const int LED = 9;

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

void loop() {
  for(byte i=0; i<256; i=i+1)
  {
    analogWrite(LED, i);
    delay(10);
  }

  for(byte i=255; i>=0; i=i-1)
  {
    analogWrite(LED, i);
    delay(10);
  }
}