int ledPin = 9; 

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

void loop() 
{
  // Increase brightness
  for (int brightness = 0; brightness <= 255; brightness++) {
    analogWrite(ledPin, brightness); // Set PWM duty cycle
    delay(8);
  }

  // Decrease brightness
  for (int brightness = 255; brightness >= 0; brightness--) {
    analogWrite(ledPin, brightness);
    delay(4);
  }
}