int PWMPin=3;  // LED pin

void setup() {

  // put your setup code here, to run once:
  pinMode(PWMPin,OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int i=0;i<=255;i++){
    analogWrite(PWMPin,i);
    delay(100);
  }
  for (int j=255; j>=0; j--){

    analogWrite(PWMPin, j);
    delay(100);
  }

}