#define ledPin 2
unsigned long last_time = 0;
int ledState = LOW;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(115200);
}

void loop() {
  unsigned long currentMillis = millis();
  if(currentMillis - last_time >= 500){
    ledState = !ledState;
    digitalWrite(ledPin, ledState);
    last_time = currentMillis;
  }
}