int led = 3;
void setup() {
  // put your setup code here, to run once:

}

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);
  }
  digitalWrite(led, LOW);
  delay(2000);
}