int const trigPin = 10;     // Defines the data pins of the ultrasonic
int const echoPin = 9;     
int const buzzPin = 2;      // Defines the buzzer pin
void setup()
{
pinMode(trigPin, OUTPUT);   // sets trig pin to have output pulses
pinMode(echoPin, INPUT);    // sets echo pin to be input and get the pulse width
pinMode(buzzPin, OUTPUT);   // sets buzz pin as output to control the sound
}
void loop()
{
  long duration, distance;
  digitalWrite(trigPin, LOW); 
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);

 distance= duration * 0.0344 / 2;
  
if (distance > 0 && distance < 51)
{
  tone(buzzPin, 8000, 500);
}
else
{
  noTone(buzzPin);
}
}