int ledPin = 9;

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  for (int i=0; i <= 255; i++){
    analogWrite(ledPin, i);
    delay(50);
  }
  for (int i=255; i >= 0; i--){
    analogWrite(ledPin, i);
    delay(50);
  }
}