const byte TRIGGER_PIN = 2; 
const byte ECHO_PIN = 3;   
 

const unsigned long MEASURE_TIMEOUT = 25000UL; // 25ms = ~8m at 340m/s

const float SOUND_SPEED = 340.0 / 1000; // In mm/µs


void setup() {
   
  Serial.begin(115200);
   
  pinMode(TRIGGER_PIN, OUTPUT);
  digitalWrite(TRIGGER_PIN, LOW); // TRIGGER pin have to be low when no activate
  pinMode(ECHO_PIN, INPUT);
}
 
void loop() {
  
  /* 1. Start a mesure by sending a 10µs HIGH Impulse on TRIGGER pin */
  digitalWrite(TRIGGER_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIGGER_PIN, LOW);
  
  /* 2. Mesure the time between the Impulse sending & is echo (if echo we are) */
  long measure = pulseIn(ECHO_PIN, HIGH, MEASURE_TIMEOUT);
   
  /* 3. Calcul the distance with the mesured time (divide by 2 time sound speed cause the return count) */
  float distance_mm = measure / 2.0 * SOUND_SPEED;
   
  /* Print the result in mm , cm & meter */
  Serial.print(F("Distance: "));
  Serial.print(distance_mm);
  Serial.print(F("mm ("));
  Serial.print(distance_mm / 10.0, 2);
  Serial.print(F("cm, "));
  Serial.print(distance_mm / 1000.0, 2);
  Serial.println(F("m)"));
   
  delay(500);
}