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Dark - Activated LED or Lamp Flasher
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Schematic design of Dark - Activated LED or Lamp Flasher
Part List :
Capacitors	Resistors
C1          =  220 μF / 25V Electrolytic Transistors Q1,Q2     =  BC337 Others SW1       =  SPST Switch B1          =  3V (2 x 1.5V AA or AAA cells in series, etc)	R1         =   Photoresistor (any type) R2         =   100K Ω ½W R3,R4    =     10K Ω ¼W R5         =     470 Ω ¼W R6         =       47 Ω ¼W Diodes D1         = 1N4148 D2         = LED  any type and color
Notes
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 100μF 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 Ω ½W Trimmer Cermet R3, R4 = 1K Ω ¼W R6       = 1 Ω ¼W C1       = 470 μF to 1000 μF / 25V Electrolytic In LED-mode operation the stand-by current consumption is less than 400 μA.   In Lamp-mode operation the stand-by current consumption is about 3mA.

Source : Electronics Lab