int led_pin = 6;
void setup() {
  pinMode(led_pin, OUTPUT);
}
void loop() {
  for(int i=0; i<250; i++){
    analogWrite(led_pin, i);
    delay(5);
  }
  for(int i=250; i>0; i--){
    analogWrite(led_pin, i);
    delay(5);
  }
}