Automatic Headlight Brightness Switch

Schematic

Parts

Part Total Qty. Description Substitutions 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

Notes

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.