int led = 3;        
int brightness = 0;  
int fadeAmount = 20;  


void setup() {
 
  pinMode(led, OUTPUT);
}

void loop() {
  
  analogWrite(led, brightness);

  brightness = brightness + fadeAmount;

  if (brightness <= 0 || brightness >= 255) {
    fadeAmount = -fadeAmount;
  }
  
  delay(30);
}