HC-SR501 PIR sensor with Arduino Uno — wiring, jumpers and code

The HC-SR501 is the white-domed motion sensor behind most automatic lights and security alarms. It costs about ₹70 in India and does all the detection on board, so the Uno only reads one pin: HIGH when something warm moves, LOW otherwise.

It runs from the Uno’s 5V pin. Its output is a 3.3 V HIGH, which is still above the Uno’s logic threshold, so it connects straight to D2 with no extra parts. D2 is also one of the Uno’s two external-interrupt pins, in case you later want motion to wake a sleeping Uno.

Most “it doesn’t work” reports are really about the two pots and the jumper on the back, so they are covered below. The sketch waits out the warm-up, then lights the on-board LED during motion and reports how long each event lasted.

Wiring

Board quirks

• Give it 60 seconds after power-up. During warm-up OUT goes HIGH at random — the sketch waits before reporting.
• The two orange pots: Tx (time) sets how long OUT stays HIGH, from about 3 s to 5 min; Sx (sensitivity) sets range, up to about 7 m. Start with both fully anticlockwise.
• The jumper: H (repeat) keeps OUT HIGH while there is movement; L (single) drops it after the time even if you keep moving. Use H for lights.
• After OUT goes LOW, the sensor ignores motion for about 3 s. That is built into the BISS0001 chip, not a bug in your code.
• PIR sees heat moving, not presence. A person sitting still at a desk disappears after the delay; a ceiling fan or sunlight on a wall can trigger it.

Code

// HC-SR501 PIR motion sensor on an Arduino Uno R3.
// VCC → 5V, OUT → D2, GND → GND. OUT is a 3.3 V HIGH, which the Uno reads fine.
// Lights the on-board "L" LED (D13) while motion is detected.
const uint8_t PIR_PIN = 2;

bool lastState = LOW;
unsigned long motionStart = 0;

void setup() {
  pinMode(PIR_PIN, INPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.println(F("PIR warming up for 60 s - keep still..."));
  delay(60000);  // the sensor calibrates to the room after power-up
  Serial.println(F("PIR ready"));
}

void loop() {
  bool state = digitalRead(PIR_PIN);
  if (state != lastState) {
    lastState = state;
    digitalWrite(LED_BUILTIN, state);
    if (state == HIGH) {
      motionStart = millis();
      Serial.println(F("Motion detected"));
    } else {
      Serial.print(F("Motion ended after "));
      Serial.print((millis() - motionStart) / 1000);
      Serial.println(F(" s"));
    }
  }
}

// HC-SR501 PIR motion sensor on an Arduino Uno R3.
// VCC → 5V, OUT → D2, GND → GND. OUT is a 3.3 V HIGH, which the Uno reads fine.
// Lights the on-board "L" LED (D13) while motion is detected.
const uint8_t PIR_PIN = 2;

bool lastState = LOW;
unsigned long motionStart = 0;

void setup() {
  pinMode(PIR_PIN, INPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.println(F("PIR warming up for 60 s - keep still..."));
  delay(60000);  // the sensor calibrates to the room after power-up
  Serial.println(F("PIR ready"));
}

void loop() {
  bool state = digitalRead(PIR_PIN);
  if (state != lastState) {
    lastState = state;
    digitalWrite(LED_BUILTIN, state);
    if (state == HIGH) {
      motionStart = millis();
      Serial.println(F("Motion detected"));
    } else {
      Serial.print(F("Motion ended after "));
      Serial.print((millis() - motionStart) / 1000);
      Serial.println(F(" s"));
    }
  }
}

Expected serial output

PIR warming up for 60 s - keep still...
PIR ready
Motion detected
Motion ended after 7 s

PIR warming up for 60 s - keep still...
PIR ready
Motion detected
Motion ended after 7 s

Common errors