#define LED_BUILTIN 2
unsigned long prevTime =0;
unsigned long currentTime;
unsigned int cycleTime=500;
bool ledstatus=0;

void setup() {
  // put your setup code here, to run once:
  pinMode( LED_BUILTIN, OUTPUT);
}

void loop() {
  currentTime=millis();
  if((currentTime-prevTime) > cycleTime){
    ledstatus= !ledstatus;
    prevTime=currentTime;
  }

  digitalWrite(LED_BUILTIN, ledstatus);
}