#define ECHOpin 5 // defines the ECHO pin of the sensor to pin 5 of Arduino
#define TRIGpin 6

long duration; // variable for the duration of sound wave travel
int distance;   // variable for the distance measurement

void setup() {
  pinMode(TRIGpin, OUTPUT); // sets the TRIG pin as OUTPUT
  pinMode(ECHOpin, INPUT);  // sets the ECHO pin as INPUT
  Serial.begin(9600);       // Serial Communication at the rate of 9600 bps
  Serial.println("Test of the Ultrasonic Sensor HC-SR04"); // text on Serial Monitor
  Serial.println("with the Arduino Mega");
}

void loop() {
  digitalWrite(TRIGpin, LOW);          // sets the TRIG pin at LOW for 2 microseconds
  delayMicroseconds(2);

  digitalWrite(TRIGpin, HIGH);         // sets TRIG pin at HIGH for 10 microseconds
  delayMicroseconds(10);
  digitalWrite(TRIGpin, LOW);

  duration = pulseIn(ECHOpin, HIGH);    // reads the ECHO pin and returns the time duration

  // distance formula
  distance = duration * 0.034 / 2;     // speed of sound wave (340 m/s) divided by 2 (forward and backward bounce)

  // To display the distance on Serial Monitor
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm"); // specified unit of distance

  delay(1000); // optional delay for better readability
}