const int LED = 12;
const long interval = 500;
int LED_STATE = HIGH;
unsigned long previousMillis = 0;

void setup() {
  pinMode(LED, OUTPUT);
}

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

  if (currentMillis-previousMillis >= interval) {
    previousMillis = currentMillis;
    LED_STATE = LED_STATE ? LOW : HIGH;
  }

  digitalWrite(LED, LED_STATE);
}