Automatic headlight dim switch.


With this cool circuit integrated to your cars headlight system , you can drive cool headed in high beam.The circuit will take over the duty of low beaming the headlight when vehicles approach against , and high beams the lights when they pass over.

The circuit is based on a photo transistor(Q1) for sensing the approaching vehicles and transistor Q2 (BC177) for switching the relays for controlling the headlight.When the light from the opposite vehicle falls on Q2 , it’s collector current increases and turns ON Q1.The relay will be activated and the head light will be dimmed.When the vehicle pass over the reverse will happen.

Circuit diagram with Parts list.

Automatic headlight dim switchjpg


  • The battery B1 can be the 12V car battery itself.
  • Adjust POT’s R1,R2,R4 to obtain optimum performance.
  • The Q1 can be any general purpose PNP photo transistor.It should be mounted in front of the car so that the light from opposite vehicle easily falls on it.
  • Relay contact A goes to high beam circuit,B to low beam circuit and C is the common point.
  • Carefully understand the wiring of your car before attempting to install the circuit.Wrong connections could easily bring trouble to your cars wiring.

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