Creating a Laser + LDR Security Alarm System is a simple and effective DIY electronics project. It uses a laser pointer, LDR (Light Dependent Resistor), LED, diode, resistor, and a buzzer or other alarm system to detect when a laser beam is interrupted — commonly used in basic intrusion or object detection systems.
🔧 Components Required:
| Component | Specification |
|---|---|
| LDR (Light Dependent Resistor) | 1x |
| Laser pointer/module | Red laser, 5V |
| Resistor | 10kΩ, 220Ω |
| Diode | 1N4007 or similar |
| LED | Any color |
| Buzzer | 5V (optional) |
| Transistor | BC547 or 2N2222 (to drive LED or buzzer) |
| Power Supply | 5V Battery or Adapter |
| Breadboard & Wires | For prototyping |
⚙️ Working Principle:
Laser continuously shines on the LDR.
The LDR has low resistance in light and high resistance in darkness.
When someone passes through the laser beam, it breaks — causing a sudden change in LDR’s resistance.
This change is used to trigger a transistor circuit which turns ON an LED or buzzer.
🧪 Step-by-Step Instructions:
🔹 Step 1: Set Up the Laser
Place the laser in a fixed position so it shines directly on the LDR.
Ensure alignment is tight so even small movements break the beam.
🔹 Step 2: Connect LDR Circuit
Connect one side of the LDR to Vcc (5V).
Connect the other side of the LDR to a 10kΩ resistor, and connect the other end of the resistor to GND.
The junction point between LDR and the resistor goes to the base of an NPN transistor via a 220Ω resistor.
🔹 Step 3: Add Diode Protection
Place a diode (1N4007) in reverse bias between the transistor’s collector and emitter to protect from reverse voltage spikes.
🔹 Step 4: Connect the LED or Buzzer
Connect an LED or buzzer in series with the collector of the transistor.
Connect the emitter to GND.
The collector goes to +5V through the LED/buzzer.
🔹 Step 5: Power Up the Circuit
Use a 5V battery pack or USB power supply.
When the laser beam is aligned and unbroken, the transistor stays off.
When the beam is interrupted, LDR resistance increases, voltage drops at the base — transistor turns ON, activating LED or buzzer.
💡 Tips for Better Performance:
Use a tube or casing for LDR to avoid ambient light.
Secure laser with clamps or mounts for stability.
You can extend this to trigger a camera or send a signal to a microcontroller like Arduino.
🧠 Applications:
Doorway intruder alert
Object counting system
DIY laser security perimeter
Tripwire for home safety projects
✅ Summary:
This Laser + LDR Security Alarm System is a great beginner-level electronics project. It teaches the fundamentals of light sensors, transistor switching, and basic security systems — all with cheap, easily available components.
