const long interval = 500;
unsigned long previousMillis;
const int ledPin = LED_BUILTIN;
int ledState = LOW;
void setup(){
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop(){
  unsigned long currentMillis = millis();
  if(currentMillis - previousMillis >= interval){
    previousMillis = currentMillis;
    if(ledState == LOW){
      ledState = HIGH;
    }
    else{
      ledState = LOW;
    }
    digitalWrite(ledPin,ledState);
  }
}