int led = 9;           
int brightness = 0;    
int fadeAmount = 5;    

void setup() {
    pinMode(led, OUTPUT);
}
void loop() {
  
  const unsigned long adjustInterval = 30;
  static unsigned long lastAdjust = 0;
  if ( millis() - lastAdjust > adjustInterval) {
    lastAdjust += adjustInterval;
    
    analogWrite(led, brightness);

    
    brightness = brightness + fadeAmount;

    
    if (brightness <= 0 || brightness >= 255) {
      fadeAmount = -fadeAmount;
    }
     

  } 

}