Laser Security Alarm Circuit

We need safety and security for our valuable things, to protect our property simple and robust security alarm circuit designed by using easily available components. We know the LASER (Light Amplification by Stimulated Emission of Radiation) light has high strength focused light rays, in this project we have used that laser light as a fence.




Laser security alarm circuit produce highly audible buzzer beep sound when the laser light blocked. Here LDR placed to sense the laser light from the laser diode.

Circuit diagram

Construction & Working

Two NPN (BC548) transistors implemented in this security alarm circuit as a switching device. LDR (Light Dependent Resistor) connected to the base terminal of Q2 transistor through R4 resistor this LDR gives low resistance when the laser light falls on it and gives high resistance when there is no laser light due to that supply voltage to the Q2 transistor varies depends on the LDR resistance.

The Q1 transistor collector terminal is connected with buzzer and Indicator LED and then base terminal of Q1 transistor connected to the Q2 transistor collector terminal. Each transistor needs some threshold voltage to get turn ON, If the laser light falls on LDR it allows supply to the Q2 base hence it becomes turn ON  so the Q1 transistor base don’t get any bias, due to that Q1  transistor stays in OFF condition so the buzzer and LED also in OFF condition.

When the laser light blocked then LDR turned to high resistance and there is no voltage flow to the Q2 transistor base so the Q2 becomes turned OFF but Q1 transistor base gets supply through R3 resistor and Q1 gets turned ON, so the buzzer and LED also gets turn ON and gives alert sound and Indication.

Note:-

  • This Circuit produce alarm sound when the laser light continuously blocked.
  • LASER lights are dangerous to the bare eyes, handle with proper protection and care.
  • This circuit requires proper positioning of LASER light and LDR.




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