DIY Intruder Alarm with LDR Sensor | Arduino Security Project

In this tutorial, we’ll build a simple light alarm system using an Arduino Uno and a 3-pin LDR sensor module. This system detects when someone blocks a light source—like breaking a beam—and triggers an LED alert, simulating a basic laser tripwire.

Light Alarm System

🔍 What Is an LDR Sensor?

An LDR (Light Dependent Resistor) is a sensor that changes its resistance based on the amount of light it receives. In brighter light, resistance drops; in darkness, resistance increases. By using the analogRead() function, we can detect the intensity of ambient light. 

📦 What You’ll Need

To get started, you will need the following components:

Components required for this tutorial

✅ Arduino UNO × 1

✅ 3-pin LDR Module × 1

✅ Jumper Wires × 1

✅ USB cable for Arduino × 1

✅ Resistor 220Ω × 1

✅ Breadboard × 1

🛠️ Circuit Wiring

Here’s how to connect each component to the Arduino:

3-pin LDR module wiring

3-pin LDR Module 

✅ VCC → 5V on Arduino

✅ GND → GND on Arduino

✅ A0 → Analog pin A0 on Arduino

LED and resistor wiring

LED and Resistor

✅ LED Anode (long leg) → Digital pin 9 on Arduino

✅ LED Cathode (short leg) → 220Ω resistor → GND on Arduino

The connection between components and Arduino

💻 Uploading the Code

After wiring everything up, connect the Arduino to your computer using a USB cable.  Then, upload the following code, or you can download the code HERE.

int ldrPin = A0;
int ledPin = 9;
int baselineLight = 800; // Adjust based on your lighting

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int ldrValue = analogRead(ldrPin);
  Serial.println(ldrValue);

  if (ldrValue < baselineLight - 200) {  // Sudden drop in light
    digitalWrite(ledPin, HIGH); // Alert
  } else {
    digitalWrite(ledPin, LOW);
  }

  delay(100);
}

Open Arduino IDE and copy-paste the code

The system reads the LDR sensor using analogRead(ldrPin) (range: 0–1023), compares it to the baselineLight value representing normal ambient light, and if the light level drops significantly—by more than 200 units—the LED is activated to indicate an alert.

🧪 Test the System

Once you've uploaded the code and completed the wiring, it's time to test how the system reacts to different light levels:

Place your hand over the sensor, and the LED will turn ON.

Remove your hand, and once the light returns to normal, the LED will turn OFF

📌 Conclusion

This project is a simple and fun way to get started with analog sensors, security concepts, and Arduino programming. You’ve now created a basic light alarm system that responds to interruptions in a light beam—just like a basic tripwire!

📦 Where to Buy

Shopee:

1. 3-Pin LDR Module
2. Jumper Wire
3. UNO R3
4. USB Cable
5. Resistor 0.25W Pack

Lazada:

1. 3-Pin LDR Module
2. Jumper Wire
3. UNO R3
4. USB Cable
5. Resistor 0.25W Pack

TikTok:

1. 3-Pin LDR Module
2. Jumper Wire
3. UNO R3
4. USB Cable
5. Resistor 0.25W Pack

Aliexpress:

1. 3-Pin LDR Module
2. Jumper Wire
3. UNO R3
4. USB Cable
5. Resistor 0.25W Pack

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