int led = 13;
int brightness = 0; // 0-255
int fadeAmount = 5;

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

void loop() {
  analogWrite(led, brightness);
  brightness = brightness + fadeAmount;

  if(brightness <= 0 || brightness >= 255){
    fadeAmount = -fadeAmount;
  }
  delay(100);
}