[sdiy] Aftertouch via Pressure

[Ian Fritz]

At 03:39 AM 7/9/2003, jhaible at debitel.net wrote:
> > Some soft silicon tubing connected to a transducer and
> > sealed at the other end is probably how this is done.
>
>Yes, this is how it looks like. These transducers hace a pipe
>end where you connect the tube.

I haven't seen this method before.  Is the pressure medium air?  It would 
seem that something less compressible (silicone oil) would work better.

>Francois Buat showed me this on his Polykobol. But I've only taken
>a brief glance because I was so occupied with the PS-3100 which he
>had also brought.
>Now, ironically, I consider using this method when I will build
>a keyboard for my PS-3200 clone. (;->)
>
>I think the electronic part is not worth mentioning (super easy).
>Amplify and level shift the output voltage of the transducer.

Right.  You have to be sure you understand what the common mode voltage of 
the transducer is, to avoid clipping.  I just use two stages of 
amplification, so this isn't a big issue.  And the output will vary with 
supply voltage, so you need to be sure your supply regulation is good 
enough. SenSym has a handbook with circuits and also some circuits on line.

An additional consideration -- you might want some kind of thresholding to 
take out ambient pressure variations.

>But the mechanical part is difficult. You must find a way to insert
>the tube (hose?) beneath the keys, such that it's equally actuated from
>blacks's and white's. And, worst of all, you have to find a way to
>get enough key travel even with the additional thickness of the
>tube. But then again, the SH-2000's keyboard also has reduced
>key travel, and it plays nicely ...

Agreed.  This sounds pretty challenging.  You also have to have a perfect 
pressure seal.  And you may have to think some about atmospheric pressure 
variations if you use air for the pressure fluid.

   Ian