const byte ledPin = 2;
const unsigned long intervalMs = 500;
unsigned long changedAt = 0;
bool lit = false;

void setup() {
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);
}

void loop() {
  const unsigned long now = millis();
  if (now - changedAt >= intervalMs) {
    changedAt = now;
    lit = !lit;
    digitalWrite(ledPin, lit ? HIGH : LOW);
  }
}