#define TRIG_PIN 13
#define ECHO_PIN 12
#define IND_LED 11
#define BUZ_PIN 9

bool entered;
long duration, distance;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(IND_LED, OUTPUT);
  pinMode(BUZ_PIN, OUTPUT);

  entered = false;
}

void loop() {
  // put your main code here, to run repeatedly:
 // Clear sensor
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);

  // Generate 10 microseconds pulse to the sensor
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  duration = pulseIn(ECHO_PIN, HIGH); // in microseconds
  distance = (duration / 2) / 29.1; // in cm

  // Assuming door width is 10cm (If someone crossed the 10cm range, alarm will be raised)
  if (distance < 10) {
    entered = true;
  }

  // Alarm raises when someone crossed the range
  if (entered) {
    // Turn on alarm light
    digitalWrite(IND_LED, HIGH);
    
    // Generate 2KHz buzzer noise
    tone(BUZ_PIN, 2000);
    delay(500);
    
    digitalWrite(IND_LED, LOW);
    noTone(BUZ_PIN);
  }
  
  Serial.print(distance);
  Serial.println(" cm");
  delay(500);
}