volatile unsigned char brightness = 0;

void tick(){
  if (digitalRead(3) == HIGH){
    if (brightness == 255){return;}
    brightness++;
    return;
  }

  if (brightness == 0){return;}
  brightness--;
  return;
}

void setup(){
  pinMode(5, OUTPUT);
  attachInterrupt(0, tick, RISING);
}

void loop(){
  analogWrite(5, brightness);
}