How to Build a Laser Tripwire Alarm Demo with Arduino

Detect a broken light beam with an LDR and sound a buzzer

Arduino Uno Security Beginner30 minutes4 components

Updated June 12, 2026

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What you'll build
Wiring diagram
Components needed
Assembly
Pin assignments
Code

What you'll build

This guide builds a safe laser tripwire alarm demo with an Arduino Uno, a light-dependent resistor, a 10k resistor, a piezo buzzer, and a low-power laser module. The laser shines continuously at the LDR. While the beam is aligned, the Arduino reads a high analogue value and stays quiet. When something blocks the beam, the reading drops below a threshold, the buzzer sounds, and the sketch prints an alert to Serial Monitor.

The project is scoped as a learning build. You will wire a voltage divider for the LDR, read the analogue value on A0, calibrate a threshold from real Serial Monitor readings, and drive the buzzer with tone(). Keeping the laser under firmware control from a digital output means you can turn the beam off in software — useful if you want to add a re-arm delay later.

Safety rule: low-power visible laser only, never aimed at eyes, faces, pets, windows, or reflective surfaces.

What you are building

The firmware has three jobs:

keep the laser on by holding LASER_PIN (D2) high,
read the LDR voltage divider on LDR_PIN (A0) every 100 ms and compare against LDR_THRESHOLD (default 400),
sound the buzzer on BUZZER_PIN (D5) with tone() at 1 kHz while the beam is broken, and silence it with noTone() when the beam is restored.

The sketch also prints the raw LDR value to Serial Monitor at 9600 baud on every loop, which makes threshold calibration straightforward. Hysteresis, a status LED, and Bluetooth notification are out of scope for this demo.

Upload and calibrate

Upload the sketch from Schematik and open Serial Monitor at 9600 baud. You will see a continuous stream of LDR values:

Laser tripwire demo ready
LDR value: 820
LDR value: 817
LDR value: 412
LDR value: 38

The first readings should be high (beam hitting the LDR), then drop when you block the beam. Note both values. The constant to adjust is:

const int LDR_THRESHOLD = 400;

Set LDR_THRESHOLD to a value midway between your beam-on and beam-blocked readings. If the buzzer never fires despite a blocked beam, the ambient light is strong enough to keep the LDR reading above the threshold — try shielding the LDR with a short cardboard tube to reduce stray light. If the buzzer fires while the beam is aligned, lower the threshold.

Once calibrated, block the beam with a card and confirm "Intruder detected: beam broken" appears in Serial Monitor.

Troubleshooting

LDR value stays flat or near zero. The LDR or 10k resistor is not making contact at the A0 node. Check that both legs share the same breadboard row connected to A0, and that the 10k goes from that row to GND, not to 5 V.
LDR value barely changes when the beam is blocked. Strong ambient light is swamping the laser contribution. Shield the LDR from room lighting with a short tube and retest.
Buzzer fires continuously even with the beam aligned. LDR_THRESHOLD is set higher than the beam-on reading. Print the beam-on value and set the threshold well below it.
Buzzer is silent even when the beam is clearly blocked. Confirm the buzzer is passive (not active). An active buzzer with a built-in oscillator will sometimes not respond to tone(). Also check the SIG pin is on D5 and GND is connected.
Laser does not turn on. Verify the SIG pin is D2 and the module VCC is on 5 V, not 3.3 V. Serial Monitor should show "Laser tripwire demo ready" once the sketch starts, confirming LASER_PIN has been set high.

Going further

This demo shows the core principle cleanly. The two most practical extensions are a re-arm delay and a status LED. A re-arm delay stops the buzzer after a fixed period and waits for the beam to be restored before arming again — this avoids a continuous alarm during a slow crossing. A status LED (green for armed, red for triggered) makes the state visible at a distance without Serial Monitor.

For a multi-beam version with note output instead of a buzzer alarm, the Laser Harp Synth guide uses three LDR channels on an ESP32 with tone-per-channel output, which is a natural next step once this single-channel circuit is solid.

Wiring diagram

Loading diagram…

Components needed

Component	Type	Qty	Buy
KY-008 650nm Laser Transmitter Module	actuator	1	€1.40
Photo Cell (CdS Photoresistor)	sensor	1	$0.95
1/4W Resistor Pack - 400 PCS	passive	1	$0.00
Piezo Buzzer	actuator	1	$1.50

Supplier links, prices, and availability are shown as a guide and may change. Schematik may earn a commission from purchases made through affiliate links.

Assembly

1

Build the LDR divider

Place the LDR so one leg goes to 5V and the other leg shares a row with A0. Add the 10k resistor from that same A0 row to GND.

Tips

Point the laser at the LDR before powering the circuit.

Warning

Use only a low-power visible laser and never aim it at eyes, pets, reflective surfaces, or people.

2

Wire the laser and buzzer

Connect the laser module signal to D2, VCC to 5V, and GND to GND. Connect the buzzer signal to D5 and its negative leg to GND.

Tips

Share a common ground across the laser, buzzer, and LDR divider.

Warning

This is a toy alarm demo, not a real security or access-control system.

3

Upload and calibrate

Upload the sketch, open Serial Monitor at 9600 baud, and note the LDR readings with the beam hitting and missing the sensor. Adjust LDR_THRESHOLD if your readings sit above or below 400.

Tips

Increase the threshold if the buzzer never fires; decrease it if it fires while the beam is aligned.

Pin assignments

Pin	Connection	Type
5V	laser-module-1 VCC	POWER
GND	laser-module-1 GND	GROUND
GPIO 2	laser-module-1 SIG	DIGITAL
5V	ldr-1 VCC	POWER
GPIO 14	ldr-1 AO	ANALOG
GPIO 14	resistor-10k-1 Divider node → ldr-1:AO	ANALOG
GND	resistor-10k-1 GND	GROUND
GPIO 5	buzzer-1 SIG	DIGITAL
GND	buzzer-1 GND	GROUND

Code

Arduino C++

OpenOpen in Schematik

#include <Arduino.h>

const int LASER_PIN = 2;
const int BUZZER_PIN = 5;
const int LDR_PIN = A0;
const int LDR_THRESHOLD = 400;

void setup() {
  Serial.begin(9600);
  pinMode(LASER_PIN, OUTPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  pinMode(LDR_PIN, INPUT);
  digitalWrite(LASER_PIN, HIGH);
  noTone(BUZZER_PIN);
  Serial.println("Laser tripwire demo ready");
}

void loop() {
  int lightLevel = analogRead(LDR_PIN);
  Serial.print("LDR value: " );
  Serial.println(lightLevel);

  if (lightLevel < LDR_THRESHOLD) {
    tone(BUZZER_PIN, 1000);
    Serial.println("Intruder detected: beam broken");
  } else {
    noTone(BUZZER_PIN);
  }

  delay(100);
}

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