//HC-SR04 - Distance Measurement
int trigPin = 12;
int echoPin = 11;
float Vs = 331.5 + 0.6 * 20; // sound velocity in m/s

void setup() {
Serial.begin(9600);
pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
}

void loop() {
// Send sound pulse 10us TTL (Time to live)
digitalWrite(trigPin, LOW);
delayMicroseconds(3);
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);
  
// Listen for echo, tS in microseconds (us)
float tS = pulseIn(echoPin, HIGH);
// Get half round-trip time, RTT_half
float RTT_half = tS / 2.0;
// Convert sound velocity Vs (m/s) to Vcmus (cm/us)
float Vcmus = Vs / 10000.0;
// Distance Dcm in centimeters (cm)
float Dcm = Vcmus * RTT_half;

// Print measured distance in cm
Serial.print(Dcm);
Serial.println(" cm");
delay(200);
}