Originally published by Paul Stenning in ETI, December 1994

• Diagrams and Schematics

Christmas just wouldn't be Christmas if your favourite electronics magazine didn't publish a hi-tech gadget for flashing your tree lights!

This year we present a straight-forward fixed pattern flasher for three sets of lights. However this is not your usual 1 - 2 - 3 chasing pattern, this unit has six steps, 1 - 1+2 - 2 - 2+3 - 3 - 3+1, and then repeated. If the three sets of lights are mixed on the tree, it is not that easy to see the pattern! On the prototype the speed was fixed to about three steps per second, but it would be a simple matter to put a control pot on the front panel to set the mood if required.

The triac outputs are driven from zero-crossing opto isolators, which virtually eliminate radio interference. The triacs are hard driven, making the outputs suitable for driving inductive loads such as the modern low voltage transformer driven Christmas lights. The outputs can drive loads of up to 3 Amps (or possibly even more if the PCB tracks are reinforced), making the unit suitable for driving higher powered outdoor lights.

THIS PROJECT OPERATES FROM THE MAINS. MAINS VOLTAGE IS POTENTIALLY LETHAL. DO NOT CONSTRUCT IT UNLESS YOU ARE CERTAIN OF YOUR ABILITY TO DO SO SAFELY.

How it Works
The complete circuit diagram is shown in figure *. The low voltage section of the circuit is powered by a small mains transformer. Although this is slightly more expensive than a mains derived circuit using a dropper resistor or capacitor, it is more reliable, cooler running than a resistor and easier to build.

IC1 (NE555) is the main oscillator. If a variable speed is required, R2 can be replaced with a 22K pot and a 1K0 resistor in series. The output drives the clock input of IC2 (4017), a decimal counter. Outputs Q1 to Q5 if IC2 go high in turn, on a clock pulse. When Q6 goes high the device resets itself due to this line being linked to the Reset pin. The outputs are decoded into the desired pattern by the three OR gates in IC3.

The outputs of IC3 drive transistors TR1, TR3 and TR5, which in turn operate the LED's within the opto-isolators (IC4, IC5 and IC6) and the front panel LED's (D1, D2 and D3).

The MOC3041 opto-isolators contain full zero crossing circuitry, and a triac output stage. They are ideally suited to driving triacs in this manner, since they do all the hard work for you! The LED current for guaranteed operation is 15mA max. Other devices in this useful family include the MOC3040 which needs a 30mA input (it is slightly cheaper), and the MOC3020 which does not have the zero crossing circuit.

The triacs used in the prototype were BT137 types, however most TO220 packaged triacs should be suitable, including C206M, C225M, C226M, BT138, BT139, BTA08-600B etc. If you are likely to be driving inductive loads (or you are not sure) use 600V devices, otherwise a 400V devices are suitable.

The outputs are individually fused. The F-500mA fuses are suitable for normal indoor light sets, but a higher value may be needed for outdoor light sets. Do not use anti-surge fuses (T type, eg T-500mA) since these will not adequately protect the triacs.

Construction.
The circuit is constructed on a single sided PCB. Construction is straight-forward, and requires little comment from me. SK1 to SK4, and X1 position are fitted with PCB mounting terminal blocks. LED's D1 to D3 should be mounted about 20mm above the board and bent forward to line up with the holes in the front panel. Don't forget the three short links near IC3.

The prototype was constructed in a plastic case, 190mm * 165mm * 68mm, see parts list for details. A suitable overlay for the front panel is shown in figure *. Two photocopies may be taken (enlarge to 162mm * 64mm), one can then be used as a drilling template while the other may fixed to the front panel with clear self-adhesive vinyl (sticky-backed plastic, as they say on Blue Peter!). Three 6.35mm (1/4") holes are needed for the LED clips.