Light Sensor Switch Circuit

Posted on Feb 6, 2014

This light sensor switch circuit allows the automatic connection of a lamp when the light is low (at nightfall) and will maintain the lamp ON for a certain period of time. From the moment that T4 and T5 are opened, relay`s LED start to light and powers the lamp. As soon as one of the transistors is blocked the lamp will go OFF. The phototransistor

Light Sensor Switch Circuit
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T3 will be the one that blocks T5 if there is light that falls on T3. The T2 ²s base-emitter junction is connected in parallel with T3 and so will be blocked as long there is light. T2 will continuously reset IC1 whose counter outputs will be in 0 ³ state. When the night falls R7 provides base current for T2 and the transistor starts to conduct. The counter can now starts to count the impulses from the internal oscillator and in this time the light will bulb will stay lit. After a time, when the output of Q13 goes in state 1 ³ T4 is blocked. This causes the relay`s LED to go off and the lamp too. There is no need for external power supply because the light sensor switch is powered directly from the 220V mains. D1 D5 diodes rectifies the voltage and C4 filters it.

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