void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT);   // external pull-down resistor
}

void loop() {
  int buttonState = digitalRead(BUTTON_PIN);

  if (buttonState == HIGH) {
    digitalWrite(LED_PIN, HIGH);   // LED ON
  } else {
    digitalWrite(LED_PIN, LOW);    // LED OFF
  }
}