Shadow Detector with OPAMP
Shadow Detector with OPAMP

Shadow Detector with OPAMP

Emergency lights are widely used to detect in household application. Many circuits published earlier have a serious drawback that only one light dependent resistor (LDR) is used for shadow detection.
in this project we will create a opamp based shadow detector circuit and as an output lights will glow.

Component required:

  • LM358
  • Resistors R1 =1k

   R2 = 10k

   R3 = 330 Ohm

  • Variable resistor VR1 = 10k
  • LED
  • Transistor T1 BC547
  • LDR sensor
  • 9v battery


Op Amp IC UA 741 is used as a voltage comparator. Its inverting input pin2 receives half supply voltage (4.5 volts) through the potential resistors R1 and R2. The non inverting input pin3 gets a variable voltage through LDR and VR. Normally when the LDR gets light, its resistance will be low and it conducts and provide a high voltage to the non inverting input of IC. This makes the output of IC high. The high output from IC is given to the base of T1 through a current limiting resistor R3. T1 is PNP transistor and it conducts only when its base becomes negative. Here normally the base of T1 will be high due to the high output from IC. So T1 remains off and Buzzer and LED connected to its emitter remains off. When a person passes in front of LDR, the shadow of the person reduces the resistance of LDR and the voltage at the non inverting input of IC decreases. This makes the output of IC low. Immediately T1 conducts activating Buzzer and LED indicating the entry of a person


  • It can be used on automatic bike headlight.
  • Dark detection lights.
  • Automatic car headlights
  • Home emergency lights