// These constants won't change:
const int sensorPin = A0;    // pin that the sensor is attached to
const int ledPin = 9;        // pin that the LED is attached to

// Variables:
int sensorValue = 0;         // the sensor value
int sensorMin = 1023;        // minimum sensor value
int sensorMax = 0;           // maximum sensor value

void setup() {
  // Turn on LED to signal the start of the calibration period:
  pinMode(13, OUTPUT);
  digitalWrite(13, HIGH);
  
  // Start serial communication for debugging:
  Serial.begin(9600);

  // Calibrate during the first five seconds
  while (millis() < 5000) {
    sensorValue = analogRead(sensorPin);

    // Record the maximum sensor value
    if (sensorValue > sensorMax) {
      sensorMax = sensorValue;
    }

    // Record the minimum sensor value
    if (sensorValue < sensorMin) {
      sensorMin = sensorValue;
    }
  }

  // Signal the end of the calibration period
  digitalWrite(13, LOW);
}

void loop() {
  // Read the sensor:
  sensorValue = analogRead(sensorPin);

  // Print sensor values for debugging:
  Serial.print("Sensor Value: ");
  Serial.println(sensorValue);
  Serial.print("Sensor Min: ");
  Serial.println(sensorMin);
  Serial.print("Sensor Max: ");
  Serial.println(sensorMax);

  // In case the sensor value is outside the range seen during calibration
  sensorValue = constrain(sensorValue, sensorMin, sensorMax);

  // Apply the calibration to the sensor reading
  sensorValue = map(sensorValue, sensorMin, sensorMax, 0, 255);

  // Fade the LED using the calibrated value:
  analogWrite(ledPin, sensorValue);
}