// Constants, variables
    const int ledPin = 2;      // the number of the LED pin
    long previousMillis = 0;   // will store last time LED was updated
    long interval = 1000;      // interval at which to blink (milliseconds)
     
    void setup() {
      // set the digital pin as output:
      pinMode(ledPin, OUTPUT);      
    }
     
    void loop()
    {
      // 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;
     
        // set the LED with the ledState of the variable:
        digitalWrite(ledPin, HIGH);
        delay(100);
        digitalWrite(ledPin, LOW);
      }
    }