> In "Red" Welte pianos and Vorsetzers (at least in early ones), the
> switch for the pump motor uses a metal cup containing mercury, and
> a metal rod.  When a lever in the spool box is pressed down against a
> spring, the rod enters the mercury, completing the circuit and starting
> the pump.  A catch holds the rod down in the mercury.

I'm not at all familiar with the mechanism that's described here, but
it would seem that a small opto-sensor or Hall-effect sensor could be
placed in the mercury cup.  Opto-sensors contain a light source and a
phototransistor that stare at each other across a gap.  When that rod
enters the gap, the resistance between the phototransistor leads rises
enough for an external electronic circuit to detect it.  The
Hall-effect sensor requires a small permanent magnet to be placed near
it, at which point its resistance changes.

I think there might be sensors that have a triac integrated into them
and are thus capable of switching a motor directly, but I'm not sure.

In any case, I'd guess that the most reliable route would be to have
any electronic circuit operate a substantial electromagnetic relay that
would switch the motor since good mechanical contacts are immune to
inductive effects.

If the sound of the relay is a problem, an electronic relay should do
the job.

Mark Kinsler
(Didn't 'Red' Welte sing on the "Grand Ole Opry"?)
Lancaster, Ohio USA
http://www.frognet.net/~kinsler

 [ Gee, I thot he played for the Army football team...!  ;-)  -- Robbie