const int LED_PIN = 13;
const unsigned long INTERVAL = 1000;

bool ledState = false;
unsigned long oldTime = 0;

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

void loop() {
  if (millis() - oldTime >= INTERVAL) {
    oldTime = millis();
    ledState = !ledState;
    digitalWrite(LED_PIN, ledState);
  }
}