Dark Activated Led or Lamp Flasher


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.

Circuit Diagram:

Dark Activated Led or Lamp Flasher-Circuit Diagram


  • R1 = LDR
  • R2 = 100K
  • R3 = 10K
  • R5 = 470R
  • R6 = 47R
  • R4 = 10K
  • C1 = 220uF-25V
  • D1 = 1N4148
  • D2 = LED any type (see notes)
  • Q1 = BC337
  • Q2 = BC337
  • B1 = 3V Battery or 2x1.5V cells in series
  • SW1 = SPST Switch


  • Best results in flashing frequency can be obtained using for C1 a value in the 100 - 1000µF ranges.
  • 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.