#define LED 2

unsigned long currentTime = 0;
unsigned long previousTime = 0;

boolean ledState=LOW;

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

void loop() {
  // put your main code here, to run repeatedly:

  currentTime = millis();

  if((currentTime - previousTime)>500){
    previousTime=currentTime;
    ledState=!ledState;
    digitalWrite(LED,ledState);

  }
}