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PIR Intruder Alarm (Arduino)

Sound an active buzzer and flash an LED when a PIR sensor sees motion. A complete little security alarm.

Beginner20 min

What you need

Wiring

Arduino Uno
PIR sensor
Active buzzer
LED

Hover a pin or a wire to trace the connection.

Code

intruder_alarm.ino
// Alarm: PIR motion triggers an active buzzer + LED for a couple seconds.
const int pirPin = 2;
const int buzzer = 8;   // active buzzer (beeps on HIGH)
const int ledPin = 13;

void setup() {
  pinMode(pirPin, INPUT);
  pinMode(buzzer, OUTPUT);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  if (digitalRead(pirPin) == HIGH) {     // motion!
    digitalWrite(buzzer, HIGH);
    digitalWrite(ledPin, HIGH);
    delay(2000);                          // hold the alarm 2s
  } else {
    digitalWrite(buzzer, LOW);
    digitalWrite(ledPin, LOW);
  }
}

Steps

  1. Wire the PIR: VCC to 5V, OUT to D2, GND to GND.
  2. Wire an active buzzer: + to D8, − to GND. LED on D13 through a 220Ω resistor.
  3. Upload, let the PIR settle ~30–60s.
  4. Walk past — the buzzer sounds and the LED lights for two seconds.