Two-wire Lamp Flasher

Ideal to operate 3 to 24V DC existing on-circuit lamps

LED operation is also possible

Circuit diagram

Two-wire Lamp Flasher-Circuit diagram


  • R1 6K8 1/4W Resistor
  • R2 270K 1/4W Resistor
  • R3 22K 1/4W Resistor
  • C1 220µF 25V Electrolytic Capacitor
  • C2 10µF 25V Electrolytic Capacitor
  • D1 1N4002 100V 1A Diode
  • Q1 BC557 45V 100mA PNP Transistor
  • Q2 BD139 80V 1.5A NPN Transistor
  • LP1 Existing filament Lamp: any type in the range 3-24V 10W max.
  • SW1 Existing On-Off switch
  • B1 Existing V DC source: any type in the range 3-24V
  • suited to the lamp adopted

Device purpose:

This circuit has been designed to provide that continuous light lamps already wired into a circuit, become flashing. Simply insert the circuit between existing lamp and negative supply. Especially suited for car or panel pilot lights, this device can drive lamps up to 10W.


  • Break lamp(s) to negative supply connection(s), then insert the circuit between existing lamp(s) connection(s) and negative supply (respecting polarities!).
  • C1 value can be varied from 100 to 1000µF or higher, in order to change flashing frequency.
  • Although rather oversized, this circuit can also drive any LED, providing a suitable resistor is fitted in series with the light emitting device.
  • The resistor should be in the range 47R to 2K2, depending on supply voltag
author: RED Free Circuit Designs
circuit from