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.