>> Heh. And here I was thinking it's just a case of "if the voltage is 
>> less than X, you don't need a resistor"...
> 
> If you want to run the LED at 10 mA you look up on the V-I curve in the 
> LED datasheet what forward voltage corresponds to 10 mA.  Say for 
> example 2.8 V. If your logic output is 5 V (and the other side of the 
> LED is connected to 0V), then you want 2.8 V across the LED and hence 
> 2.2 V across the series resistor.  So your resistor needs to drop 2.2 V 
> with a current of 10 mA, that makes it a 220 Ohm resistor.
> 
> Connecting the LED directly with no series resistor to a voltage source 
> is usually a bad idea (even if you think the voltage is correct), 
> because even slight changes in the voltage cause large differences to 
> the current the LED draws which might damage your circuit.

With my old electronics kit, if I'm remembering this rightly, you 
connect through a resistor if you're using the 3V power source, and skip 
the resistor if you're using the 1.5V source.

Oh, and *don't* connect the LED to the 9V source at all. I did that once 
by mistake... that particular LED doesn't do anything now.

Ah, life seemed so much simpler back then. :-(

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