#define PIN 11      // control pin
#define DELAY 10    // 20ms internal delay; increase for slower fades

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

void loop() {
  // fade in
  for(int i=0; i<255; i++) {
    analogWrite(PIN, i);
    delay(DELAY);
  }

  // fade out
  for(int i=0; i<255; i++) {
    analogWrite(PIN, 255-i);
    delay(DELAY);
  }
}