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// constants won't change. Used here to 
// set pin numbers:
const int ledPin =  13;      // the number of the LED pin
 
// Variables will change:
int ledState = LOW;             // ledState used to set the LED
long previousMillis = 0;        // will store last time LED was updated
 
// the follow variables is a long because the time, measured in miliseconds,
// will quickly become a bigger number than can be stored in an int.
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);
   }
}