const int LED_PIN = 3;
const int DELAY_TIME = 15;
const int SPEED_FACTOR = 5;

void setup() {
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  
  // Increasing brightness
  for(int brightness = 0; brightness < 256; brightness += SPEED_FACTOR) {
    analogWrite(LED_PIN, brightness);        
    delay(DELAY_TIME);
  }

  // Decreasing brightness
  for(int brightness = 255; brightness >= 0; brightness -= SPEED_FACTOR) {
    analogWrite(LED_PIN, brightness);
    delay(DELAY_TIME);
  }
}