Thursday, August 28, 2014

Dark activated LED or Lamp Flasher

This schema adopts the rather unusual Bowes/White emitter coupled multivibrator schema. 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.





Dark-activated




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________________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.)




Notes:

* 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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