This example shows the simplest thing you can do with an Arduino to see physical output: it blinks an LED.
To build the circuit, attach a 220-ohm resistor to pin 13. Then attach the long leg of an LED (the positive leg, called the anode) to the resistor. Attach the short leg (the negative leg, called the cathode) to ground. Then plug your Arduino board into your computer, start the Arduino program, and enter the code below.
Most Arduino boards already have an LED attached to pin 13 on the board itself. If you run this example with no hardware attached, you should see that LED blink.
In the program below, the first thing you do is to initialize pin 13 as an output pin with the line
In the main loop, you turn the LED on with the line:
This supplies 5 volts to pin 13. That creates a voltage difference across the pins of the LED, and lights it up. Then you turn it off with the line:
That takes pin 13 back to 0 volts, and turns the LED off. In between the on and the off, you want enough time for a person to see the change, so the
delay() commands tell the Arduino to do nothing for 1000 milliseconds, or one second. When you use the
delay() command, nothing else happens for that amount of time. Once you’ve understood the basic examples, check out the BlinkWithoutDelay example to learn how to create a delay while doing other things.
For more detail: Blink LED using Arduino