#define led 11

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

void loop()
{
  for(int i = 0; i <= 255; i++)
  {
    analogWrite(led, i);
    delay(20);
  }
  for(int j = 255; j >= 0; j--)
  {
    analogWrite(led, j);
    delay(20);
  }
}