Simplest circuit to control an LED, depending on light, using LDR

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Light dependent resistors (LDR) have many applications in our daily life. Street lights, mobile phone screen lights, car head lights etc works based on the light intensity using light dependent resistors. Many ways are there to interface light dependent resistors in circuits. One of such circuit is given below.

This circuit will turn on LED if the light intensity around LDR is more. Similarly, LED will turn off, if light intensity around LDR is less. Here, we are using a ULN2803 darlington driver IC to control the circuit. Proper modifications have to me made in the circuit to reverse the operation.

1 74165 using Arduino (copy)

Working of circuit

Current will flow through the LED if and only if voltage at pin 1 of ULN2803 is HIGH. When light falls on LDR, it’s resistance will drop which will increase the voltage across pin 1 of ULN2803. This will result in current flow through LED which will turn on LED. When there is no light on LDR, it’s resistance will be more which will reduce the voltage across pin 1 of ULN2803. This will block the current flow through LED. Video demonstration of the working of circuit is given below.

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A deep electronic enthusiast who spent most of my time for electronic experiments. I am also interested in publishing my experiments in my blog for helping other electronics enthusiasts.