The below circuit is Light operated Relay circuit.In this post we will learn about light activated relay circuit.In this circuit we have 3 transistor.Transistor T1 (Til 78) is a photo transistor.which toggled On/OFF a relay in the response to the light or darkness over a photo sensitive device. On the other hand transistor T2 and T3(BC 547) which makes a darlington pair for activating the relay.
The light sensitive relay circuit presented here has been configured such that it increases the sensitivity of an attached transistor many folds. In this circuit the relay can be any type rated between 12V to 6V.The circuit also have a photo sensitive device TIL78. But we can use any similar photo transistor to serve the purpose.Even we can used a stripped 2N3055 power transistor. to do this take a good 2N3055 transistor and very carefully grind of the cap surface such that the internal junction gets exposed, but make sure you do not damage the entire cap or the junction while scraping the device.
The 2M2 is used for increasing or decreasing the light sensitivity of the photo transistor.
Circuit Diagram :
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