Especially designed for Christmas tree lamps
Replaces old thermally-activated switches
R1___________100K 1/4W Resistor
R2,R5__________1K 1/4W Resistors
R3,R6________470R 1/4W Resistors
R4____________12K 1/4W Resistor
C1__________1000΅F 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 2200΅F instead.
Best performances are obtained with C1=470 or 1000΅F 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 SCRs. 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.
Source: Electronics Lab