Building your own DIY laser tripwire alarm system with Arduino is an enjoyable electronics project that teaches you a lot about lasers, sensors, microcontrollers and coding. This alarm system will alert you when someone crosses the laser beam, useful for home security or just for fun.

I will explain step-by-step how to build the circuit on a breadboard, program the Arduino code, and set up the laser and sensor. I’ll also provide tips on how to configure the system so it works reliably. With just a few common components, in an afternoon you can have your own alarm system up and running!

What You Will Need

To build the DIY laser tripwire alarm, you will need:

How a Laser Tripwire Alarm Works

The basic operating principle of the laser tripwire alarm is simple. A focused laser beam shines across a path at a photoresistor sensor. Normally the laser hits the sensor, so the sensor has high resistance in the bright light. But when someone crosses the beam, the laser is temporarily blocked. This causes the sensor resistance to increase.

The change in resistance is detected by the Arduino. Its code sounds the buzzer alarm and optionally flashes an LED whenever the beam is broken. Once the obstruction passes, the alarm stops and resets for the next crossing.

How to Build a DIY Laser Tripwire Alarm System With Arduino That Alerts You When Someone Crosses It

By carefully setting the sensor threshold and alarm duration, you can make the alarm trigger reliably whenever the beam is interrupted. The idea is similar to how a store entrance sensor works, except homemade!

How to Build the Circuit

Building the circuit on a breadboard is straightforward:

Refer to the schematic below. Make sure to match the connections:

Schematic diagram of circuit

To assemble it:

Check your work carefully before powering it on!

How to Program the Arduino

With the hardware assembled, now you need to upload the Arduino sketch. This contains code to:

The complete Arduino sketch is shown below in the code snippet.

It uses the analogRead() function to measure the sensor and tone() to control the buzzer. Customize the threshold and alarmDuration variables as needed.

// Tripwire alarm sketch

int sensorPin = A0; // photoresistor on analog pin A0
int buzzer = 9; // buzzer on digital pin D9

int threshold = 600; // sensor threshold voltage
long alarmDuration = 1000; // 1 second alarm

void setup() {
pinMode(buzzer, OUTPUT);

void loop(){

int sensorValue = analogRead(sensorPin);

if(sensorValue > threshold) { // beam broken

long startTime = millis();

while(millis() - startTime < alarmDuration) {
  tone(buzzer, 500); 



Upload this sketch to the Arduino board. Now it will monitor the photoresistor and sound the alarm when triggered!

How to Set Up and Test the Tripwire

To complete the laser tripwire alarm:

Once set up, the DIY tripwire alarm should function just like a commercial system. Walking through the laser beam will activate the buzzer alarm!

For best reliability, use tape to hold the components in their aligned positions. Box or enclose the Arduino and wires for a finished look.


Building your own laser tripwire alarm with Arduino is an enjoyable DIY electronics project. With just a handful of components and some basic Arduino skills, you can construct an alarm system in an afternoon.

Set it up across a doorway or hallway for simple home security. Or use multiple tripwires to create an interactive laser maze! What will you use your DIY laser tripwire alarm for?

Let me know in the comments if you have any questions about the project. And if you build your own laser tripwire alarm, please share photos!