float distance;
float duration;
void setup(){
Serial.begin(9600);
pinMode(18,OUTPUT);
pinMode(17, INPUT);
pinMode(4, OUTPUT);
}
void loop(){
digitalWrite(13, HIGH);
delay(10);
digitalWrite(18, LOW);
duration=pulseIn(17,HIGH);
distance=duration *0.017;
if (distance<50)
{
  digitalWrite(13,HIGH);
}
else
{
  digitalWrite(4, LOW);
}
Serial.print("distance:");
Serial.print(distance);
Serial.print(" cm ");
}