Dark-activated 230V Lamp

Compact circuitry using small-size parts only

Can be wired in parallel to existing switches

Circuit diagram:

Dark-activated Lamp


R1_____________Photo resistor (any type)
R2____________100K     1W Resistor
R3____________200K   1/2W Trimmer Cermet
R4,R7_________470R   1/4W Resistors
R5_____________12K   1/4W Resistor
R6______________1K   1/4W Resistor

C1_____________10nF   63V Polyester Capacitor

D1_________TIC106D   400V 5A SCR
D2-D5_______1N4007  1000V 1A Diodes

Q1___________BC327    45V 800mA PNP Transistor
Q2___________BC337    45V 800mA NPN Transistor

SK1__________Female Mains socket

PL1__________Male Mains plug & cable


This device allows one or more lamps to illuminate at sunset and turn off at dawn.
Q1 and Q2 form a trigger device for the SCR, providing short pulses at 100Hz frequency. Pulse duration is set by R2 and C1.
When the light hits R1, the photo resistor assumes a very low resistance value, almost shorting C1 and preventing circuit operation. When R1 is in the dark, its resistance value becomes very high thus enabling circuit operation.