HC-SR04

// HC-SR04 ultrasonic distance sensor example
// Wiring:
// HC-SR04 Vcc -> 5V
// HC-SR04 Gnd -> GND
// HC-SR04 Trig -> D9
// HC-SR04 Echo -> D10

const int trigPin = 9;
const int echoPin = 10;

void setup() {
  Serial.begin(9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  digitalWrite(trigPin, LOW);
  Serial.println("HC-SR04 ready");
}

void loop() {
  // Send a 10 microsecond pulse to start a measurement
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  // Read the echo pulse length (timeout after 30 ms)
  unsigned long duration = pulseIn(echoPin, HIGH, 30000UL);

  if (duration == 0) {
    Serial.println("Out of range or no echo");
  } else {
    // Convert time to distance in centimeters
    // Speed of sound approx. 343 m/s => 29.1 us per cm round trip
    float distanceCm = duration / 58.0;
    Serial.print("Distance: ");
    Serial.print(distanceCm, 1);
    Serial.println(" cm");
  }

  delay(500);
}