// Pin Definitions
const int TRIG_PIN = 16;
const int ECHO_PIN = 17;
const int LED_PIN  = 23;

// Constants
const int TARGET_DISTANCE = 50; 
const int TOLERANCE = 1;             // Allows for 49, 50, or 51cm
const long DURATION_REQUIRED = 10000; // 5 seconds

// Variables
unsigned long detectionStartTime = 0; 
bool isDetecting = false;

void setup() {
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);
}

void loop() {
  // 1. Measure Distance
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  long duration = pulseIn(ECHO_PIN, HIGH);
  int distance = duration * 0.034 / 2;

  // 2. Strict Range Logic
  // This checks if the distance is specifically between 49 and 51
  if (distance >= (TARGET_DISTANCE - TOLERANCE) && distance <= (TARGET_DISTANCE + TOLERANCE)) {
    
    if (!isDetecting) {
      detectionStartTime = millis(); // Start timing
      isDetecting = true;
      Serial.println("Object held at exactly 50cm. Timing...");
    } else {
      if (millis() - detectionStartTime >= DURATION_REQUIRED) {
        digitalWrite(LED_PIN, HIGH); // Turn on after 5 seconds of stability
      }
    }
    
  } else {
    // Distance is < 49 or > 51, so we reset everything
    isDetecting = false;
    digitalWrite(LED_PIN, LOW);
    
    // Optional: Print distance for debugging
    if (distance > 0) {
      Serial.print("Out of range: ");
      Serial.println(distance);
    }
  }

  delay(100); 
}