float distance;
float duration;
void setup()
{
  Serial.begin(9600);
  pinMode(18, OUTPUT);
  pinMode(17, INPUT);
  pinMode(4, OUTPUT);
}
void loop()
{
  digitalWrite(18, HIGH);
  delay(10);
  digitalWrite(18, LOW);
  duration=pulseIn(17,HIGH);
  distance=duration*0.017;
  if(distance<100)
  {
    digitalWrite(4, HIGH);
  }
  else
  {
    digitalWrite(4, LOW);
  }
//  Serial.print("distance");
}