// constants won't change. Used here to set a pin number:
const int ledPin =  LED_BUILTIN;// the number of the LED pin
// Variables will change:
int ledState = LOW;             // ledState used to set the LED
// Generally, you should use "unsigned long" for variables that hold time
// The value will quickly become too large for an int to store
unsigned long previousMillis = 0;        // will store last time LED was updated
// constants won't change:
const long interval = 1000;           // interval at which to blink (milliseconds)
void setup() {
  // set the digital pin as output:
  pinMode(ledPin, OUTPUT);
}
void loop() {
  // here is where you'd put code that needs to be running all the time.
  // check to see if it's time to blink the LED; that is, if the difference
  // between the current time and last time you blinked the LED is bigger than
  // the interval at which you want to blink the LED.
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    // save the last time you blinked the LED
    previousMillis = currentMillis;
    // if the LED is off turn it on and vice-versa:
    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;
    }
    // set the LED with the ledState of the variable:
    digitalWrite(ledPin, ledState);
  }
}