Before we solder the wire to the LED, just one thought. Ok, all we did until now was just our first step into Arduino programming. We can finish it right here after the prototyping. If you still want to finish our project keep in mind, that we'll need the Arduino for the next project... And maybe you want to use the LED for something more useful. Just do want you want to do :)
Now it's time for prototyping....
STOP!!!! First some information about LEDs (yes, after all that fun, we just have to learn something...), let's see what wikipedia tells us about LEDs:
A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting. Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.
When a light-emitting diode is forward-biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. LEDs are often small in area (less than 1 mm2), and integrated optical components may be used to shape its radiation pattern. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, and faster switching. LEDs powerful enough for room lighting are relatively expensive and require more precise current and heat management than compact fluorescent lamp sources of comparable output.
Light-emitting diodes are used in applications as diverse as aviation lighting, automotive lighting, advertising, general lighting, and traffic signals). LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology. Infrared LEDs are also used in the remote control units of many commercial products including televisions, DVD players, and other domestic appliances.
Ok, that's about the theory. If you want to know even more about LEDs visit this: http://en.wikipedia.org/wiki/Light-emitting_diode
Disconnect your Arduino.
Now let's grab the LED, there's a longer leg and shorter one. The longer leg is called Anode and is always connected to the "+" pole, the shorter one also called Cathode is connected to "-" or ground. We will connect the Anode now to pin 13 of the Arduino and the Cathode to "GRND", after that we can connect our Arduino again. After a second it will start to blink.
You may think "Hey just wait a minute, why don't we add a resistor, every electronic book tells us, that we always have to use the right resistor with LED". That's right, but the Arduino has a built-in resistor on pin 13, so it's ok for this example.
Well done "The Internet" is now online.... ;)