Laser Security Alarm Circuit using LDR and BC547 Transistors

This project is for Laser Security Alarm Circuit using LDR and BC547 Transistors.

It uses LDR, two BC547 transistors, one diode and a buzzer.

When laser light breaks an alarm rings.

Circuit is cheap and very easy to build.

Good for door security, box security and drawer protection.

Circuit Working:

Parts List:

Laser light always falls on LDR.

When light falls on LDR the LDR resistance becomes low.

Low resistance keeps Q1 and Q2 OFF.

So beeper stays OFF.

When someone blocks the laser light then LDR resistance becomes high.

High resistance creates voltage drop across R1.

This voltage passes through diode D1 and R2 and drives the base of Q1.

Q1 turns ON and also turn ON Q2.

Q2 gives ground connection to buzzer.

Buzzer starts ringing.

Alarm continues until laser beam returns.

How to Build:

To build a Laser Security Alarm Circuit using LDR and BC547 Transistors following are the steps to follow:

• Take all the parts as shown in circuit diagram.
• Battery positive goes to R1 and buzzer positive.
• R1 second pin goes to LDR top end.
• LDR bottom goes to ground.
• R1 and LDR junction goes to diode D1 anode.
• Diode D1 cathode goes to R2 one end.
• R2 other end goes to ground.
• Q1 BC547 base connects to D1 and R2 node.
• Q1 emitter goes to ground.
• Q1 collector connects to buzzer negative.
• Q2 emitter goes to ground.
• Q2 collector joins with Q1 collector and buzzer negative.
• Laser pointer must point to LDR directly.

Conclusion:

This Laser Security Alarm Circuit using LDR and BC547 Transistors is very easy project.

It works on simple LDR light change.

Two BC547 transistors amplify small change and activate beeper.

Good project for beginners.

Low cost and good for home security demonstration.

References:

Simple DIY laser alarm