Having built the power supply, the chaser board and, perhaps, the dark space/strobe board we now come to the output interfaces.

Interface Board

The interface board is capable of switching 1kw of lighting per channel. Unfortunately the standard mains socket/plug combination restricts us to a total of 13 Amps — about 750 watts per channel (3/4kw). In practice a 650w, quartz halogen lamp (eg A1 247) gives a hell of a lot of light!

Parts List

8 off BC173 or BC108 NPN transistor
8 off 1N4148 diode
4 off ISOLATED TAB Triacs 400V, 4A or heavier, eg TXAL228B
4 off 220 ohm miniature horizontal preset
4 off 47 ohm resistor, 1 watt
4 off 1 Kohm resistor
4 off 4.7 Kohm resistor, All ¼ watt
4 off 10 Kohm resistor
0.1" pitch Veroboard 20 rows of 36 holes
Insulated hook up wire

Construction

The board (see Figs one and two) contains four identical circuits — one for each channel — and contains all the components except the triacs. The only exceptions to the symmetry of the layout are the five wire links. The board contains 20 track cuts and these should be made first, being particularly careful to make them in exactly the correct places — there is no room for error! Marking out, prior to cutting, with a felt tip pen is highly recommended.

Points to note

1. All the diodes and all the resistors except the 1kohm are mounted vertically.

2. All the diodes are mounted so that the painted band sits on the board.

3. The BC173 transistors look like a D when viewed from above — make sure they are the correct way round. If NC108s are used the leg nearest the tag is the emitter.

4. The lower left hand tag on the presets is bent away and is not connected to the board. This tag can be cut off with sidecutters if space is tight.

5. Soldering of diodes and transistors should not be prolonged as they are damaged by excessive heat — blow them cold and allow time to cool between soldering legs. Stand the transistors as high as you can off the board.

6. The input and gate connections for each channel are grouped and it is sensible to colour code the wires.

7. Be extra vigilant about solder bridges between veroboard tracks — a desolder pump, desolder braid or heating the joint and then rapping the board on a hard surface all work in clearing them.

Testing

Hook up the nine volt and 0 volt power rails by connecting a wire from chaser board A32 to interface board C3 and another from chaser board N9 to interface board R3. Connect a test LED and resistor (see last month) to the 0 volt track on the board (track R). Touch the flying lead to the C32-K32 link to test the LED — it should light. Now touch the flying lead to each of the gate wires in turn — the LED should not light. Touch the flying lead to one of the gate leads and then touch the relevant input lead to the nine volt supply (eg C32-K32 link) — the LED should light and turning the preset fully clockwise should cause a fractional increase in brightness. Repeat the tests for the other three gate/input combinations.

Fitting the triacs

The triacs must be isolated tab type — don't allow a shop to fob you off with non isolated types plus an insulating kit — unless you are experienced at using these they spell danger. If there is any doubt get the shop to test the isolation with a meter. The cost of triacs varies enormously so it pays to shop around — the best I've found were isolated tab, TXAL228B at just under 80p each (TK Electronics, (Contact Details)).

Fit the triacs to the metal case as shown in Figure three using 4BA bolts, nuts and shakeproof washers. Tighten them down hard, preferably using a drop of screwlock adhesive — you don't want them to shift! A smear of Vaseline between the case and the metal tab helps heat sinking. Carefully bend the triac legs as shown making sure that they are bent away from the metal of the case.

Mains sockets

The type of mains outlets you use is a matter of personal choice. The two options I would recommend are either:-

1. Bulgin type fingerproof, three pin, 250 volt, three amp (4 off) e.g. RS Components stock no 488-567 cable plug. 488-573 chassis socket.

or

2. Bulgin P552 eight pin socket and plug (1 off) eg RS Components stock no 487-384 cable plug. 487-378 chassis socket.

The second type is used on many commercial units and, provided you stick to the wiring diagram, sometimes gives you the option of plugging the inhouse lighting rig into your chaser.

Connecting it up

1. Strip the outer covering from a suitable length of 13 amp, three core mains cable to give three separate cables; brown, blue and yellow/green.

2. Using a sharp knife pare away ¼" of insulation, at appropriate places, to match the triac spacing along the blue cable. Tin the bared wire with solder.

3. Solder these points to the triacs as shown to provide the neutral rail, being careful not to strain the triac legs.

4. Take the end of the cable to the mains terminal block and connect to the neutral terminal.

5. If you are using three pin sockets, use similar techniques to link the earth and live pins, once again returning the cables to the mains terminal block.

With the P552 socket connect the earth and live wires to pins one and seven/eight respectively, returning them to the mains terminal block.

6. Connect the centre leg of each of the triacs to the appropriate pins using blue three amp cable (taken from two or three core mains cable).

7. Link the gate wires from the interface board to the triacs as shown.

8. If the footswitch option is used, connect the all/chase lead from E33 of the interface board to the empty terminal (Z) on the footswitch terminal block. If not, E33 can be wired through a toggle switch to the nine volt line (eg interface board C33) or discarded if the chase override function is not wanted.

9. Connect up the inputs on the interface board as shown in Table One. If the dark space/strobe board is used then the inputs connect to the rotary switch, if not they connect directly to the chaser board.

Setting up

WARNING: KEEP YOUR FINGERS AWAY FROM THE TRIACS AND MAINS SOCKETS — THEY ARE LIVE!

1 Use a small screwdriver to turn the four presets fully anticlockwise.

2. Plug in the mains lamps and plug the unit into the mains.

3. If you have a footswitch, switch on (LED off) and select all (LED off) — if not select all with the toggle switch. (Individual lamps may or may not come on due to variations in the triac driving requirements).

4. Slowly turn the preset for channel A until the lamp comes up to full brightness — don't set the preset any further clockwise than you need to.

5. Repeat for the other channels.

6. Cancel the all switch and check that all the lamps are triggered full on by the chase sequence(s). If not, trim the presets as required. If a lamp flashes dimly at the same time as another is full on then the dimly flashing lamp's preset needs backing off slightly by turning a whisker anticlockwise.

Rope light and LED Interfaces

These are based on a similar circuit to the one we've just used. By replacing the right hand transistor of each pair with a BFY51, and putting the LEDs, each with an 820 ohm resistor in series, in place of the 47 ohm and preset, the board converts to an LED driver board. For a rope light board the thyristors (CI06D) can be driven directly by the other circuitry. The outputs from the chaser board or rotary switch are taken (through diodes as on the triac board) directly to the gates of the thyristors. Each of the all/chase diodes are taken to the footswitch via 1 kohm resistors. The thyristor cathodes are commoned to the mains neutral rail and the anodes are taken to the rope lights, the other side of which are commoned to the mains live rail. I would recommend that you use wire ended neons with 100kohm resistors in series as the lamps in your rope light. If every fourth neon is wired to the same channel then the familiar rope effects are available. Some thyristors tend to be over-sensitive and the neons may suffer from the odd flash that is out of place. If this happens simply strap a 1 kohm resistor between the gate and cathode of each thyristor and this will suppress it. The thyristors SHOULD NOT BE BOLTED TO THE CASE as the tabs are live! Simply stand them upright on the circuit board and make sure the tabs don't touch anything.

Bring back the rope

The neons can be wired up into four channels with a common live rail and then fitted into a suitable length of tubing. I found mine in Woolworth under the guise of clear red garden hose but plumbers' merchants and aquarium supplies shops are also handy sources of clear plastic tubing. To make life easy for yourself make sure the internal diameter of the tubing is at least ½" so that the neons slide in easy (I love that bit!). The unit will run up to 100 ft of rope but it's probably more sensible to build five 20 ft ropes if you really want to fill the stage with lights.

So there we are, everything you always wanted to know about stage lighting but were afraid to ask.

Next month:- Drumatix owners reserve your copy of the January edition now — you'll kick yourself if you miss next month's Workbench.