How To Make An Automatic Light Fence Circuit with Alarm

 The grating circuit is used to detect the presence of any person or object in a specific area. The detection range of Light Fence Circuit is approximately 1.5 to 3 meters. Using LDR and operational amplifier to design the circuit is quite simple. This portable circuit can be easily operated with a commonly used 9V battery, and the warning sound generated by the buzzer is loud enough to detect people, cars or objects. 

Hardware Required:

  • Resistor
  • Capacitor
  • LDR
  • IC 555 timer
  • PNP Transistor
  • Op-amp IC
  • Potentiometer
  • Buzzer
  • Breadboard
  • Battery

  • LED



The circuit can run on a 9-volt battery. The operational amplifier IC is used as a voltage comparator in the circuit, and the 555 timer IC is placed in an astable mode. The voltage divider circuit is generated by the LDR and potentiometer. The output of the frequency divider circuit can vary according to the amount of light falling on the LDR. The voltage divider is connected to the inverting board of the operational amplifier IC. A 5.7K ohm resistor connects the non-inverting pin to the source and sets the voltage value at the same phase.

To change the voltage as needed, you should replace the resistor with a 10 K potentiometer. Whenever a person or entity approaches a prohibited place, the LDR detects the shadow and the circuit activates an alarm. You can use a potentiometer connected in series with the LDR to adjust the sensitivity of the device.

Post a Comment

Previous Post Next Post