const int echoPin = 2;
const int triggerPin = 3;

long duration;
int distance;

void setup() {
   pinMode(triggerPin, OUTPUT);
   pinMode(echoPin, INPUT);
   Serial.begin(9600);
   Serial.println("Ultrasonic Sensor HC-SR04 Test");
   Serial.println("with Arduino UNO R3");
}

void loop() {
   // Set the trigger to LOW
   digitalWrite(triggerPin, LOW);
   delayMicroseconds (2);
   // Set the trigger to HIGH
   digitalWrite(triggerPin, HIGH);
   delayMicroseconds (10);
   digitalWrite(triggerPin, LOW);
   // Read ECHO - sound wave travel time in µs
   duration = pulseIn (echoPin, HIGH);

   if (duration == 0) {
      Serial.println("No echo from sensor");
   } else {
      // Calculate the distance
      distance = duration * 0.034 / 2;

      Serial.print("Distance:");
      Serial.print(distance);
      Serial.println("cm");
   }
}