Automatic Headlight Brightness Switch Project

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.

Automatic Headlight Brightness Switch Circuit Diagram

Parts

Part    Total Qty    Description
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

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.
Adjust all the pots for proper response by testing on a deserted road.
S1 enables and disables the circuit.
B1 is, obviously, in the car already.
Before you try to connect this circuit, get a wiring diagram for your car. Some auto manufacturers do weird things with wiring.
Connection A goes to the high beam circuit, B goes to the headlight switch common and C connects to the low beam circuit.