#define LED_PIN 9
#define BUTTON_PIN 3
volatile bool ledState = false;
void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT);
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), blinkLed, RISING);
}
void loop() {
  // nothing here!
}
void blinkLed() {
  ledState = !ledState;
  digitalWrite(LED_PIN, ledState);
}