Building an Ultrasonic Parking Sensor

// Ultrasonic parking sensor: the buzzer beeps faster as an object gets closer.
// HC-SR04 Trig on pin 9, Echo on pin 10; active buzzer on pin 8.

const int TRIG_PIN = 9;
const int ECHO_PIN = 10;
const int BUZZER_PIN = 8;

void setup() {
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  // send a short trigger pulse
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // time the echo and convert to centimetres (speed of sound ~343 m/s)
  long duration = pulseIn(ECHO_PIN, HIGH);
  int distance = duration * 0.0343 / 2;

  Serial.print(distance);
  Serial.println(" cm");

  if (distance > 0 && distance < 50) {
    int gap = distance * 6;          // closer = shorter gap between beeps
    digitalWrite(BUZZER_PIN, HIGH);
    delay(30);
    digitalWrite(BUZZER_PIN, LOW);
    delay(gap);
  } else {
    digitalWrite(BUZZER_PIN, LOW);   // nothing close -> silent
    delay(60);
  }
}