LIght activated switch Circuit

Light activating and deactivating switches are simple to build when you are provided with a LDR (sensing element) and 555 (switch activator). These kind of sensor circuits are widely employed in many applications and may come in handy when you intend to build a simple light sensing circuit. LED is used in the above circuit to indicate switching, you can use device of your desire provided ample amount of power supply.

The circuit comprises of two divisions one is LDR , the light sensing element and another one is 555 activator. The LDR is paired with Resistor R1 to form a potential divider to feed the trigger voltage to the IC 555. It gives maximum voltage when no light is incident in it and minimum voltage when light falls above LDR.

The activator is build around a 555 IC which was wired as a monostable multivibrator. Monostable Multivibrator gives a high output in the presence of negative trigger at the pin 2 whereas low output in the presence of positive trigger. This principle was used here to make the 555 acts as a switch when light incident in LDR.
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