R1___________100K 1/4W Resistor R2,R5__________1K 1/4W Resistors R3,R6________470R 1/4W Resistors R4____________12K 1/4W Resistor C1__________1000µF 25V Electrolytic Capacitor D1-D4_______1N4007 1000V 1A Diodes D5__________P0102D 400V 800mA SCR Q1___________BC327 45V 800mA PNP Transistor Q2___________BC337 45V 800mA NPN Transistor PL1__________Male Mains plug SK1__________Female Mains socket
This circuit is intended as a reliable replacement to thermally-activated
switches used for Christmas tree lamp-flashing. The device formed by Q1, Q2 and
related resistors triggers the SCR. Timing is provided by R1,R2 & C1. To
change flashing frequency don't modify R1 and R2 values: set C1 value from 100
to 2200µF instead.
Best performances are obtained with C1=470 or 1000µF and R4=12K or 10K. Due to low consumption of normal 10 or 20 lamp series-loops intended for Christmas trees (60mA @ 220V typical for a 20 lamp series-loop), very small and cheap SCR devices can be used, e.g. C106D1 (400V 3.2A) or TICP106D (400V 2A), this last and the suggested P0102D devices having TO92 case.
For proper operation it's absolutely necessary to employ high Gate-sensitive
If you are unable to find these devices you can use Triacs instead. In this case the circuit operates also with relatively powerful devices. A recommended Triac type is the ubiquitous TIC206M (600V 4A) but many others can work.
Note that in spite of the Triac, diode bridge D1-D4 is in any case necessary.
This circuit was awarded with publication in ELECTRONICS WORLD "Circuit Ideas", June 2000 issue, page 458.