← All projects

Motion Alarm

A small home alarm that goes off when someone walks in: the HC-SR501 PIR sensor senses the change in the heat a human body gives off, the buzzer plays a two-tone siren and the red LED blinks; every alarm is counted in the Serial Monitor. It teaches reading a sensor's digital output, catching only the moment motion starts, sounding for a set time with millis(), and gathering the job into its own function. No extra library needed.

What you need

• 1 × Arduino Uno R3
• 1 × Breadboard 830 Points (MB-102)
• 1 × HC-SR501 PIR Motion Sensor
• 1 × Passive Buzzer (Piezo)
• 1 × LED - Red (5mm)
• 1 × Resistor 220 Ω (1/4W)
• 5 × Jumper Wire - Male/Male
• 3 × Jumper Wire - Male/Female

Wiring steps

1. 1

   Connect the power rails (5V and GND)

   

   Plug a black male-male jumper wire into one of the GND pins on the Arduino's power header and connect its other end to the breadboard's top blue (−) rail. Plug a red wire into the 5V pin on the same header and connect its other end to the top red (+) rail. The PIR sensor is powered from these two rails, and the LED's resistor and the buzzer take their ground from the (−) rail.

   • Arduino GND → the top blue (−) rail (black wire)
   • Arduino 5V → the top red (+) rail (red wire)
2. 2

   Add the warning LED and its resistor

   

   Plug the red LED's long leg (+) into column 15 in the top half of the breadboard and its short leg (−) into column 16. Put one end of the 220 Ω resistor in column 16 and the other end in the top (−) rail. Then connect the Arduino's pin 12 to column 15 with a blue jumper wire (LED_PIN 12). The resistor limits the current through the LED.

   • red LED long leg (+) → column 15, top half
   • red LED short leg (−) → column 16, top half
   • 220 Ω resistor (leg 1) → column 16, top half
   • 220 Ω resistor (leg 2) → the top blue (−) rail
   • Arduino pin 12 → column 15, top half (blue wire)
3. 3

   Add and wire the buzzer

   

   Plug the passive buzzer's + leg into column 8 in the top half and its other leg into column 9; the + leg is usually the longer one and the body has a + mark. Connect the Arduino's pin 8 to column 8 with an orange jumper wire (BUZZER_PIN 8). Connect column 9 to the top (−) rail with a short black wire.

   • buzzer + leg → column 8, top half
   • buzzer − leg → column 9, top half
   • Arduino pin 8 → column 8, top half (orange wire)
   • column 9, top half → the top blue (−) rail (black wire)
4. 4

   Connect the PIR sensor

   

   Push the female ends of male-female jumper wires onto the HC-SR501's three pins (take off the white dome and the pin names are printed on the board). Connect VCC to the top (+) rail with a red wire, GND to the top (−) rail with a black wire, and OUT straight to the Arduino's pin 2 with a yellow wire (PIR_PIN 2). Of the two small screws on the board, one sets how far it senses and the other how long the output stays HIGH: to start, turn the time screw fully anticlockwise. With the corner jumper on H, the output stays HIGH for as long as the motion goes on.

   • PIR sensor VCC → the top red (+) rail (red wire)
   • PIR sensor GND → the top blue (−) rail (black wire)
   • Arduino pin 2 → PIR sensor OUT (yellow wire)

Arduino sketch

// PIR, buzzer and LED pins
const int PIR_PIN = 2;
const int BUZZER_PIN = 8;
const int LED_PIN = 12;

// PIR warm-up (ms): it learns the room
// meanwhile, so keep still
const unsigned long WARM_UP = 30000;
// How long one alarm sounds (ms)
const unsigned long ALARM_TIME = 3000;

int lastState = LOW;
int alarms = 0;

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

  Serial.println("PIR warming up, wait 30 s...");
  delay(WARM_UP);
  Serial.println("Alarm armed.");
}

// Siren: two tones in turn, LED blinks
void soundAlarm() {
  unsigned long start = millis();
  bool high = true;
  while (millis() - start < ALARM_TIME) {
    tone(BUZZER_PIN, high ? 880 : 660);
    digitalWrite(LED_PIN, high ? HIGH : LOW);
    high = !high;
    delay(250);
  }
  noTone(BUZZER_PIN);
  digitalWrite(LED_PIN, LOW);
}

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

  // Only the moment motion starts
  if (state == HIGH && lastState == LOW) {
    alarms++;
    Serial.print("Motion detected! Alarm: ");
    Serial.println(alarms);
    soundAlarm();
  }
  lastState = state;
}

What the Serial Monitor shows

PIR warming up, wait 30 s...
Alarm armed.
Motion detected! Alarm: 1
Motion detected! Alarm: 2

What you will see

At start-up the Serial Monitor says "PIR warming up"; wait 30 seconds without moving. Once it says "Alarm armed.", walk past the sensor: the buzzer sounds a two-tone siren for 3 seconds, the red LED blinks and the alarm count is printed. While the motion goes on the alarm does not sound again; it does once the sensor has calmed down and sees motion anew. If the alarm never sounds, check that the yellow wire is on pin 2 and VCC is in the (+) rail, and really move in front of the sensor (a PIR sees heat; it cannot see through glass). If the alarm keeps sounding or stops too late, turn the time screw on the board anticlockwise.