[sdiy] Magnetic field

[harrybissell]

Ian Fritz wrote:

> Think and guess what you like.  :-)
>
> I've worked with both.  Photo sources and detectors have a lot of part-to
> part variability.  Also a large temperature dependence.  You can find some
> discussion of this at my website, where I show a servo compensated photo
> system used for an opto-based breath-pressure sensor.
>
> Magnets don't have any significant temperature drift, and the Hall sensors
> you get nowadays have built-in compensation.  I like the idea of having a
> software calibration. Mechanical adjustment of this stuff is a pain,
> especially if you have to do a full keyboard.

LOL !!! Hah I still agree with me.

If you are using the photosensors as analog circuits it would be very difficult
to set up, however
I think slotted optical switches with keyboard activated "vanes" is the way to
go.  Key trip point
can be precisely set mechanically, there is no possibility of adjacent key
interference.  I'd sense
velocity by the time-of-flight of the vane from one sensor to the next... there
were sensors with
dual photoconductors at one time.

I don't think temperature dependancy is an issue at all in digital sensing, nor
is LED life, aging
etc.  Its not like we're gonna stick it in our mouth or anything (like Ian does
!!!)   :^P

H^) harry

>
>
> Good luck!
>
>    Ian