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Dark-activated LED or Lamp Flasher

Simple photo-sensitive circuit

3V battery supply

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.


  • 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.

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