const int trigPin = D1;
const int echoPin = D3;
long duration;  int distance; // VARIABLES
void setup() {
pinMode(trigPin, OUTPUT);     // TRIGPIN IS OUTPUT
pinMode(echoPin, INPUT);      // ECHO PIN IS INPUT
Serial.begin(9600);           // 9600 BITS TRANFERING TO PC PER SECOND
}
void loop() {
digitalWrite(trigPin, LOW);   // SENDING LOW PULSE FOR 2uSECONDS TO TRIGGER
delayMicroseconds(2);
digitalWrite(trigPin, HIGH);  // SENDING HIGH PULSE FOR 10uSECONDS TO TRIGGER
delayMicroseconds(10);
digitalWrite(trigPin, LOW);   
duration = pulseIn(echoPin, HIGH); // TO READ THE PULSE WHICH RETURN BACK THROUGH ECHO PIN
distance= duration*0.034/2;        // CALUCATING DISTANCE BASED ON SOUND VELOCITY

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