How to build Dark-activated LED or Lamp Flasher

Simple photo-sensitive circuit
3V battery supply

Circuit diagram

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.

Best results in flashing frequency can be obtained using for C1 a value in the 100 to 1000µF range.
To drive a filament lamp the following changes must be made:
Use a 2.2 to 3V, 250-300mA lamp in place of the LED
R2 = 10K 1/2W Trimmer Cermet
R3, R4 = 1K 1/4W Resistors
R6 = 1R 1/4W Resistor
C1 = 470 to 1000µF 25V Electrolytic Capacitor
In LED-mode operation the stand-by current consumption is less than 400µA.
In Lamp-mode operation the stand-by current consumption is about 3mA.

Author: RED Free Circuit Designs

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