> 12VAC power if fed by 2 slip rings.

Hmm.  You may be in for some long term problems with this approach.  There
is a commercial product out there that uses slip rings.  They have problems
with both noise and reliability.

> The 50Hz AC wave form also suppies timing for the clock.
>
> 100 pulses/sec = no press
> 50 pulses/sec = press

As someone else already pointed out, you could use a FET in the right place
to achieve the same effect.  Another possibility is to pass DC to the rotary
board and then superimpose a digital data stream on it.  This can supply
both clock and control information.


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