// LECTURA DE BOTÓN

int pin_led = 21;
int pin_boton = 19;

void setup() {
  pinMode(pin_led, OUTPUT);
  pinMode(pin_boton, INPUT_PULLUP);
}

void loop() {

  if (digitalRead(pin_boton) == LOW) {

    digitalWrite(pin_led, HIGH);
    delay(100);

    digitalWrite(pin_led, LOW);
    delay(100);

  } else {

    digitalWrite(pin_led, LOW);
  }
}