#define LED_BUILTIN 2


unsigned long previousTime=0;
unsigned long currentTime;
unsigned int cycleTime=500;
bool ledStatus=0;

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

void loop() {

  currentTime = millis();
  if((currentTime-previousTime) > cycleTime){
    ledStatus = !ledStatus;
    previousTime=currentTime;
  }

  digitalWrite(LED_BUILTIN, ledStatus);

}