scott wrote:
>> Presumably the resistence of the LED is finite and fixed.
> 
> No.  Here's a graph of voltage against current for a typical LED:
> 
> http://www.cq.cx/pics/int-led-vi.png

Where do you *find* this stuff??

> Note that for a fixed value resistor this would be a straight line 
> starting at the origin.

OK, now I'm confused. A graph of current against voltage? But current is 
completely *determined by* voltage! o_O

>> Now, see, I've never been able to comprehend stuff like this. To me, 
>> this diagram just looks like when you close the switch, all the 
>> current will flow straight from one rail to the other, shorting out 
>> the battery and not providing any current at all to the input of the 
>> gate.
> 
> Yes, *some* current will flow from one rail to the other, which is why 
> it is important to use a *high* value resistor.  If you use a 10kOhm 
> resistor across 5 Volts, that's I=V/R=0.5mA which is hardly going to 
> cause a problem. But the important thing is, that point in the middle 
> that is directly connected to the input will be at 5V when the switch is 
> closed.

OK... so... why not an open circuit then? That would have infinite 
resistence?

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