Wednesday, 11 July 2012


It is the circuit for automatic street light. The main part is a photodiode. And light activated relay. It is nothing but a circuit which operate a relay according to the light input. This circuit set up such a way that the LIGHT will glow until the ligth is incident on the photo diode.
In the absence of light, current into the transistor through the base is sufficient enough to turn the transistor on. This makes the 5V relay operate and the LIGHT gets 230V ac  and it glows. When the light falls on photo diode , its resistance decreases and the current through it increases. This cause the current through the transistor to decrease and the transistor turns off. Then the current through the relay stops and it gets deactivated . In turn , LIGHT turn by the absence of 230V ac.
Componets for wiring this circuit
R1 = 10K
photo diode
5V relay
230V ac

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