HC-SR501 PIR with Wemos D1 mini — interrupt-driven motion

A PIR sensor and a D1 mini make a motion-triggered light, an intrusion alert or a room-occupancy logger. Power the HC-SR501 from the 5V pin, connect OUT to D5, and the ESP8266 can react the instant someone walks in.

The sketch uses an interrupt, which brings out two ESP8266 rules: the handler must be in IRAM, and D0 can’t do interrupts at all. It also covers the most common D1 mini PIR complaint — false triggers caused by the board’s own Wi-Fi.

Wiring

Board quirks

• Interrupt handlers on the ESP8266 must be marked IRAM_ATTR. Without it the board crashes (or refuses to attach the interrupt) the moment motion is seen.
• D0 (GPIO16) has no interrupt on the ESP8266, which is why the PIR goes on D5. D1–D8 all support attachInterrupt.
• The HC-SR501 is known to false-trigger when an ESP8266 transmits close by — its amplifier picks up the 2.4 GHz bursts. Keep the PIR at least 10–15 cm from the D1 mini’s antenna end, or put it on a short cable.
• After power-up the sensor needs about a minute to settle; expect a few motion events in that time. The on-board time and sensitivity pots set how long OUT stays high and how far it sees.
• The built-in LED is on D4 and lights when the pin is LOW, so the sketch writes LOW for “motion”.

Code

// HC-SR501 PIR motion sensor on a Wemos D1 mini (ESP8266), read with an interrupt.
// VCC → 5V, OUT → D5 (GPIO14), GND → GND. The built-in LED (D4) lights while motion is seen.
const uint8_t PIR_PIN = D5;          // GPIO14 - D0 can't do interrupts on the ESP8266
const uint8_t LED_PIN = LED_BUILTIN; // D4 / GPIO2, lit when LOW

volatile bool changed = false;

// Interrupt handlers must live in IRAM on the ESP8266, or the board crashes when one fires.
void IRAM_ATTR onPirChange() {
  changed = true;
}

void setup() {
  Serial.begin(115200);
  Serial.println();
  pinMode(PIR_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, HIGH);  // LED off
  attachInterrupt(digitalPinToInterrupt(PIR_PIN), onPirChange, CHANGE);
  Serial.println("PIR on D5 - give it about 60 s to settle after power-up");
}

void loop() {
  if (changed) {
    changed = false;
    bool motion = digitalRead(PIR_PIN);
    digitalWrite(LED_PIN, motion ? LOW : HIGH);
    Serial.printf("[%lus] %s\n", millis() / 1000, motion ? "Motion detected" : "Motion ended");
  }
}

// HC-SR501 PIR motion sensor on a Wemos D1 mini (ESP8266), read with an interrupt.
// VCC → 5V, OUT → D5 (GPIO14), GND → GND. The built-in LED (D4) lights while motion is seen.
const uint8_t PIR_PIN = D5;          // GPIO14 - D0 can't do interrupts on the ESP8266
const uint8_t LED_PIN = LED_BUILTIN; // D4 / GPIO2, lit when LOW

volatile bool changed = false;

// Interrupt handlers must live in IRAM on the ESP8266, or the board crashes when one fires.
void IRAM_ATTR onPirChange() {
  changed = true;
}

void setup() {
  Serial.begin(115200);
  Serial.println();
  pinMode(PIR_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, HIGH);  // LED off
  attachInterrupt(digitalPinToInterrupt(PIR_PIN), onPirChange, CHANGE);
  Serial.println("PIR on D5 - give it about 60 s to settle after power-up");
}

void loop() {
  if (changed) {
    changed = false;
    bool motion = digitalRead(PIR_PIN);
    digitalWrite(LED_PIN, motion ? LOW : HIGH);
    Serial.printf("[%lus] %s\n", millis() / 1000, motion ? "Motion detected" : "Motion ended");
  }
}

Expected serial output

PIR on D5 - give it about 60 s to settle after power-up
[64s] Motion detected
[69s] Motion ended
[112s] Motion detected

PIR on D5 - give it about 60 s to settle after power-up
[64s] Motion detected
[69s] Motion ended
[112s] Motion detected

Common errors