Light emitting diode
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Commonly abbreviated LED.
A diode specially fabriacted to produce light, usually in the visible spectrum for illumination or indicative purposes. Some, however produce UV or IR used for other devices, with IR used for communication. All diodes emit some sort of light, LEDs are specifically produced for light output.
Contents |
Different types
| Type | Vf | If |
|---|---|---|
| Standard red | 2.1 V | 15 mA |
| Standard green | 2.1 V | 30 mA |
| High Brightness red (500 mcd) | 1.8 V | 30 mA |
| Super High Brightness red (20 000 mcd) | 2.1 V | 50 mA |
| High Brightness white (7000 mcd) | 3.5 V | 30 mA |
| UV (200 mcd) | 3.7 V | 10-20 mA |
Driving
To use an LED you need to know how much voltage it drops in use (the forward voltage, or Vf) and how much current it needs (the forward current, or If). Both of these are available on datasheets for the specific LED you are using.
In the simplest and most common LED driver, the LED should be put in series with a resistor (see diagram). The resistance should have a value determined by: 
This is a rough value and LEDs are fairly tolerant. 9-12V and a 330-560Ω resistor should be fine in many cases.
Polarity
Wrong polarity can damage a LED, some types can only handle a reverse voltage of 5 V or less. So be careful if you select a random orientation to find the correct polarity.
For a new LED, the longer lead should be connected to the positive side of the supply. The flattened side of the LED belongs to the negative side of the supply. If you have an old LED with leads snipped off and/or is missing a flattened side, you can look inside. The lead connected to the larger bit inside should be connected to the negative. If all else fails you can use a diode tester.
Using a LED as a light sensor
LEDs can be fairly efficient as a solar cell or light sensor. It is even possible to use a LED as both the transmitter and reciever in a short range communication system.
