🚨 Build a Motion-Activated LED Alarm with Arduino

PIR (Passive Infrared) sensors detect the infrared radiation emitted by warm bodies. The HC-SR501 outputs HIGH when motion is detected, LOW when clear.

// HC-SR501 PIR Sensor Wiring:
//
//  PIR Sensor   Arduino Uno
//  ──────────   ───────────
//  VCC  ───────  5V
//  OUT  ───────  Digital Pin 7
//  GND  ───────  GND
//
// LED wiring:
//  Anode (+, long leg) → 220Ω resistor → Digital Pin 13
//  Cathode (-, short leg) → GND
//
// Buzzer wiring (optional):
//  Positive (+) → Digital Pin 8
//  Negative (-) → GND
//
// PIR calibration: let the sensor sit powered for 60 seconds before
// using it — it needs time to stabilise to the ambient IR level.

Read the PIR pin — HIGH means motion detected, LOW means clear.

#define PIR_PIN  7
#define LED_PIN  13

void setup() {
  Serial.begin(9600);
  pinMode(PIR_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);

  Serial.println("PIR sensor warming up — please wait 10 seconds...");
  delay(10000);  // Give PIR time to calibrate
  Serial.println("Ready! Watching for motion...");
}

void loop() {
  int motion = digitalRead(PIR_PIN);

  if (motion == HIGH) {
    digitalWrite(LED_PIN, HIGH);
    Serial.println("⚠️  MOTION DETECTED!");
  } else {
    digitalWrite(LED_PIN, LOW);
  }

  delay(100);
}

PIR sensor warming up — please wait 10 seconds...
Ready! Watching for motion...
⚠️  MOTION DETECTED!
⚠️  MOTION DETECTED!
⚠️  MOTION DETECTED!

#define PIR_PIN   7
#define LED_PIN   13
#define BUZZ_PIN  8

bool lastState    = false;
unsigned long alarmCount = 0;

void beep(int times) {
  for (int i = 0; i < times; i++) {
    digitalWrite(BUZZ_PIN, HIGH); delay(100);
    digitalWrite(BUZZ_PIN, LOW);  delay(80);
  }
}

void setup() {
  Serial.begin(9600);
  pinMode(PIR_PIN,  INPUT);
  pinMode(LED_PIN,  OUTPUT);
  pinMode(BUZZ_PIN, OUTPUT);

  Serial.println("Motion Alarm — warming up (10s)...");
  delay(10000);
  Serial.println("Ready.\n");
}

void loop() {
  bool motion = digitalRead(PIR_PIN);
  unsigned long seconds = millis() / 1000;

  if (motion && !lastState) {
    alarmCount++;
    String time = String(seconds / 60) + "m " + String(seconds % 60) + "s";
    Serial.print("[" + time + "] ALARM #");
    Serial.print(alarmCount);
    Serial.println(" — Motion detected!");

    digitalWrite(LED_PIN, HIGH);
    beep(2);
  }

  if (!motion && lastState) {
    digitalWrite(LED_PIN, LOW);
    Serial.println("             — All clear.");
  }

  lastState = motion;
  delay(100);
}

Motion Alarm — warming up (10s)...
Ready.

[0m 15s] ALARM #1 — Motion detected!
             — All clear.
[0m 43s] ALARM #2 — Motion detected!
[0m 45s] ALARM #3 — Motion detected!
             — All clear.

# Run this on your computer while the Arduino is connected via USB
# pip install pyserial

import serial
import datetime

PORT = 'COM3'      # Windows: COM3, COM4, etc.
                   # Mac/Linux: /dev/ttyUSB0 or /dev/ttyACM0
BAUD = 9600

ser = serial.Serial(PORT, BAUD, timeout=1)
print(f"Listening on {PORT}...")

with open('motion_log.txt', 'a') as log:
    while True:
        line = ser.readline().decode('utf-8', errors='ignore').strip()
        if line:
            timestamp = datetime.datetime.now().strftime('%Y-%m-%d %H:%M:%S')
            entry = f"[{timestamp}] {line}"
            print(entry)
            log.write(entry + '\n')
            log.flush()

Listening on COM3...
[2026-08-09 14:32:15] [0m 15s] ALARM #1 — Motion detected!
[2026-08-09 14:32:22] — All clear.
[2026-08-09 14:33:01] [0m 43s] ALARM #2 — Motion detected!