const int PIR_PIN = 2;
const int LED_PIN = 8;

int lastState = LOW;
unsigned long tRise = 0;

void setup() {
  pinMode(PIR_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);
  Serial.begin(115200);
}

void loop() {
  int s = digitalRead(PIR_PIN);
  if (s != lastState) {
    if (s == HIGH) {
      tRise = micros();
      digitalWrite(LED_PIN, HIGH);
      unsigned long tCmd = micros();
      Serial.print("RESP_US=");
      Serial.println(tCmd - tRise);
    } else {
      digitalWrite(LED_PIN, LOW);
    }
    lastState = s;
  }
}