Dark-activated LED or Lamp Flasher

Posted in Light sensor, LED, LED & Light, on 2015-10-23

Circuit diagram:

Dark-activated LED or Lamp 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 - 1000µF range.
  • To drive a filament lamp make the following changes:
    • Use a 2.2 to 3V, 250-300mA bulb 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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