const int ledPin = 11;

int ledState = LOW;
unsigned long previousMillis = 0;

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

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

  if(currentMillis - previousMillis >= 1000) {
    previousMillis = currentMillis;
    ledState = !ledState;
    digitalWrite(ledPin, ledState);
  }
}