[sdiy] 61 Note Keyboard from Electronic Goldmine

[Byron G. Jacquot]

>The membranes are cup shaped.  Around the 'rim' of the cup is one conductor
>which switches one diode.  In the 'bottom' of the cup is another conductor
>which switches the other diode.  Now, I'm not sure about this, but it seems
>that the 'rim' conductor switches the first diode earlier in the travel of
>the key, and the diode in the 'cup' switches the other diode later in the
>travel of the key.   This could be my imagination - I'll know better when I
>can put two DMM's on it and check, but judging by eye, that seems to be
>what's happening.
>
>Could this be used for some type of velocity or after-touch function?  I'm
>not familiar with this arrangement at all (I don't get a chance to dissect
>too many keyboards).  If anyone knows, please enlighten me!

This is the most common velocity implementation, two staggered switches.
They're usually scanned with a microprocessor, and when the first switch it
found closed, the processor starts a timer.  The amount of time between the
two switch closures tells you how hard the key was hit...and it workes in
reverse for key-up velocity.

Aftertouch is often with some touch sensitive foam under the keybed, with
one sensor shared between all of the keys...you might look to see if it's there.

Byron Jacquot