void setup() {
pinMode(3, INPUT);
pinMode(5, OUTPUT);
pinMode(13, OUTPUT);
Serial.begin(9600);
}

void loop() {
digitalWrite(5, LOW);
delayMicroseconds(2);
digitalWrite(5, HIGH);
delayMicroseconds(10);
digitalWrite(5, LOW);
long Duration = pulseIn(3,HIGH);
int distance = Duration*0.034/2;
int b = map(distance, 2,510,2,1021);
analogWrite(13,b);
Serial.print("Distance:");
Serial.print(distance);
Serial.println("CM");
delay(1000);
}