int trigPin = 7;
int echoPin = 4;
int Ledpin = 2;
int (distance);
long (duration);
void setup() {
  pinMode(trigPin, OUTPUT); 
  pinMode(echoPin, INPUT);
  pinMode(Ledpin, OUTPUT);
  Serial.begin(9600); 
}
void loop() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  
  duration = pulseIn(echoPin,HIGH);
  distance = duration * 0.034/2;

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

  if (distance <= 51){
    digitalWrite(Ledpin, HIGH);
  }else{
    digitalWrite(Ledpin, LOW);
  }

}