>>>> So what might be a good value for a pull-down resistor then?
>>>>
>>>> (4.5V supply, 74HCxx ICs.)
>>>
>>> A very common choice is 10 kOhm.
>>
>> OK. That should only be a few pence...
> 
> Unless you are connecting a switch to the input, I don't see why you'd 
> need a pull-down resistor, just connect it directly to ground.

Indeed. So it looks like I'm going to end up needing a seperate 
pull-down resistor for each "external" input to the logic system. I was 
hoping I could get away with just one, but that apparently won't work at 
all...

Oh well, I guess I'm not going to have all that many manual inputs.

The other problem is that I suspect connecting an LED to an output drags 
the voltage out of spec, so I probably can't connect the same output to 
an input. I guess I'll just buy myself a hex buffer and be done with it.

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