On Thu, 11 Mar 2004 02:19:57 +0100, you wrote:

>>
>> Permanent magnets can have significantly differing strengths due to =
the way
>> they are magnetised. Hall effect sensors have significant  tempco, and=
 of
>> course there is the earth's magnetic field, and local magnetic fields =
to
>> contend with. This would also be somewhat expensive.
>>
>
>zero point adjustment is the magic word... but you're right, this is =
awful to
>realize.
>
>> My money's still on my earlier suggestion of a simple tuned circuit in=
 each
>> piece, and coils etched into a PCB under the board - cheap & easy to =
build.
>> Even if you couldn't figure out a way to do it in a matrix, the =
switching
>> would only need nine 8-1 analogue multiplexers (ultra-cheap 4000 CMOS)=
 ,
>> and the detection hardware would be simple and cheap - PC generates =
the
>> frequencies, and a simple detector generates a DC level for the PIC =
ADC to
>> detect the presence of a piece - maybe a handful of passives and an =
opamp.
>> Can't see how you could get much cheaper than this.
>>
>> You could even make it retro-fittable to existing pieces, as you could=
 usea
>> PCB coil with a cutout for the capacitor, forming a simple disc shape =
that
>> can be stuck to the bottom of the piece.
>>
>> Proximity of the piece will not cause false detection (e.g. if piece
>> hovers) - you are simply looking for a resonance at one of 12 =
frequencies.
>> The position of the piece within the square should also not be much of=
 a
>> problem. The proximity of the board coil to the (pcb) coil in the base=
 of
>> the piece would be so close (2-3mm max) that cross-coupling to an =
adjacent
>> piece should not be an issue.
>>
>> Even if PCB coils turn out to not give enough coupling at a suitable
>> frequency, small surface-mount inductors could be used - still cheap, =
and
>> very good coupling - I've recently used these to do an inductive =
datalink
>> into a potted product.
>>
>
>I've got another Idea... Resonant  frequencies aren't that perfect (lot =
of
>tuning and choosing the right frequency for each pice without =
constructing
>overcutting resonants).
>But we can still go forth with coils. We can put a resonant coil (only =
one
>frequency for all) in each figure in parallel to a load resistor.
>If we choose different load resistors, then each figure would draw a =
different
>amount of power from the coils under the chessboard, this could be
>measured...  You have to admit, that this is a little bit easier than =
using
>multiple resonant frequencies, that all want to be scanned on each =
square of
>the board.

No it isn't.
We are back to all the possible variations of signal strength giving =
false results, in particular
distance from piece to coil.=20
With different resonant frequencies, we only have to detect presence or =
absence of loading at a
given frequency for each square, which should give a much more reliable =
and definite result.

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