Laser alarm circuit, ldr based security system (Assembled), 100% Tested


A laser-based security system is a type of security alarm that uses a light sensor and laser light. In this tutorial, we are going to demonstrate a circuit of a laser security alarm system that will detect any irregular activity and will produce sound using a buzzer. One of the main components of this circuit is LDR. It will detect dark and light laser beams and will change its resistance to send a signal to the buzzer.

Laser based Security System is a type of security and alarm system that uses laser light and a light sensor. A security system protects our homes, offices, banks, lockers etc. from intrusion and unauthorised access. There are different types of security systems available and laser based security system is an important and efficient type.

A Laser security system can acts as a standalone system, which makes some sound or noise when it detects any irregular activity, or can be part of a much bigger security and home automation system, which can send messages, call the owner etc.

In this project, we have designed a simple DIY lased based security system, which acts as a tripwire like security system and triggers an alarm when the laser in interrupted.

WARNING: We have used a laser pointer in this project. Direct exposure of laser light on eyes can be very dangerous. Even though it is a low power laser, avoid direct eye exposure of laser.

Circuit Design

The design of the laser security system circuit is very simple. We will see the design of the circuit in this section and in the working explanation we will see the working of the individual component.

Coming to the design of the circuit, first, the LDR and a 10 K Ω resistor are connected in a voltage divider fashion and its output (common point) is connected to the pin 3 (non – inverting) of the Op- Amp IC LM358.

For the inverting terminal (pin 2), connect the wiper of a 10 KΩ potentiometer (other two terminal of the POT are connected to VCC and GND).

The output of the Op – Amp (Pin 1) is connected to the base of the transistor (BC547) through a resistor.
The trigger pin of 555 (Pin 2) is pulled high using a 10 KΩ resistor.

The reset pin (pin 4) of the 555 is connected to VCC through a 10 KΩ resistor and a push button is connected between Pin 4 of 555 and GND. A bypass capacitor of 100 nF is connected between pins 5 and GND. A buzzer is connected to pin 3 of 555 IC.

Rest of the connection are shown in the circuit diagram.

Working of the Project

A simple, cheap and effective laser based security system is developed in this project. Let us see the working of this project.

First, the Op – Amp circuit acts as a comparator i.e. it compares the voltages at the inverting and non – inverting terminals and produces an output accordingly.

The LDR – 10 KΩ resistor Voltage divider is connected to the non – inverting terminal of Op –Amp and a POT is connected to the inverting terminal.

Assume, the laser pointer is placed directly in line of sight to the LDR and the light from the laser is continuously being incident on LDR.

In this situation, the resistance of LDR falls down to few Ohms (or tens of Ohms) and as a result, the voltage at the non – inverting terminal will be less than that at the inverting voltage. The output of the Op –Amp is low and the transistor is OFF.

If the laser light is blocked by an intruder from falling on the LDR (even for a small duration), the resistance of the LDR goes to few hundreds of Ohms and as a result, the output of the Op –Amp will be HIGH. This will turn on the Transistor.

As the output of the transistor is connected to the Trigger Pin (Pin 2) of the 555 Timer IC, if the transistor is ON, the trigger pin gets a short low pulse and as a result, the output of the 555 becomes HIGH. This will activate the alarm by turning ON the buzzer.

Since, the 555 Timer IC is configured as a Bi – Stable Multivibrator, a small active low trigger pulse at the trigger pin will set its output to HIGH and in order to reset it we need to push the reset button.


Components Required

  • LM358 (Op – Amp IC)
  • NE555 (Timer IC)
  • LDR
  • 3 x 10 KΩ Resistors (1/4 Watt)
  • 220 Ω Resistors (1/4 Watt)
  • 10 KΩ Potentiometer
  • BC547 (NPN Transistor)
  • Small Buzzer
  • 100 nF Capacitor (Ceramic Disc Type Capacitor – Code 104)
  • Push Button
  • Laser Pointer
  • 9V Battery
  • Connecting Wires
  • Breadboard (Prototyping board)





  • The circuit, construction and setup for the Laser Security System is very simple.
  • If used with a battery, the laser security system can work even when there is a power outage.


  • The laser security system works only if the laser is obstructed. If the intruder passes without obstructing the laser, it is considered as a failure.
  • In order to secure a larger area, we need more lasers and corresponding sensors.