呼吸燈(for)(analogWrite)(~3.5.6.9.10.11)

void setup() {
  pinMode(3, OUTPUT);
}
void loop() {
  for(int a=0;a<=255;a=a+5)
  {
  analogWrite(3, a);
  delay(25);
  }
  for(int b=255;b>=0;b=b-5)
  {
  analogWrite(3, b);
  delay(25);
  }                              
}