> Only drawback I can see is no multiple buttons can be pressed
> at a time and give a valid value, but this is not a requirement.

Hi Ariel, two buttons together would give you "a" value, because
of the Rs in parallel. For 10 buttons I think the number of button
pairs would be (10 * 9)/2. The parallel result of some pairs might
be very close to single buttons though. Two low Rs in parallel
would be rejectable because the effective R gives an ADC result
below that of the lowest single R. And two high Rs in parallel for
example might be somewhere between two lower buttons

It would help if you use good quality buttons and fairly tight ADC
ranges. As buttons get dirty over time that might cause problems

I know you said no external chips, but perhaps you can do a little
multiplexing and free up just one more pin ?

http://home.clear.net.nz/pages/joecolquitt/0pots.html

In that top circuit, using an RC you can drive the 4017 with just
1 pin. A C on Reset and an R between Reset and Clock. Short
pulses will clock the 4017, a long pulse will reset it

Substitute pushbuttons or switches for the pots and detect a "1"
on a PIC input, noting the clock position to tell you the switch
number

A similar multiplexing technique here, which I used before the
do-everything PICs came along

http://home.clear.net.nz/pages/joecolquitt/mixer.html

> Do you see any problem with this approach? Anybody used
>something like this before ?

I believe Bob Axtell uses ADC this way

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