How to Build a DIY Laser Tripwire Alarm with Arduino for Under


Building your own DIY laser tripwire alarm with Arduino is an enjoyable electronics project that can be completed on a budget under $20. This alarm creates an invisible laser beam that triggers an audible alarm when broken.

This project is perfect for learning basic skills in Arduino programming and electronics. It also has practical applications for home security, sensing intruders, and more. With just a few common components, I'll show you how to construct the hardware and code the software for your own laser tripwire alarm.

Required Components

To build the laser tripwire alarm, you will need the following components:

Arduino Uno Board

The Arduino Uno is the microcontroller board where all of the logic and processing happens. The Uno provides digital and analog pins to connect components and can be programmed via USB.

Laser Diode Module

A laser diode module contains a laser diode that produces a focused beam of light. These modules typically run on 5V and can be purchased very cheaply.


A photoresistor or light-dependent resistor (LDR) is a sensor that changes resistance based on light exposure. It detects the laser beam in this project.


A simple piezo buzzer that beeps when triggered by the Arduino. This provides the audible alarm.


Jumper wires

For making connections between components.


To easily prototype and connect the circuit before soldering.

Battery pack

A 4xAA battery holder provides 6V power to the Arduino and laser module.

Circuit Diagram

The following circuit diagram shows how to connect the components to the Arduino:

The photoresistor and 10k ohm pull-down resistor create a voltage divider. When the laser hits the photoresistor, the resistance drops, increasing the voltage. This voltage is read by an analog input pin on the Arduino.

The buzzer and laser diode are connected to digital pins. The Arduino can turn them on and off based on the code.

Laser Safety

Use caution when working with laser diodes. Never point a laser directly at anyone's eyes or stare directly at the beam. LASERs can cause permanent eye damage and blindness.

For this project, the low power 5mW laser modules are reasonably safe if handled carefully. Avoid shining the beam at reflective surfaces that could redirect it.

Arduino Code

The Arduino code for this project reads the analog voltage from the photoresistor, and triggers the alarm when the beam is broken.

// Define pin connections
const int buzzer = 9;
const int laser = 10;
const int sensorPin = A0;

// Threshold voltage to trigger alarm
const int threshold = 500;

void setup() {

// Setup serial monitor

// Set laser and buzzer pins as outputs
pinMode(laser, OUTPUT);
pinMode(buzzer, OUTPUT);


void loop() {

// Read analog voltage from photoresistor
int sensorValue = analogRead(sensorPin);

// Print value to serial monitor

// If value rises above threshold, trigger alarm
if (sensorValue > threshold) {
digitalWrite(buzzer, HIGH);
digitalWrite(laser, LOW);
else {
digitalWrite(buzzer, LOW);
digitalWrite(laser, HIGH);

delay(10); // Small delay

This code turns the laser on and buzzer off normally. When the sensor value rises above the threshold, it turns the buzzer on and laser off to trigger the alarm.

The threshold value may need to be adjusted based on the specific photoresistor used and light conditions.


To build the tripwire alarm:

The alarm can be enclosed in a cardboard box to better detect the beam. Make sure the laser and photoresistor face each other across the box opening where you want to detect motion.

Position the box such that the laser beam crosses a doorway or corridor. When the beam is broken, the buzzer will sound the alarm!

Usage Examples

This DIY laser tripwire has many useful applications:

With a bit of creativity, this inexpensive alarm system can provide enhanced security and detection for your specific needs.

The tripwire is easy to reposition and configure for different spaces. Adjust the box opening or angle the components to control the laser crossing point as required.


Building a laser tripwire alarm with Arduino provides a fun electronics project that teaches useful skills. With around $20 of easily accessible components, you can construct a versatile security sensor.

The Arduino code exemplifies reading analog values and controlling digital outputs. The circuit illustrates Ohm's law and voltage dividers.

With the instructions provided above, you now have all the information needed to successfully create your own DIY laser tripwire alarm. Feel free to customize the code and circuit for enhancements. And always use laser diodes safely.

This is an enjoyable, budget-friendly project for learning Arduino prototyping and creating a unique alarm system with practical applications.