Light Detector Circuit using Wheatstone bridge

It is said that the eyes are sensing things which our mind reads and the same is case with the LDR known as light dependent resistor. The LDR is sensing the light and giving an output. The statement is right that you can either make any switch ON or OFF depending on the requirements however, we are often showing carelessness which is resulting an increase bill of the Electricity and energy is also wasted. For this purpose the light detector circuit is being designed using a Wheatstone bridge, which is very useful in the aim of energy saving.


The light detector circuit using Wheatstone bridge is working in a manner when light is sensed by the Light Dependent Resistor, the circuit switch off all the lights inside a building. A relay is being used for the smooth operation of the circuit. All the light sources inside a building are being controlled through LDR for the minimization of the electricity use and bill. The circuit is found to be very useful in the purpose of saving energy and minimizing the electricity bill.


Basic Circuit Diagram


The following is the basic block diagram of the light detector circuit using Wheatstone bridge.


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Safety precautions


The safety precautions were recommended by the well PCBs and were followed. The light dependent resistors are very sensible to all sources of lights, therefore you need to be very careful if you are having the sensor circuit in house or room you have to take care of the artificial sources of lights e.g. electric bulbs. This is because the sensor circuit will trigger the all lights off when it senses lights each time it sense light.




  1. The circuit is working on even very low light sources.
  2. There is no need of having additional capacitors in the circuit.
  3. There is a need of variable DC supply in the circuit.


Main Working of the Circuitry

The light dependent resistor is a resistor which is varying its resistance with the strength of light at different places. The resistor is made of the cadmium sulphide. When the resistor is not sensing any light source, its resistance is huge and is in Mega ohms. Whereas, when there is a light source sensed by the light dependent resistor, the resistance automatically reduces. This simply means that when there is a presence of light, the light dependent resistor is having an output and when there is no light, the light dependent resistors has no output. It can be simplified that the light dependent resistors can be used to differentiate among light and darkness, or day and night.


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The Wheatstone bridge is used for the detection of the voltage drop in between the different resistors. In case of the light dependent resistors, the Wheatstone bridge is consisting of an LDR connected with a potentiometer in the first arm which is having a resistance of 10K ohms or more and also a resistor in the second arm. When the light is incident on the light dependent resistor, the resistance is getting lowered and in return the voltage is changed at the potentiometer and detected and hence the required decision is made.


The light dependent resistor is working on the principle of the conductivity of photons. As the light is falling on the surface of the light dependent resistor, the resistance of the sensor is decreasing and in turns it gives an output. This is due to the transfer of electrons from the valence band to that of the conduction band and due to the high energy of the photons present in the incident light.]


Components Required


The following are the basic components required in order to make the light detector circuit using Wheatstone bridge.


  1. LDR
  2. Transistor (BC547)
  3. LM741op-amp IC
  4. Potentiometer (10k)
  5. Resistance (10k, 330ohm)
  6. Led (red)
  7. Battery (9v)




  • The resistance of the cell is only 400 ohms and can vary up to 9 kilo ohms.
  • The resistance of the LDR is in mega ohms in darkness.
  • The rise time is about 48 to 120 milli seconds and fall time is about 2.8 to 1.2 milli seconds.
  • This has a wide spectral range.
  • The project is economical when cost is considered.
  • The project is working perfectly even in high ambient temperatures.



  • The project can be used in automated street lights.
  • It can be used in the position sensor.
  • It can be used in the intensity meters for lights.
  • It be used in the burglar detection circuits.
  • It can be used in the automated bedroom lighting.
  • It can be used in the LED based obstacle detection systems. 


  Implemented Photo of the Light Detector Circuit using Wheatstone bridge


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