LDR Controlled LED Circuit

LDR Controlled LED circuit
LDR Controlled led circuit

This LDR circuit diagram shows how you can make a light detector. An LDR or “Light Dependent Resistor” is a resistor where the resistance decreases with the strength of the light.

LDR Controlled Led Circuit

Circuit Diagram

LDR controlled led circuit
LDR Controlled led circuit

Light Dependent Resistors

Light Dependent Resistors (LDR) is also called photoresistors. They are made of high resistance semiconductor material. When light hits the device, the photons give electrons energy. This makes them jump into the conductive band and thereby conduct electricity.

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How The LDR Circuit Diagram Works

The LDR circuit diagram works like this:

When it’s dark, the LDR has high resistance. This makes the voltage at the base of the transistor too low to turn the transistor ON.

Therefore, no current will go from the collector to the emitter of the transistor. All the current will instead pass through the LDR and the potentiometer.

When it’s light, the LDR has low resistance. This makes the voltage at the base of the transistor higher. High enough to turn the transistor ON.

Because the transistor is turned on, current flows through the transistor. It flows from the positive battery terminal, through R1, the LED, and the transistor down to the negative battery terminal.

This makes the LED light up.

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Note: You can also make the LED turn ON when it is dark instead of when it is light. To do this, replace the NPN transistor with a PNP transistor like this:

Build it yourself


  • LDR (Light Dependent Resistor)
  • LED (Any Color)
  • Resistor (100K and 100 Ohms)
  • BC547 NPN Transistor
  • 9V Battery

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