Automating Lighting with a Twilight Switch
A twilight switch is an electronic device that facilitates the automatic activation of a lighting circuit as natural light diminishes in a given setting. Its primary application lies in the automatic illumination of various areas, such as streets, roads, highways, gardens, courtyards, and more, once daylight falls below a certain threshold. Twilight switches are employed to seamlessly control lighting systems based on ambient light levels. This circuit represents one of the simplest twilight switches ever featured in this publication.
Switching on as Darkness Falls
As darkness descends, the resistance of the light-sensitive resistor R1 increases, causing transistor T1 to turn off. Subsequently, transistor T2 switches on, energizing relay Rel. Simultaneously, a voltage drop of approximately 1 V occurs across R4, constituting the switch’s hysteresis. Capacitor C1 plays a crucial role in ensuring the switch remains unfazed by brief fluctuations in ambient light, like those caused by a passing vehicle with its headlights on. The only requirement for the transistors is high current amplification, necessitating the use of type C transistors.
Selecting the Appropriate Varistor
The varistor used here is an eco-friendly type (Piher) that contains no cadmium and is roughly the size of a match head. If an alternative varistor is used, it should exhibit a daylight resistance in the range of a few hundred ohms, which should then increase to about 10 kΩ at twilight. If needed, the value of P1 can be adjusted within reasonable limits. During the calibration process, C1 should be temporarily unsoldered from the ground to achieve a quicker response from the circuit.
The relay should be a 12 V type that needs an energizing current of ≤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 the long life of the relay contacts. The circuit draws a current of not more than 5 mA plus the relay current.