int red = 11;
int LED_state = LOW;
unsigned long current_millis = 0;
unsigned long previous_millis = 0;
const unsigned long blink_period = 500;

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

void loop() {
  current_millis = millis();
  blink_LED();// put your main code here, to run repeatedly:
}

void blink_LED(){
  if (current_millis - previous_millis >= blink_period){
    previous_millis = current_millis;
    if (LED_state == LOW){
      LED_state = HIGH;
    }
    else{
      LED_state = LOW;
    }
  digitalWrite(red, LED_state);
  }
}