const byte ledPin = 13 ;
const byte interruptPin = 2 ;
volatile byte state = LOW ;


void setup() {
  pinMode ( ledPin , OUTPUT ) ;
  pinMode ( interruptPin , INPUT_PULLUP) ;
  attachInterrupt ( digitalPinToInterrupt ( interruptPin ), glow, CHANGE ) ;
}


void loop() {
  digitalWrite( ledPin, state ) ;
}


void glow() {
  state = !state ;
}