Christmas just would not be Christmas if your favourite electronics magazine did not publish a hi-tech gadget for flashing your tree lights.
How it WorksThe 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.
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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.
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