int trigPin = 2;
int echoPin = 3;
int buzzer  = 13;
void setup() {
  Serial.begin (9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(buzzer, OUTPUT);
}

void loop() {
  
  long duration, distance;
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = (duration / 2) / 29.1;
  
  if (distance <= 399)// if the distance is less than 30 or = 30
                    //than the buzzer will make sound
  {
    digitalWrite(buzzer,HIGH);
    Serial.print(distance);//to print the distance in serial moniter
    Serial.println("");
  }
  else {
   digitalWrite(buzzer,LOW);//else it will turn off
   Serial.print(distance);//to print the distance in serial moniter
   Serial.println("");
  }
  delay(10);
}