#define trigPin 13   // This defines a constant "trigPin" with the value "13".
#define echoPin 12   // This defines a constant "echoPin" with the value "12".
#define led 11       // This defines a constant "led" with the value "11".

void setup() {
  Serial.begin(9600);             // Initializes the serial communication at baud rate of 9600 bits per second.
  pinMode(trigPin, OUTPUT);       // Sets the "trigPin" (pin13) as an output. This pin will send a signal to the ultrasonic sensor.
  pinMode(echoPin, INPUT);        // Sets the "echoPin" (pin12) as an input.
  pinMode(led, OUTPUT);           // Sets the "led" (pin11) as an output. This controls the LED.
}

void loop() {
  long duration, distance;        // Declared two variables "duration" and "distance" of type 'long' to store the time and distance.
  digitalWrite(trigPin, LOW);     // Sets the "trigPin" to 'LOW', ensuring there is no lingering signal from previous readings.
  delayMicroseconds(2);           // Waits for 2 microseconds to ensure a clean signal start.
  digitalWrite(trigPin, HIGH);    // Sets the "trigPin" to 'HIGH', sending a 10 microsecond pulse to the ultrasonic sensor.
  delayMicroseconds(10);          // Waits for 10 microseconds while the pulse is being sent.
  digitalWrite(trigPin, LOW);     // Stops the pulse by setting the "trigPin" to 'LOW'.
  duration = pulseIn(echoPin, HIGH); // Measures the time in microseconds that "echoPin" is set to 'HIGH'.
  distance = (duration/2) / 29.1; // The distance is divided by 2 because the pulse travels to the object and back.
  
  if (distance < 100) {
    digitalWrite(led, HIGH);      // If the distance is less than 100 cm, LED is turned on.
  } else {
    digitalWrite(led, LOW);       // If the distance is 100 cm or more, LED is off.
  }

  Serial.print(distance);         // Prints distance in value.
  Serial.print("cm");             // Prints cm after distance.
  delay(500);                     // Pauses the loop for 500 milliseconds before the next measurement.
}