int trig=13,echo=12,duration,distance;
// HC- SR04 = ULTRASONIC SENSOR 
// It's minmum range = 2cm
// maximum range = 4 metres
// accuracy = 3 millimetre
void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);
pinMode(trig, OUTPUT);
pinMode(echo, INPUT);
}
void loop() {
  // put your main code here, to run repeatedly:
digitalWrite(trig,0);
delayMicroseconds(2);
digitalWrite(trig,1);
delayMicroseconds(10);
digitalWrite(trig,0);
duration = pulseIn(echo,1); //in microsecond
distance = duration*0.034/2;
Serial.print(" Distance in cm = ");
Serial.println(distance);
}