InterruptibleLED.ino

int led = 9;
int btn = 2;
int buttonState = 0;
int lastButtonState = 0;

void setup() {
  pinMode(led, OUTPUT);
  pinMode(btn, INPUT);

  Serial.begin(9600);
}

void loop() {
  buttonState = digitalRead(btn);

  if (buttonState != lastButtonState) {
    if ( buttonState == HIGH) {
      for (int fadeval = 0; fadeval <= 255; fadeval += 5 ) {
        analogWrite(led, fadeval);
        Serial.println(fadeval);
        delay(30);
      }

    } else {
      for (int fadeval = 255; fadeval >= 0; fadeval -= 5 ) {
        analogWrite(led, fadeval);
        Serial.println(fadeval);
        delay(30);
      }
    }
    delay(20);
  }

  lastButtonState = buttonState;

}