected to place these four switches with the double bar toward the top.

The display PC board is mounted by soldering it directly onto the switch terminals.

Now that the lid is finished, the case itself requires a fair amount of drilling. Mounting holes are required for the controller and filter PC boards, the regulator, Triac, earthing lug and the transformer. As well, one end of the case requires holes for the power switch S9, fuse F1, cord-grip grommet and the surface mount General Purpose Outlet (GPO) mains socket. Two holes are required to mount the socket and three holes are for the grommets that allow the wires to pass through the panel into the GPO terminals.

Transformer T1 is mounted on the side of the case at an angle so that it does not foul the display PC board when the lid is placed on the box. One of the mounting feet needs to be bent up so that it fits against the integral pillar in the case. Use countersunk screws to mount the transformer so that they are flush with the outside of the box.

Installing the hardware

We are now in the final assembly stage where everything is wired together, as shown in Fig.7.

Install the mains cord first. Strip back about 200mm of the outer sheath of the mains cord so that there is sufficient lead length for all the mains wiring. Cut off 50mm and 100mm lengths of the brown Active wire from the mains cord and solder the 50mm length to provide the connection between the filter board and the controller PC board. The 100mm length provides the connection from the fuseholder to the controller board (shown as "X" on Fig.7).

Apply a smear of heatsink compound to the underside of the Triac tab and regulator tab before bolting them down. Note that the specified Triac has an isolated tab which means that the metal flange on the device is not connected to any of the pins. You can check this by measuring the resistance between the tab and the pins with a multimeter.

If the resistance shows a short circuit then the Triac is not an isolated tab type and should not be used. An open circuit measurement will show if the Triac is the correct isolated tab device.

Mount the controller PC board on 6mm Nylon spacers using 15mm long screws. Note that the screw near Triac1 must be a Nylon screw (see Fig.7). Secure the regulator and Triac to the case with 8mm long screws and nuts.

Fit the mains cord into the hole in the case with a cordgrip grommet. Make sure that this cord cannot be pulled out from the case once secured in place. The green-yellow earth wires are attached to the case using crimp eyelets and a screw, two star washers and two nuts - see Fig.6.

Fit switch S9, fuseholder F1, the GPO socket and its grommets and wire switch S9 and fuseholder F1 as shown. Be sure to sheath the fuseholder terminals, with 10mm diameter heatshrink tubing and the switch terminals with 5mm heatshrink tubing.

Mount the transformer now. Note that the switch terminals will need to be bent over slightly to allow clearance for the transformer.

Connect the wiring to the filter PC board and to the GPO before mounting the filter PC board on 25mm screws and 15mm Nylon spacers. Note that a Nylon screw and nut is used for the corner mounting position on the filter PC board. Do not use a metal screw in this position otherwise there is a risk of flashover. This applies to any position where we have specified Nylon screws - see Fig.7.

Connect the transformer secondaries to the Controller PC board as shown. The mains wiring needs to be tied so that if one wire should come adrift it will still be secured in place with another wire connection. Use cable ties to strap the wires in place on the switch, the back of fuseholder F1 and the wires on the filter PC board.

Finally, attach four rubber feet to the base of the case.

PLEASE NOTE!

The scope waveformes in this article are shown to explain the operation of the circuit. DO NOT try to reproduce these waveforms yourself - it is much too dangerous.

Presspahn insulation

As shown in the accompanying photo, a folded piece of Presspahn or Elephantide insulation is used to cover the high-voltage section of the circuitry (apart from the mains switch). This serves to isolate the low-voltage cable wiring from those components operating at mains potential when the lid is attached and is important for long-term reliability. Fig.8 shows the cutout and folding details for the insulation material.

Testing

Check your wiring very carefully, making sure all the wires are connected to the correct positions. Also check that the mains Earth lead is connected to chassis and that the Earth terminal on the GPO is also connected to chassis. You can check that there is continuity from the Earth pin on the mains plug to chassis and to the Earth connection on the GPO using a multimeter.

The Automatic Dimmer is best initially tested with the lid secured in place with its mounting screws. Apply power and check that the display LEDs light. If the LEDs do not light, switch off power immediately and disconnect the power plug. Check that the fuse is intact and that the switch was turned on.

Check that the LED bargraphs have been installed correctly. Also you may need to check the supply voltages on IC1 and IC2 for 5V. There should be 5V between pins 5 and 14 on IC1 and between pins 8 and 16 on IC2. Be careful when checking voltages since the righthand side of the PC board has mains voltages on it. This includes the MOC3021 (IC4), L1 and the adjacent 360Ω and 470Ω resistors and the Triac leads.

If the displays are operating, you should be able to set the dim and flash levels and the rates using the up and down set level switches. Also the LED lamp indicator should light according to the dimming level. Check operation of the autodim function and the flash.

You can set the filament preheat by connecting up an incandescent lamp via the GPO socket. Note that you must use a lamp that is at least 60W. A smaller bulb rating may flicker since the Triac holding current is too high for these.

Press all three lefthand switches (Store Settings, Set UP and Dim Up switches) and wait as the slow increase in voltage applied to the lamp is sufficient to provide a soft glow in the filament. Release the switches and press the Store Settings switch to retain this value of preheat.

Note that the dim up and set up switches may have altered settings at the time the filament preheat adjustment was made. You can store the dim and flash levels and A and B rates that you set on the display at any time using the store settings switch.

Parts List

1 display PC board, code 10104021, 89 x 102mm
1 controller PC board, code 10104022, 84 x 96mm
1 filter PC board, code 10104023, 52 x 59mm
1 front panel label, 116 x 168mm
1 diecast aluminium box, 171 x 121 x 55mm
2 10mm x 51.5 x 2mm clear red Perspex or Acrylic
1 sheet of Elephantide or Presspahn insulation, 100 x 160mm
1 30V centre-tapped 5VA mains transformer with internal thermal fuse (T1) (Jaycar MM-2007 or equivalent)
1 iron powdered toroid, 25 x 13 x 10mm approx, AL 77 (L1) (Neosid 17-730-22 or Jaycar LO-1244)
1 Ferrite toroid, 37 x 22 x 16mm approx, AL 7390 (L2) (Philips 4330 0303 4220 or Jaycar LO-1238)
2 SPST ultra-mini rocker switch (S1,S4) (Jaycar SK-0975, Altronics S-3202 or equiv.)
4 SPDT centre off spring return rocker switch (S3,S6,S7,S8) (Jaycar SK-0987 or equiv.)
2 SPST mini rocker switch (S2, S9) (Jaycar SK-0984 or equiv.)
1 snap action keyboard switch (S5) (Jaycar SP-0721, Altronics S-1096 or equiv.)
1 20MHz parallel resonant crystal (X1)
1 10A surface mount mains outlet (HPM Cat. NO.35)
1 2AG panel-mount safety fuse holder
1 10A 2AG fuse
1 10A mains plug and lead
1 cord grip grommet to suit mains lead
4 DIP20 IC sockets (for LEDs1-40)
1 DIP18 IC socket (for IC1)
3 9mm rubber grommets
8 8-way 0.1-inch polarised header plugs (Jaycar Cat. HM-3408 or equivalent)
4 8-way 0.1-inch polarised pin headers (Jaycar Cat. HM-3418 or equivalent)
4 8-way 0.1-inch polarised right-angle pin headers (Jaycar Cat. HM-3428 or equivalent)
1 550mm length of 8-way rainbow cable
2 M3 x 10mm countersunk screws
2 M3 x 8mm pan head screws
1 M3 x 12mm pan head screw
2 M3 x 20mm pan head screws
1 M3 x 15mm Nylon screw
3 M3 x 15mm metal screws
1 M3 x 25mm Nylon screw
1 M3 x 25mm metal screw
2 15mm M3 tapped plastic standoffs
4 6mm M3 tapped plastic standoffs
12 M3 nuts
2 M3 Nylon nuts
2 M3 star washers
2 crimp eyelets
4 rubber feet
1 45mm length of 10mm diameter heatshrink tubing
1 20mm length of 4mm diameter heatshrink tubing
10 100mm long cable ties
1 2.3m length of 1mm diameter enamelled copper wire
1 150mm length of 0.8mm tinned copper wire
3 PC stakes

Semiconductors

1 PIC16F84A-20P microcontroller programmed with autodim.hex
1 4017 decade counter (IC2)
1 ULN2803 8-way Darlington driver (IC3)
1 MOC3021 Triac driver (IC4)
1 7805 3-terminal regulator (REG1)
1 BTA41.600B 40A 600V insulated tab Triac (TRIAC1)
5 BC328 PNP transistors (Q1-Q5)
12 1N914, 1N4148 diodes (D1-D12)
2 1N4004 diodes (D13,D14)
4 10-red LED bargraph displays (LED1-LED40)
1 10mm red LED (LED41)

Capacitors

1 470μF 25VW PC electrolytic
2 10μ;F 25VW PC electrolytic
1 1μF bipolar electrolytic
2 0.1μF 250VAC X2 class
2 0.1μF MKT polyester
1 .047μF 250VAC X2 class
2 22pF NP0 ceramic

Resistors (0.25W 1%)

1 4.7MΩ Philips VR37 high-voltage type (DO NOT substitute)
1 100kΩ
2 470Ω
2 10kΩ
1 360Ω
8 4.7kΩ
2 220Ω
2 2.2kΩ
5 47Ω
5 680Ω

Table 1: Resistor Colour Codes

No.	Value	4-Band Code (1%)	5-Band Code (1%)
1	4.7MΩ (VR37)	yellow violet green yellow
1	100kΩ	brown black yellow brown	brown black black orange brown
2	10kΩ	brown black orange brown	brown black black red brown
8	4.7kΩ	yellow violet red brown	yellow violet black brown brown
2	2.2kΩ	red red red brown	red red black brown brown
5	680Ω	blue grey brown brown	blue grey black black brown
2	470Ω	yellow violet brown brown	yellow violet black black brown
1	360Ω	orange blue brown brown	orange blue black black brown
2	220Ω	red red brown brown	red red black black brown
5	47Ω	yellow violet black brown	yellow violet black gold brown