[sdiy] Keyboard Encoder Experiments...

[Peter Keller]

On Mon, Feb 11, 2008 at 10:24:34PM +0100, Ben Stuyts wrote:
> How about hall-effect sensors?

How about an aligned IR emitter/detector pair where the emitter is
attached to the key and the detector is fixed to the case and one or both
have a narrow field of power. Then when you move the key, the beam power
varies as they misalign and can be read as a position sense. Encase the
tiny contraption in a black box to remove outside light influence.

It would look like this:


+---------/\/\---------+                                 fingers go here
|DETECTOR]     [EMITTER|-----------------------------------------------
+---------/\/\---------+       ^
                             fulcrum

the /\/\ isn't a resistor, but instead a bendy part of the box the sensor
is contained in.

Calibration would kinda suck though.... A) If when the key is up the beam
is fully on, you'd have to ensure that when the key is down, you wouldn't
have marked fully off before the key actually touched the bottom--this is
a physical effect.  B) after ensuring A is correct, you'd have to then
read the position when the key is up, and then when the key is down,
for each key and then store the information (after ADC conversion) in a
bank somewhere--this is an electrical effect since the IR pairs might
be slightly different so you can't use a single calibration value for
all of them. If you sealed the IR sensors right and were *sure* about
mechanical tolerances, you'd probably only have to calibrate it once if
you stored the original results in NVRAM or something.

Hall effect sensors have the nasty habit of picking up stray fields and
your speakers might cause your board to think it has depresed the keys. :)
I think it is easier to shield the IR emitter than the magnetic source.

Later,
-pete