Dark-activated LED or Lamp Flasher

Simple photo-sensitive circuit

3V battery supply

Circuit diagram:

Dark-activated Flasher


R1_________________Photo resistor (any type)
R2________________100K   1/2W Trimmer Cermet
R3,R4______________10K   1/4W Resistors
R5________________470R   1/4W Resistor
R6_________________47R   1/4W Resistor

C1________________220µF   25V Electrolytic Capacitor

D1______________1N4148    75V 150mA Diode
D2_________________LED    Any type and color (See Notes)

Q1,Q2____________BC337    45V 800mA NPN Transistors

SW1_______________SPST Switch

B1__________________3V (Two 1.5V AA or AAA cells in series, etc.)

Circuit operation:

This circuit adopts the rather unusual Bowes/White emitter coupled multivibrator circuit. The oscillation frequency is about 1Hz and is set by C1 value. The LED starts flashing when the photo resistor is scarcely illuminated. The onset of flashing can be set by trimming R2.