Arduino parking sensor on my knapsack

This is a “parking sensor” that I made for my knapsack, but you can use it on your car.

You’ll need an arduino board, a ultrasonic range sensor, a 9v battery, a switch and some wires.

This is the code:

//pin 6 = ultrasonic sensor pin

//pin 11 = 5v buzzer

const int pingPin = 6;
int SPKR = 11;
long previousMillis = 0;

void setup() {
  pinMode(SPKR, OUTPUT);
  Serial.begin(9600);
}

void loop()
{
  long duration, inches, cm;
  pinMode(pingPin, OUTPUT);
  digitalWrite(pingPin, LOW);
  delayMicroseconds(2);
  digitalWrite(pingPin, HIGH);
  delayMicroseconds(5);
  digitalWrite(pingPin, LOW);
  pinMode(pingPin, INPUT);
  duration = pulseIn(pingPin, HIGH);
  duration = map(duration,  0, 15000, 0, 5000);
  Serial.println(duration);
  unsigned long currentMillis = millis();

  if(currentMillis – previousMillis > (duration/2)) {
    previousMillis = currentMillis;  
  if (duration!=0) {
    if (duration<2000) {
    for (int i=0; i<500; i++) {
      digitalWrite(SPKR, HIGH);
      delayMicroseconds(150);
      digitalWrite(SPKR, LOW);
      delayMicroseconds(150);
    }
    }
  }
}
  delay(100);
}