Automatic headlight dim switch.

admin April 7, 2010 24 Comments


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.



  • 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.

  • goldenflower
    February 23, 2015

    what will be the range of distance from which the phototransistor can sense the light? is it good enough to sense the upcoming vehicle at a safer distance?how optimum performance is achieved through the pots?

  • Seetharaman
    March 27, 2014

    The collector and emitter of Q2 should be interchanged. Q1 emitter should return to +ve. why so many pots? The circuit should be made in such a way it should have an head light auto / bypass switch, in the event of something going wrong in the circuit. Author please respond.

  • aditya
    March 26, 2014

    sir . can we make the same circuit using a 8051 microcontroller? if yes , please do reply.

  • January 4, 2014

    Guys.. can you tell me which software is used for designing this circuit?
    I am confused.. Multisim, Proteus or something else?


  • June 23, 2013

    sir,can i have the all diagram of the automatic headlight because i need it ?if its okay for you.

  • venkat
    March 10, 2013

    pls send abstract&circuit diagram about automatic vehicle dim/bright for miniproject

  • Naveen
    November 23, 2012

    Sir i need complete details of this project, please send the details to my email.

  • Naveen
    November 23, 2012

    Sir i need complete details of this project, please send the details to my email

  • Ricky
    October 31, 2012

    does anyone have idea how to transport light from headlight to sensor? what should e the size of sensor so that it can have sufficient light to trigger the dimmer mechanism?

  • April 12, 2012

    Hi,Please send the brief description about Automatic Flash Headlight using Dim and Dipper control(Not more than lines) Please send the answer to this mail id.

  • mathew
    December 30, 2010

    can anyone please tell me where to find automatic headlight dimmer circuit with a micro controller

  • seetharaman
    March 7, 2010

    PNP photo transistors (OCP71) may not be available readily in the market. if you have patience you can make yours by modifying a standard PNP TO18 metal transistor BC177. just carefully file the top off, use alchohol and clean the jel inside, fit a small round plstic cap and seal it with anabond. i use this in all my opto projects. (BC107 for NPN and BC177 for PNP). with a small modification you can use the standard LDRs freely available in this circuit.

  • […] headlight dim switch… Hi All! Does anybody tried to bulid this diviceAutomatic headlight dim switch.. I have tried but no results i need more details about Phototransistor. Someone have more […]

  • IG
    March 4, 2010

    Hi to all.

    I tryed this shematic but i have problem with Phototransistor a need more infotmation for it. I build a PCB. Any body have more information?

  • vivek jain
    January 11, 2010

    can any one send me the circuit diagram of blue tooth jamer

  • March 25, 2009

    hi any body tell briefly about this project.

  • Aditya
    January 29, 2009

    Can anyone please send me the Circuit diagram for Multilevel Car Parking Systems on my email address :


  • agung
    July 9, 2008

    please give me some mode and type of the operation circuit, cause i’d like to know much.


  • ashir
    May 16, 2008

    i’d like to know much on the mode of operation of the circuit

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