int led = 13;
volatile int i = 0;
int sw = 2;
void setup() {
  attachInterrupt(0, changeLED, FALLING);
  pinMode(sw, INPUT_PULLUP);
  pinMode(led, OUTPUT);
}
void changeLED() {
  i = 1;
}
void loop() {
  if (i == 1) {
    digitalWrite(led, 1);
    delay (500);
    digitalWrite(led, 0);
    delay(500);
    i = 0;
  }
}