How to use Ultrasonic Sensor?

Arduino Sketch Code is as below:

// Pin 13 has an LED connected on most Arduino boards.
// give it a name:
int sig = 7;
int led = 3;

// the setup routine runs once when you press reset:
void setup() {
  // initialize the digital pin as an output.
  Serial.begin(9600);
  pinMode(led, OUTPUT);
}

// the loop routine runs over and over again forever:
void loop() {
  pinMode(sig, OUTPUT);
  digitalWrite(sig, LOW);   // turn the LED on (HIGH is the voltage level)
  delayMicroseconds(2);               // wait for a second
  digitalWrite(sig, HIGH);    // turn the LED off by making the voltage LOW
  delayMicroseconds(5);               // wait for a second
  digitalWrite(sig, LOW);

  pinMode(sig, INPUT);
  long duration = pulseIn(sig, HIGH);
  int distance = duration / 74 / 2;

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

  if (distance <50){
    tone(8, 1000, 500);
    digitalWrite(led, HIGH);
    delay(100);
  }

  digitalWrite(led, LOW);
  delay(100);

}

Note: I am using circuits.io for simulating the task,