const int led = 3;

void setup() {
  // put your setup code here, to run once:
  pinMode(led, OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly:
  for(int i = 0; i < 256; i++) {
     analogWrite(led, i);
     delay(10);
  }

  for(int i = 255; i >= 0; i--) {
    analogWrite(led, i);
     delay(10);
  }
}