>> What you mean is, they draw *up to* 40 mA?
>
> No idea. The spec sheet just says "forward current".

It's probably under a section titled "typical operating conditions" or 
"absolute maximum ratings".  There should also be a graph showing a curve of 
current against voltage, from zero up to the absolute maximum current 
rating.  Usually with LEDs the light output is fairly linear with respect to 
current, but certainly not with voltage.

>> How much they actually draw depends on what value series resistor you put 
>> in there.  Using Ohm's law and the V-I curve of the LED from the 
>> datasheet you can work out exactly what resistor size you need for a 
>> given current.
>
> 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.

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