LED stands for Light Emitting Diode. LED's convert an electrical current directly into light. The light emitted by an LED is directly proportional to current through the LED. This means LED's are ideal for transmission of information. However, LED's need direct line of sight and they usually have a short range of light emission. Because LED's are current dependent they need to be protected from excessive current with a resistor. For most robotic applications with power sources of around 9 volts I find that a 1K resistor will always to the trick. You'll notice one lead is longer than the other, in most cases a longer lead indicates that it is the positive lead. It might be easy to dismiss the LED and assume all it does is light up, but LED's are very interesting electronics components and have some surprising functions. For instance if you take a Jumbo LED (of any color) and connect your Multimeter up to it for voltage reading and point the LED towards a very bright light source you'll get a voltage reading. In direct sunlight you can get a reading of 1.5 volts from a normal LED! Some people even think it is possible to make a color sensor using a LED like this, however after much experimenting with this It is found that LED's to not be suitable for use as a sensor.
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