Plastic Twilight Switch Circuit

Twilight Switch is a light dependent switch, which automatically turns ON when it gets dark. This is one of the simplest light dependant switch, which automatically turns on when it gets dark. It is based on the characteristics of light sensitive resistor R1. During the day time, the value of R1 is very low. But in dark, it has very high value. This feature makes this simple circuit to operate as an automatic twilight switch.
About the Circuit :

• When it gets dark, the value of R1 increases, whereupon transistor T1 switches off. Transistor T2 then switches on and relay connected at the collector of T2 becomes on. Also LED D2 glows, which gives the visual indication.
• Capacitor C1 serves to make the switch insensitive to brief changes in ambient darkness such as caused by passing a car with its headlights blazing.
• The light dependent resistor R1 should be such that its day light resistance should be of the order of a few hundred ohms; this should increase to about 10KW at twilight. In any case, the value of P1 may be increase.

Calibration :

• During calibration, unsolder C1 from earth, the circuit then resets faster. Preset P1 is used to increase the sensitivity of the circuit.
• The circuit draws a current of 5mA plus the relay current.