#define echoPin 7// attach pin D2 Arduino to pin Echo of HC-SR04
#define trigPin 8 // attach pin D3 Arduino to pin Trig of HCSR04                                        
long duration;  // Variable to store time taken to the pulse to reach receiver
int distance; // Variable to store distance calculated using formula
void setup()
{
 pinMode(trigPin, OUTPUT); // Sets the trigPin as an OUTPUT
  pinMode(echoPin, INPUT); // Sets the echoPin as an INPUT
  // Serial Communication is starting with 9600 of baudrate speed
 Serial.begin(9600);
  // The text to be printed in serial monitor
  Serial.println("Distance measurement using Arduino Uno.");
 delay(500);
}
void loop()
{
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2); // wait for 2 ms to avoid collision in serial monitor
 digitalWrite(trigPin, HIGH); // turn on the Trigger to generate pulse
  delayMicroseconds(10); // keep the trigger "ON" for 10 ms to generate pulse for 10 ms.
 digitalWrite(trigPin, LOW); // Turn off the pulse trigger to stop pulse generation
 // If pulse reached the receiver echoPin become high Then pulseIn() returns the time taken
  //        by the pulse to reach the receiver
  duration = pulseIn(echoPin, HIGH);
distance = duration * 0.0344 / 2; // Expression to calculate distance using time
Serial.print("Distance: ");
Serial.print(distance); // Print the output in serial monitor
Serial.println(" cm");
delay(100);
}