[ATM] Web page on basic LED circuits

[Mark Holm <mdholm@telerama.com>]

Hey, Google turned up a pretty good page right off the bat.
http://www.kpsec.freeuk.com/components/led.htm

The most obvious omission on this page is any discussion of variable
brightness.  The whole trick to basic variable brightness with an LED is
that you use both a variable resistor (potentiometer) and the fixed
resistor that is shown for the fixed brightness case.  Put them in
series.  That way, even if you turn the variable resistor down to 0
ohms, the fixed one will still be there to keep your LED from blowing out.

Say, using the formula from the web page, that you calculate a fixed
resistance of 41 ohms.  You happen to have 22 ohm resistors on hand, so
you put two of them in series to get 44 ohms, close enough (a little
high is safe, a little low may not be).  Also put in series the variable
resistor.  Try a 0 - 500 ohm unit to start.  That will let you cut the
current to less than 10% of the full brightness amount.  If that isn't
dim enough, try 0 - 5000.

As the web site shows, the LED and batteries have polarities that must
matched up, but resistors have no polarity, and the polarity of
potentiometers, such as it is, isn't important for this application
They can go in any way round, and anywhere between the batteries and LED.

Resistors have colored stripes that code their resistance, but it is
easier for a novice to buy them at Radio Shack in packages that have the
 resistance in nice black numerals.  Once out of the pack, store the
extras in zip lock bags that you label.

Resistors also have power ratings.  1/4 watt are common and will work
for most LED applications.  1/2 watt are also common.  You will be hard
pressed to overheat a 1/2 watt resistor in an LED circuit.

-- 
Mark Holm
mdholm@telerama.com

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