Solid-state keyboards

[Chris Stecker]

I am currently restoring several Buchla 400 instruments, which use a  
solid-state/switchless/touch-sensitive keyboard.   The keyboard  
itself is very robust: after years of storage, they still work, and I  
haven't been spending any time repairing the keyboards themselves.  


They work on the principle of sensing a change in the capacitance of  
between key elements as your skin makes contact.  I'll have a look at  
the schematics and see if I can tell you more about how this is  
accomplished.  


As an aside, some of the 400's were built with weighted-action piano  
keyboards.  I've had a look at the insides of one, and it operates in  
the same way (!) as the touch keyboard.  Rather than skin contact,  
however, a little copper strip (one for each key) is bent by the key  
action.  These strips rest on a steel or aluminum plate connected to  
ground.  The capacitance between the copper strip and the plate  
changes as the key moves the strip.  The remainder of the sensing  
logic is the same as for the touch keyboard: look for abrupt changes  
in this capacitance, and you've got a note.  Somehow, it gets  
velocity from the same signal (that part I haven't yet figured out).

-Chris Stecker