const int LED = 9;
const int BUTTON = 2;
bool lastButton = LOW;
bool currentButton = HIGH;
bool ledOn = false;

void setup() {
  Serial.begin(9600);
  pinMode(LED, OUTPUT);
  pinMode(BUTTON, INPUT);
}

bool debounce(bool last) {
  bool current = digitalRead(BUTTON);
  if (current != last) {
    delay(5);
    current = digitalRead(BUTTON);
    return current;
  }
}

void loop() {
  currentButton = debounce(lastButton);
  if (lastButton == LOW && currentButton == HIGH) {
    ledOn = !ledOn;
  }
  lastButton = currentButton;
  digitalWrite(LED, ledOn);
}