Hi, take a look at this nice Microchip document, in the tip #5 you have the
answer http://ww1.microchip.com/downloads/en/DeviceDoc/40040C.pdf

HTH


2007/11/28, Rob Robson <rob@silk.net>:
>
> Here's a head-scratcher for the brain trust.  I want to connect four
> momentary pushbuttons to a single A/D input such that each and every
> combination of pressed buttons will present a unique voltage to the A/D.  To
> minimize errors, it seems there should be a wide and fairly even
> distribution between the fifteen possible voltage levels (not including the
> "no-buttons-pressed" option).
>
> The proposed circuit consists of four different resistor values, each one
> tied between the A/D input and a pushbutton.  The other side of each
> pushbutton is tied to ground.  A fifth resistor ties the A/D input to +5V to
> complete the voltage divider.
>
> My limited math abilities are hampering my attempts to optimize the five
> resistor values for a nice safe spread between all the resulting single- and
> multiple-button voltages.  Has anyone seen or done something like
> this?  Googling hasn't turned up anything so far.  I'm completely open to
> changing the circuit if there's a more clever arrangement.
>
> Many thanks in advance for your skulduggery,
> RR
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-- 
Ariel Rocholl
Madrid, Spain
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