Darkness sensitive switch

Darkness sensitive switch: The circuit presented here enables the automatic switching on of outside lighting when it gets dark and, what’s more, it does so for a predetermined period. A new period can be begun only when it has been light again.

The switch is a solid-state relay. From the instant that T4 and T5 are on, the LED in the relay lights and lamp L1 is powered. As soon as one of the transistors switches off, the lamp will go out.

Whether T5 is on depends on phototransistor T3. If light falls on to this, it is switched on and removes the base current from T5. In other words, T5 can be on only when it is dark.

The base-emitter junction of transistor T2 is also connected in parallel with T3 and it, too, will, therefore, be off when it is light. This causes a constant reset on IC1, all of whose counter outputs are then low.
As soon as it gets dark, base current for T2 is provided by R7 and the transistor switches on. The counter can then count the pulses of its internal oscillator, while the lamp remains on. When, after a short time, output Q13 goes high, transistor T4 switches off. This causes the LED in the solid-state relay to quench and the lamp goes out. Since at the same time the oscillator is stopped via T1, Q13 remains high. This state is maintained until it gets light again and IC1 is reset, whereupon a new cycle can be started.

The period the lamp is on may be set between 1 and 5 hours with P1.

No special transformer is needed for the power supply, which may be derived directly from the mains. Diodes D1-D5 rectify the mains voltage and the result is smoothed by C4. Capacitor C5 operates as a resistance, and it should, therefore, have a working voltage rating of not less than 400 V, although 630 V is preferred.

Note that mains voltage exists at several points in the circuit: great care should be taken to insulate the switch unit adequately

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