Sunday, January 16, 2011

3 Volt LED Flasher Using 2 Transistor

Applications due to its simplicity and versatility. This Flasher circuit is great for beginners! If you build it, it will flash. And you can easily change the on-time and flash rate.

The two resistors on the base of the PNP set a threshold voltage and when power is applied the capacitor begins charging toward this voltage. When the capacitor voltage is high enough the two transistors begin to conduct. The current flow causes the voltage across the circuit to drop slightly and this drop causes a drop in the threshold voltage. The lower threshold voltage causes even more current and this positive feedback causes the circuit to rapidly turn on. It stays on until the capacitor discharges at which point a reverse process causes the circuit to suddenly switch off.
Note:
  • Flashing frequency can be varied by changing R1 value in the 1M - 4M7 range.
  • This circuit is very efficient when driving a small 3.2V incandescent lamp. In this case omit the LED and R3, connecting the bulb across Q2 Collector and positive supply, further reducing parts counting.
  • Maximum current drawing of the bulb type used should not exceed 100mA.
  • In order to facilitate oscillation when a bulb is driven, R2 value should be lowered to 82R or 68R.
Lis Componet

R1  : 1M   1/4W Resistor
R2  : 100R  1/4W Resistor
R3  : 22R   1/4W Resistor
C1  : 1µF/63V 
D1  : LED  
Q1  : 2N3906  PNP Transistor
Q2  : 2N3904  NPN Transistor
SW1 : SPST Switch
B1  : 3V (Two 1.5V AA or AAA battery)  
 

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