[sdiy]Touch pads?

[Tony Clark]

> Has anyone come up with a simple and affordable implementation of this?
> I've heard of pressure sensitive resistors, but can't seem to find a 
> source for them. Then there are various breeds of strain gauges but 
> they appear prohibitavely expensive and/or inaccurate.

   Well I can vouch for strain gages being very useful for your 
application.  Kinda helps that my business is the leading provider of 
strain gage force sensors to the educational market!  ;)
   You are right that most are expensive.  In fact, for low volume 
applications I'd tell you to forget it!  Strain gages are extremely 
fragile and require not only delicate handling but a very clean working 
environment.  You'll be dealing with adhesives, ovens, and clamping in 
order to bond the strain gages to whatever material you want.
   As for accuracy, they are extremely accurate.  I don't know of _any_ 
that aren't accurate.  Usually the cause for inaccuracy is due to faulty 
mounting, handling, or wrong mounting medium.  If you mean tolerances, 
yes, you can have a huge variety in tolerances between gages, but it 
doesn't really affect the use as you can easily balance them.
   I have used strain gages to do some cool stuff like a joystick with no 
moving parts!  A solid shaft of aluminum with four milled mounting points 
for two bridges (X, Y).  Works very well but I haven't actually used it 
for anything.

   For your application, you might try a device called a Hall Effect 
sensor.  It's a crude device and not very linear, but you may not want it 
to be.  It works off of magnetism, so you'll mount the sensor some 
distance away from your control surface that has a magnet glued to the 
backside.  As the plate moves, the sensor will pick up the increase in 
magnetic field, and produce a voltage accordingly.  It'll be much cheaper 
and less frustrating than dealing with strain gages.

   Tony

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