Automotive Circuit DiagramsLights and Display Board Circuits

Automatic twilight switch

This is one of the simplest twilight switches ever published in this magazine. When it gets dark, the value of light-sensitive resistor R1 increases. whereupon T1 switches off. Transistor T2 then switches on and this energizes relay Rel. At the same time, a voltage drop of about 1 V develops across R4: this is the hysteresis of the switch. Capacitor C1 serves to make the switch insensitive to brief changes in ambient darkness, such as caused by a passing car with its headlights blazing.

The only requirement on the transistors is high current amplification, which means the use of C types.

The varistor is a new type (piher) that is environment-friendly (since it Contains no cadmium) and comparable in size to the head of a match. If another type is used, it’s daylight resistance should be of the order of a few hundred ohms; this should increase to about 10 kΩ at twilight. In any case, the value of P1 may be increased (within reason). During calibration, unsolder C1 from the earth: the circuit then reacts faster.

The relay should be a 12 V type that needs an energizing current ≤50 mA.: its contact should be able to switch 8 A. The load current, however, should not exceed 4 A. When they are switched on, most lamps, and certainly halogen types, draw a very large current. Keeping the load current down ensures a long life of the relay contacts. The circuit draws a current of not more than 5 mA plus the relay current.

Automatic twilight switch Schematic diagram

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