> I thought you could just treat a 7400 as a black box. Apparently not...

You can, but you must read the spec sheet of the "black box" to see what it 
is expecting as its inputs and what it is going to give as the output.  The 
input spec will include the voltage range that it accepts as "low" and 
"high", and also what the input resistance is.

> The LEDs actually worked just fine. It's the gate driving them that didn't 
> do what it was ment to.

And the "output" spec will tell you within what range of voltages it will 
output as "low" and "high", and what the output resistance / maximum output 
current is.  If you try to connect something that is using more current than 
the IC can supply, there are going to be problems.  Usually a 7400 should be 
able to drive a single small LED directly, but anything bigger you need to 
start using a small buffer circuit (often just a discrete transistor, or you 
can get dedicated ICs to drive bigger loads).

> Presumably a debounce circuit is merely a low-pass filter?

Sometimes, but often you hook it up to a double throw switch, then you can 
only "switch" the circuit by hitting the respective side.  Simply bouncing 
on and off of one contact will only switch the circuit once.  As an 
exercise, work out the logic for that :-)

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