int led =  6;

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

void loop() {
  for (int x = 0 ; x <= 255; x += 5) 
  {
    analogWrite(led, x);
    delay(30);
  }

  for (int x = 255 ; x >= 0; x -= 5) 
  {
    analogWrite(led, x);
    delay(30);
  }
}