Light activated switch circuit using LDR and IC 555

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.

Setup of Light activated switch

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.

When there is no presence of light the potential divider gives a positive trigger to the pin 2 of 555 IC. This results in low signal output at the pin 3 of 555. Incident of light in LDR makes the potential divider to give out negative trigger to the second pin of timer IC and this in turn forces the output at pin 3 to go high. Thus the combination of LDR and 555 works as a light activated switching device.


  • To Switch high power devices such as Relay, speakers etc use transistors to turn it on by drawing power directly rather than sourcing it from 555.


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Dangerously ExplosiveFrank DonaldAnonymous Recent comment authors
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Dangerously Explosive
Dangerously Explosive

hello, is it possible to power ths with a 3.5V li-poly rechargeable battery? or would I need to change the resistors/capacitors for that?


thank you…

Frank Donald

You are welcome


where do we use this circuit i mean applications of this circuit???

Frank Donald

Light activated switched do have their applications in some places we need to initialize or get a work done when the circuit detects an external light that’s where this exactly comes into play say for example “parking lights”. It depends on our imagination and thinking to implement simple circuits to do a particular application.