#define US_TRIG 13
#define US_ECHO 12

long distance_cm, echo_duration;

void setup() {
  //Serial port
  Serial.begin(115200);

  //Ultra sonic pins
  pinMode(US_TRIG, OUTPUT);
  pinMode(US_ECHO, INPUT);
}

void loop() {
  // start measurment by a short high pulse of 10 microseconds length at the trig pin ...
  digitalWrite(US_TRIG,LOW);
  delayMicroseconds(2);
  digitalWrite(US_TRIG,HIGH);
  delayMicroseconds(10);
  digitalWrite(US_TRIG,LOW);

  // measure the duration in microseconds of the echo pin HIGH - this is the time the echo needs to be back at the sensor
  pinMode(US_ECHO,INPUT);
  echo_duration = pulseIn(US_ECHO,HIGH);

  // convert into cm ... 344m/sec is the speed of noise - thus 34400cm/sec ... or 34,400cm/milisec ... or 0,0344cm/microsec
  // the echo has to go the distance twice - forth and back - so the duration has to be the half of the measured one
  // distance_cm = echo_duration/2 * 0,0344   or   distance_cm = echo_duration/2 / 29,1
  distance_cm = (echo_duration/2) / 29.1;

  //print this result at the screen
  Serial.print("Distance in cm: ");
  Serial.println(distance_cm);

  //wait before next measurement
  delay(250); //ms
  
}