enum led {
  VILLOG_KI,
  VILLOG_BE,
  LED_BE,
  LED_KI
};

enum led led_allapot;
const int ledPin = 13;
long sebesseg = 500;
unsigned long ido_regi = 0;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  unsigned long ido = millis();

  if (Serial.available() > 0) {
    char kapcsolo = Serial.read();
    // turn off LED
    if (kapcsolo == '-')
      led_allapot = VILLOG_KI;

    // turn on LED
    if (kapcsolo == '+')
      led_allapot = VILLOG_BE;
  }

  if (led_allapot > VILLOG_KI) {
    if ((unsigned long)(ido - ido_regi >= sebesseg)) {
      if (led_allapot == LED_BE) {
        led_allapot = LED_KI;
        digitalWrite(ledPin, LOW);
      } else {
        led_allapot = LED_BE;
        digitalWrite(ledPin, HIGH);
      }
      ido_regi = ido;
    }
  } else if (led_allapot == VILLOG_KI) {
    digitalWrite(ledPin, LOW);
  }
}