float distance;
float duration;
void setup() {
  Serial.begin(115200);
  pinMode(18,OUTPUT);
  pinMode(16, INPUT);
  pinMode(4,OUTPUT);
}

void loop() {
 digitalWrite(18, HIGH);
 delay(10);
 digitalWrite(18, LOW);
 duration = pulseIn(16,HIGH);
 distance = duration*0.017;
 if(distance>50){
  digitalWrite(4, HIGH);
 }
 else{
  digitalWrite(4, LOW);
 }
 Serial.print("Distance: ");
 Serial.print(distance);
 Serial.print( " cm");
}