Light detector circuit

Light detector circuit

A Light Detector or a Light Sensor is a device or circuit that detects the intensity of the light incident on it. Different types of light detectors are LDRs (or Light Dependent Resistors), Photo Diodes, Photo Transistors, etc. All these devices are called as Photoelectric Devices as they convert light energy to electric energy. These Light Detectors or Sensors can detect different types of light like visible light, ultraviolet light, infrared light etc.



Components required:

  • 2 x BC547 NPN Transistors
  • Small LDR
  • 1 KΩ Resistor
  • 10 KΩ Potentiometer
  • White LED
  • 330 Ω Resistor
  • Connecting wires
  • Power supply (9V Battery)


Working: In the previous circuit, we have seen a simple Light Detector using LDR and an Op – Amp. If you don’t have an Op – Amp, then the above circuit will be helpful. It uses just two transistor to perform the light detection operation.

In this circuit, we have used two transistor in the form of Darlington Pair. Even though a single transistor would be suffice, Darlington Pair ensures that there is enough current at the output.

The construction of the circuit is very simple. A 10 KΩ Potentiometer and the LDR form the biasing resistors of the first Transistor. When there is light incident on the LDR, its resistance will become less.

Hence, there will not be any biasing voltage to the first transistor. As a result, it will not be turned. As there is no output from the first transistor, the second transistor will not be turned on and hence, the LED stays OFF.

When the light incident on the LDR is removed i.e. under darker conditions, the resistance of the LDR will become very high. Hence, the voltage from the POT will take a less resistance path i.e. it turns ON the first transistor.

This will in turn ON the second transistor as its input is connected to the output of the first transistor. As a result, the LED, which is connected to the output of the second transistor will be turned ON.



  • Simple Light Detector Circuit can be used in many applications like automatic switching of appliances up on detecting light, security systems, etc.
  • This circuit can be used in alarm system, where increasing intensity of light will trigger the alarm.
  • It can also be used in cupboards or wardrobes. When the door is opened, the light will automatically turned on.