int triggerpin=9;
int echopin=10;

long duration;
int distance;

void setup() {
  pinMode(triggerpin, OUTPUT);
  pinMode(echopin, INPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(9, LOW);
  delayMicroseconds(2);
  digitalWrite(9, HIGH);
  delayMicroseconds(10);
  digitalWrite(9, LOW);

  duration = pulseIn(10, HIGH);
  
  distance = duration * 0.0343/2;

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

  if (distance <= 20){
    digitalWrite(13, HIGH);
  }

  else{
    digitalWrite(13, LOW);
  }

  delay (1000);
}