July 01, 2009

Automatic Headlight Brightness Switch


Driving the highway with your high-beam headlights can really increase your visibility, but can be a blinding hazard for other drivers. This simple circuit can be wired into your headlight switch to provide automatic switching between high and low beam headlights when there is oncoming traffic. It does this by sensing the lights of that traffic. In this way, you can drive safely with your high-beams on without blinding other drivers.

circuit diagram:-

Schematic for Automatic Headlight Brightness Switch


  1. Q1 should me mounted in such a way so it points toward the front of the car with a clear line of site. Suitable places are on the dashboard, in the front grill, etc.
  2. Adjust all the pots for proper response by testing on a deserted road.
  3. S1 enables and disables the circuit.
  4. B1 is, obviously, in the car already.
  5. Before you try to connect this circuit, get a wiring diagram for your car. Some auto manufacturers do weird things with wiring.
  6. Connection A goes to the high beam circuit, B goes to the headlight switch common and C connects to the low beam circuit.
parts required

Total Qty.
R115K 1/4W Resistor
R2, R3, R435K Pot
Q11NPN Phototransistor
Q212N3906 PNP Transistor
K11Low Current 12V SPST Relay
K21High Current 12V SPDT Relay
S11SPST Switch
B11Car Battery
MISC1Case, wire, board, knobs for pots


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Electronics is the study and use of electrical that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves. and semiconductors. The pure study of such devices is considered as a branch of physics, while the design and construction electronic circuits to solve practical problems is called electronic engineering.

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