Rain Detector

This circuit uses a sensor made of a small piece of etched PC board and a simple SCR circuit to detect rain and sound a buzzer. The SCR could also be used to activate a relay, turn on a lamp, or send a signal to a security system.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
R1	1	1K 1/4 W Resistor
R2	1	680 Ohm 1/4 W Resistor
D1	1	1N4001 Silicon Diode
BZ1	1	12V Buzzer
S1	1	SPST Switch
SCR1	1	C106B1 SCR	106CY
SENSOR	1	See Notes
MISC	1	Board, Wire, Case, PC Board (For Sensor)

Notes

1. The sensor is a small piece of PC board etched to the pattern showen in the schematic. The traces should be very close to each other, but never touching. A large spiral pattern would also work.
2. Make sure to use a loud buzzer.

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.

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.