// Define pin numbers
const int trigPin = 9;
const int echoPin = 10;
const int greenLedPin = 5; // New Green LED Pin
const int redLedPin = 6;   // New Red LED Pin
const int buzzerPin = 7;   // New Buzzer Pin

// Define variables
long duration;
int distance;

// Define safety threshold (half of max 400cm range)
const int safetyThreshold = 200; 

void setup() {
  // Set pin modes for ultrasonic sensor
  pinMode(trigPin, OUTPUT); 
  pinMode(echoPin, INPUT);  
  
  // Set pin modes for new alert hardware
  pinMode(greenLedPin, OUTPUT);
  pinMode(redLedPin, OUTPUT);
  pinMode(buzzerPin, OUTPUT);
  
  // Initialize serial communication
  Serial.begin(9600);
  Serial.println("Ultrasonic Distance Sensor with Alerts Initialized!");
}

void loop() {
  // Clear the trigPin by pulling it LOW
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  
  // Trigger the sensor by sending a 10-microsecond HIGH pulse
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  
  // Read the echoPin; returns the sound wave travel time in microseconds
  duration = pulseIn(echoPin, HIGH);
  
  // Calculate the distance in centimeters
  distance = duration * 0.0343 / 2;
  
  // Print the distance to the Serial Monitor
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");
  
  // Check if something is in the half-range zone
  // Note: pulseIn can return 0 if out of range, so we ensure distance > 0
  if (distance > 0 && distance <= safetyThreshold) {
    // ALERT CONDITION: Object is close!
    digitalWrite(redLedPin, HIGH);    // Turn ON Red LED
    digitalWrite(greenLedPin, LOW);   // Turn OFF Green LED
    digitalWrite(buzzerPin, HIGH);    // Turn ON Buzzer
  } else {
    // NORMAL CONDITION: Path is clear
    digitalWrite(redLedPin, LOW);     // Turn OFF Red LED
    digitalWrite(greenLedPin, HIGH);  // Turn ON Green LED
    digitalWrite(buzzerPin, LOW);     // Turn OFF Buzzer
  }
  
  // Wait half a second before the next reading
  delay(500);
}