Especially designed for Christmas tree lamps

Replaces old thermally-activated switches

 

Parts:

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

Device purpose:

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.

Important Note:

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.

This circuit was awarded with publication in ELECTRONICS WORLD "Circuit Ideas", June 2000 issue, page 458.