EDs (Light-emitting diode) can be found in many colors and sizes. This example shows the simplest thing you can do with an Arduino to see physical output: it blinks an LED!In this tutorial you will also learn how to use pinMode(), digitalWrite() and delay() functions.
If you’re new to Arduino this tutorial will help you get started and make your first Arduino project!

What you will need – Hardware

For this tutorial you will need:

• 1 x Arduino uno
• 1 x Breadboard ( optional )
• 1 x LED
• 1 x 220 Ohm resistor

If you would like you can also use breadboard shield for arduino uno.

The Circuit

The connections are pretty easy, see the image above with breadboard circuit schematic.

Common leds have two pins. The positive end of a led (larger pin) is called anode, and the negative end is called cathode.

Arduino uno board have a built-in LED on pin 13, but in this tutorial we are going to be adding our own on digital pin 3.

The code

Keep in mind that setup( ) routine runs only once after power on / re-program or press the reset button. In the program below, the first thing you do is to initialize pin 3 as an output pin with pinMode( ) function in setup( ) routine.
The loop( ) routine runs over and over again, forever. In the main loop, you turn on or off LED with digitalWrite( ) function and “pause” the program for three seconds with delay( ) function. (3 sec are 3000 ms)

/* Learn to use and blink an LED with Arduino - Tutorial */

//Constants
const int ledPin = 3; 

void setup() {     
  //Initialize the digital pin as an output with pinMode()
  pinMode(ledPin, OUTPUT); 
}

void loop() {
  digitalWrite(ledPin, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(3000);                  // wait for 3 seconds
  digitalWrite(ledPin, LOW);    // turn the LED off by making the voltage LOW
  delay(3000);                  // wait for 3 seconds
}

Now start making your own modifications to the code. For example you can add a second led or change the delay time.