/*The ultrasound transmitter (trig pin) emits a high-frequency sound (40 kHz).
The sound travels through the air. If it finds an object, it bounces back to the 
module.
The ultrasound receiver (echo pin) receives the reflected sound (echo).
The time between the transmission and reception of the signal allows us to calculate 
the distance to an object. This is possible because we know the sound’s velocity in
 the air. Here’s the formula:
distance to an object = ((speed of sound in the air)*time)/2
*/
#define echoPin 2 // it defines the ECHO pin of the sensor to pin 2 of Arduino  
#define trigPin 3 // it defines the TRIG pin of the sensor to pin 3 of Arduino  
long duration;  // variable for the duration of sound wave travel  
int distance;  // variable for the distance measurement 
void setup() {
  // Serial Communication at the rate of 9600 bps  
  Serial.begin(9600); 
  //Define inputs and outputs
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}
void loop() {
 // It first sets the TRIG pin at LOW for 2 microseconds 
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
// It now sets TRIG pin at HIGH for 20 microseconds  
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(20);
  digitalWrite(trigPin, LOW);
 // It will read the ECHO pin and will return the time  
  duration = pulseIn(echoPin, HIGH);
  // distance formula  
  distance = duration * 0.034 / 2; // (speed in microseconds)  
  // 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);
}