LED

Light Emitting Diode. All diodes emit some light when forward-biased. LEDs are made from a special semiconductor (like gallium arsenide phosphide) which optimizes this light output. Unlike light bulbs, LEDs rarely burn out unless their current limit is passed. A current of 0.02 Amps (20 mA) to 0.04 Amps (40 mA) is a good range for LEDs (never go past 50 mA). LEDs have a forward voltage drop of about 1.6 V.

LEDs have a cathode and an anode just like regular diodes. To determine an LED's polarity, you can do one of three things:
  • Look for a line in the metal inside the LED (it may be difficult to see). This line is closest to the anode of the LED.

  • Find a flat spot on the edge of the LED -- this flat spot is on the cathode.

  • The anode of an LED is generally longer (at least, when it's a new, non-salvaged, LED).

When current is flowing through an LED the voltage on the positive leg is about 1.4 volts higher than the voltage on the negative side (this varies with LED type -- infrared LEDs have a lower forward voltage requirement, others may need up to 1.8 V). Remember that there is very little resistance to limit the current, so a resistor must be used in series with the LED to avoid destroying it (note, though, that some panel-mount LEDs come from the factory with a current-limiting resistor soldered to them).

Note that LEDs can also be used as photodiodes (tho' their sensitivity is relatively low, so they're only useable this way in very bright conditions).

LEDs are also photovoltaic (they generate voltage when exposed to light); in a high impedance (i.e., low current demand) circuit, an LED will generate a voltage dependent on its color:

Red
1.2 V
Yellow
1.4 V
Green
1.7 V
Blue
2.8 V

Data Display Products has a very helpful page on LED properties here. See also the EncycloBEAMia diode discussion here, and the BEAM From the Ground Up article on LEDs here. Duane Johnson has a large selection of LED light sensor circuits here.


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Page author: Eric Seale  
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