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.

Part

R1                 1 5K 1/4W Resistor

R2, R3, R4 3 5K Pot

Q1                 1 NPN Phototransistor

Q2                 1 2N3906 PNP Transistor

K1                 1 Low Current 12V SPST Relay

K2                 1 High Current 12V SPDT Relay

S1                 1 SPST Switch

B1                 1 Car Battery

MISC         1 Case, wire, board, knobs for pots