LED Flasher Circuit - Flip Flop

The classic LED flasher flip-flop circuit. Two circuits using NPN and PNP transistors (August 2, 2009)


Two Transistor Led Flasher This two-transistor LED flasher circuit is very popular, it is also known as "flip-flop" oscillator. Perhaps you already know this circuit but here is presented in two versions: Using NPN transistors and PNP transistors.


Bill of materials:


  • 2 transistors
  • 2 470 ohms resistors
  • 2 10 uF capacitors
  • 2 100k variable resistors
  • 2 LEDs


    Two Transistor Led Flasher This circuit works fine using 3 volts and Red, Green or Yellow LEDs. In case you want to use any other LED color, more voltage is needed.


    Adjust the variable resistors to change the frequency and width of the pulse. Pulses are basically sinusoidal waves.


    How it works:


    The circuit is basically a transistor flip-flop. When the power is applied, both transistor are triggered through the 100k resistor, but only one can be activated as the other one cuts the signal for the other one. So the LED will turn ON and the base of the other transistor will be grounded through the capacitor. This grounding keep the first transistor to be activated and the second one to stay off until the capacitor is charged above the bias voltage needed to activate the second transistor. When the needed voltage is reached, the second transistor will be switched on, this will switch off the first transistor and the LED on its collector will lit. This alternating process will be repeated forever until the power supply is turned off.


    Capacitors can be replaced by another value (22uF for example) without any change to the circuit. LEDs can be replaced by 3V light bulbs and removing the 470 ohms resistors.


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