LED Blink without Delay

মাঝেমধ্যে আমাদেরকে একসাথে দুইটি কাজ করতে হয়। যেমন ধরুন LED blinking করার সময় আপনি "Button press" read করাতে চান। এই ধরনের ক্ষেত্রে আপনি delay() ব্যাবহার করতে পারবেন না কারন delay() ফাংশন execute করার সময় Arduino প্রোগ্রামকে থামিয়ে রাখে এবং delay time শেষ হওয়ার জন্য অপেক্ষা করে। এই সময় আপনি button press করলেও Arduino সেটা read করবে না।

This sketch demonstrates how to blink an LED without using delay(). It turns the LED on and then makes note of the time. Then, each time through loop(), it checks to see if the desired blink time has passed. If it has, it toggles the LED on or off and makes note of the new time. In this way the LED blinks continuously while the sketch execution never lags on a single instruction.

An analogy would be warming up a pizza in your microwave, and also waiting some important email. You put the pizza in the microwave and set it for 10 minutes. The analogy to using delay() would be to sit in front of the microwave watching the timer count down from 10 minutes until the timer reaches zero. If the important email arrives during this time you will miss it.

What you would do in real life would be to turn on the pizza, and then check your email, and then maybe do something else (that doesn't take too long!) and every so often you will come back to the microwave to see if the timer has reached zero, indicating that your pizza is done.

In this tutorial you will learn how to set up a similar timer.

  Turns on and off a light emitting diode (LED) connected to a digital pin,
  without using the delay() function. This means that other code can run at the
  same time without being interrupted by the LED code.
 
Codes:
                                  // 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);
  }
}

Source:  http://www.arduino.cc/en/Tutorial/BlinkWithoutDelay: