Light Detector With Sensitivity Control Circuit

Simple Light Detector With Sensitivity Control Circuit is designed by using LDR (Light Dependent Resistor) and operational amplifier. Here LDR acts as Light detecting sensor and it is placed in a balanced wheatstone bridge, A 10KΩ Variable Resistor connected with LDR can control the sensitivity of LDR.

This light detector circuit is designed to indicate the presence of light through LED, you can connect Relay switch or any output actuator depends on your requirement.

Circuit Diagram

Components Required

  1. LDR (Light Dependent Resistor)
  2. Operational amplifier IC 741
  3. Transistor BC547 (NPN)
  4. LED
  5. Variable Resistor 10KΩ
  6. Resistor 220Ω, 2KΩ each one
  7. Resistor 10KΩ = 2
  8. Battery 9V

Circuit Construction & Working

We know wheatstone bridge helps us to find unknown Resistance value from a quadrilateral with three known resistances and applying a voltage between a pair of opposite corners. The voltage difference from wheatstone bridge is applied to the operational amplifier inputs and the difference between inverting pin and non inverting pin is amplified by the op-amp and gives output through pin 6. This output is fed into transistor switch Q1 and the LED glows depends on the Q1 ON and OFF conditions.

IC 741 (Operational Amplifier)

This Operational Amplifier IC LM741 has 8 Pins, and we don’t use pin 1 and 5 in this circuit they are called offset null. Bias for this IC applied through pin 7 and pin 4. Inverting input pin 2 and Non inverting input pin 3 are considered as input terminals and pin 6 acts as output terminal.

Transistor BC547 (NPN)

Transistor BC547 is a NPN transistor and have three terminals called collector, Base and Emitter. This transistor can acts as Switch and low power general purpose amplifier.

Light Dependent Resistor (LDR)

LDR is a component that changes its Resistance depends on light intensity, it has two terminals and don’t have any polarity, it is made by cadmium sulfide Material, Resistance between two terminal will be very high when there is no light and Resistance between two terminal will be very low when there is near light source.

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