LED, or light emitting diode is as the name suggests a device which can emit light. It has the electrical characteristics of a diode which means that it will pass current in one direction, but block it in the other. Standard types of LED emit red coloured light, but other types are available that emit orange, yellow, green, or infra-red types of light.

LEDs require typical forward operating voltage of about 2V and forward currents of 10 to 20 mA. They are widely available in sigle-LED packages, but are also available in multi-LED styles. 2-LED packages, for example, are available in either bi-colour or tri-colour forms. In the bi-colour device only one LED can be illuminated at a time, so that the device emits either a red or a green colour. IN the tri-colour device, both LEDs can be illuminated at the same time generating a yellow colour in addition to the original red and green. Multi-LED packages are also available in bargraph form, in 5 by 7 dot matrix form, and in seven segment display form.

In use, the operating current of a LED must be limited to a safe value. This can be achieved via a series resistor connected to either the anode or cathode.

Part of the Node Your Homework! project.

The Light emitting diode is a special type of diode, where the PN junction emits light. This is due to recombination of charge carriers (electrons). The most widely-used semiconductor in LEDs is gallium.

By using gallium arsenide(GaAs) we get a diode with a infrared to red emission. The GaAs LED has a forward voltage drop of about 1.4V.

Gallium arsenide phosphide (GaAsP) is used in diodes with frequencies between red and yellow. they have a voltage drop of ~2V.

Gallium phosphide diodes have a green to blue-green colour, and a voltage drop of about 3V.

By using different amounts of arsenide and phosphide in the manufacturing proccess it is possible to make many different colours.

Blue LEDs have proven to be harder to make, and are therefore expensive. They are made from silicon carbide or gallium nitride but are less efficient than other LEDs. But development of alternative production proccesses is underway.

LEDs are available in several shapes and sizes. You can also buy multicolor LEDs, that have both a red and green chip so you can get any colour between red and green. RGB diodes are also available but are very expensive and not very widely-used. White LEDs have also been developed and are available.

Visible light LEDs are used for indicator lights. High intensity LEDs can be used as lighting, but only at short ranges. Infrared LEDs are used for data transmission.

A lesser known fact about LEDs is that they are also light sensitive, and have a high sensitivity to light of the same frequency that they emit. Therefore a LED can be used as a light sensor.

LEDs must always be used in series with a resistor to limit the current. The LED must be connected the right way to work, usualy anode(+) is longer than the cathode(-). Modern LEDs usually have built in protection against connection of wrong polarity, but older ones might burn out. On used LEDs it might be hard to identify the electrodes. but most LEDs have the following configuration:(The cathode is marked by a flat edge on the package).

   /    \                    
  |.  __ |                  
  ||\ \ ||         
  |||  |||                
  ======== - Flat edge.         
   ||  ||                  
 A ||  || C         
   ||--- Longer leg

LED lights are fantastic gadgetry. Unfortunately, for those not well versed in the laws of physics and electronic circuit theory, it can be slightly complicated to implement them in the real world. All LEDs should have a resistor in series in order to limit the current, otherwise they may explode. For a resistor placed in series, only one resistor is needed, as opposed to in parallel, where each LED needs its own. LEDs in series are where each LED is connected directly to the next: ...--o--o--o--... LEDs in parallel are similar to a ladder, with an LED on each rung and the connecting wood the wires.
The formula below is for determining the resistor needed for a number of LEDs in series.

R = resistance required, in ohms
V = voltage supplied by the battery in volts
V(led) = forward voltage required by each LED in volts, usually found on the packaging
n = number of LEDs
I = maximum current rating in Amps, also found on the packaging

R = (V - n*V(led)) / I

For example:
I want 3 white LEDs to light up, each with a forward voltage of 3.6 Volts and a maximum current rating of 25 milliamps. The equation would thus be:
R = (12V - 3*3.6V) / .025 A = 48 ohms.
A resistor larger than 48 ohms is required to keep the current at a level that won't blow up the LED.

Of course, if a negative number is obtained, you need to supply a larger voltage.

Led (?), imp. & p. p.

of Lead.

Led captain. An obsequious follower or attendant. [Obs.] Swift. -- Led horse, a sumpter horse, or a spare horse, that is led along.


© Webster 1913.

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