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Dark-activated LED or Lamp Flasher

Dark-activated LED or Lamp Flasher

Parts:

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________________220΅F 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.

Notes:

* Best results in flashing frequency can be obtained using for C1 a value in the 100 to 1000΅F 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 1000΅F 25V Electrolytic Capacitor
* In LED-mode operation the stand-by current consumption is less than 400΅A.
* In Lamp-mode operation the stand-by current consumption is about 3mA.

Source: Electronics Lab

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