Yamaha CS80

> On Fri, 1 Oct 2004, David Rogoff wrote:
> 
> > What is in the CS-80's sensors?  I know it's a rubber cushion and,
> > according to the schematics, electronically it acts like a
> > potentiometer, outputing a voltage proportional to the pressure, but I
> > don't know what's going on inside it.  Are there (widely, cheaply)
> > available force sensors that could be used to add pressure sensing to
> > an existing keyboard (if you could figure out where to mount them)?

--- In yamahacs80@yahoogroups.com, The Old Crow <oldcrow@o...> wrote:
> 
>   They are "force-sensing resistors" which are sold in a variety of 
> configurations these days.  I'll have to find out if any are
suitable as 
> aftertouch sensors.

Scott,

Thanks for the info.  I looked up some of these on google.  They all
have various voltage vs. pressure responses which made me wonder how
the CS-80's sensors responded.  So, I decided to do a little
experiment.  After trying various ideas involved postage scales and
bath scales, I thought of using a 2-liter coke bottle (actually diet
root beer) as a weight.  I soldered a wire to the pressure sensor
output of a key (see pic cs80_pressure.jpg in Files page).  I placed
the bottle, upside-down, at the front edge of a white key.  I kept
adding more water and recording the voltage.

The results are in the files pressure_vs_voltage.xls and
aftertouch_response.jpg in the Files page.  It's a kind of log
response, toping out with a little under 7 volts for any force over 70
oz (about 2Kg).  The response is a little weird: it would stay the
same for several ounces and then jump.  I'm not sure why it did this.
 Maybe it's stiffness in the rubber piece over the sensor. 

I also ran the output to an oscilloscope and looked at the curve as a
I pressed down on the key (sorry - no picture).  It definitely looked
smooth and continuous, without the steps I had recorded.  Any ideas?

Anyway, this is way more than most people want to know, but it could
be useful if someone was trying to build a new keyboard, or looking to
replace broken sensors on a CS-80.

 David