ivp wrote:
> > For example, the brass crank could move a brass "pointer" along the
> > surface of the PCB, almost but not quite touching it. Sensor pads built
> > into the PCB artwork (sealed underneath the soldermask) would detect
> > the presence of the pointer at particular locations
> 
> I'd be interested to see any schematic you have of a simpler sensor, as
> anything that has no touching parts would have a servicing advantage

There's a very clear writeup of the general-purpose I/O technique in
this contest entry:

http://www.circuitcellar.com/microchip2007/winners/DE/MT2239.html

Microchip also has some dedicated chips for multiple proximity sensors,
and pretty much any cheap PC keyboard these days uses capacitive sensing
internally -- it may be possible to hack the circuitry out of one of those.

YES NOPE9 wrote:
> Or perhaps the plate size could be determined by tracking down a  
> triangular strip of copper.  If plate distance proved to be a problem  
> you could reference strips on each side of the triangle

Mow that's actually a very interesting idea. There's a very simple technique
for converting a differential variable capacitance (even a tiny one) directly
into a DC voltage that varies by about a couple of volts or so. You could
arrange your brass finger to span across two triangular copper areas on a
PCB, such that when it moves, it covers more of one area and less of the
other. (Think of a long rectangular area with a diagonal split through it.)

I wrote the circuit up for a Circuit Cellar EQ question a while back
(INK #124, November 2000), but unfortunately, it's no longer avaialble
online. There was also information about the technique in the INK #61
(August 1995) ConnecTime column. If you're interested in pursuing this,
let me know.

(Note to self: One of these days, I need to put all of the old EQ material
up on my own website.)

-- Dave Tweed
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