>> OK, now I'm confused. A graph of current against voltage? But current 
>> is completely *determined by* voltage! o_O
> 
> Of course it is determined, but it's only a linear relationship (V=IR) 
> for resistors.  LEDs are diodes and are certainly not linear, so you 
> need a graph.  You could draw a graph for a resistor if you wanted like 
> I said, but it would be a bit pointless because we know V=IR for a 
> resistor.

Wait a sec - so you're saying there are devices which actually violate 
Ohm's law?

>> OK... so... why not an open circuit then? That would have infinite 
>> resistence?
> 
> Then the input would be equally connected to 0V and V+ with near 
> infinite resistances, that's called a "floating" input, because it can 
> be persuaded very easily to either go to 0V or V+, depending on the 
> exact details inside the IC (and maybe some state of the other inputs, 
> who knows?!).  Best to ensure you have at most around 100K of resistance 
> to one of the rails to fix the input reliably.

Uh... so have it always connected to one rail, but through such a high 
resistance that the rail can't affect it?

Well, you're the one with the engineering degree, but it *still* doesn't 
make any sense to me...

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