Tuesday, 10 July 2012


A light activated relay is nothing but a circuit which operate a relay according to the light input. This circuit set up such a way that the LED will glow until the ligth is incident on the photo diode.
Photo diode is special semiconductor diode operated in reverse biased condition. In no light condition it offer very high resistance. Only current existing in it is the reverse satuaration current due to the minority carriers. When the light falls on it reverse through it will increase.
In the absence of light, current into the transistor through the base is sufficient enough to turn the transistor on. This makes the relay operate and LED circuit gets completed 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 , LED circuit opens and it glowing.
Components for wiring this circuit
R1 = 10K
R2 = 100Ω
Relay 5V
photo diode
Vcc = 10V

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