[sdiy] EDP gnat keyboard

[Tim Stinchcombe]

Hi Dan,
	I can answer directly (though re-ordered):

> is it simply a button underneath a key "button" or is it a 
> resistor chain like in a regular keyboard?  
> (ic10,1c25,1c33///those are 4051's right?)

There are no mechanical contacts (at least for the ordinary Gnat and the
'Special'), it is all done with capacitive touch plates, so the note
annotations at the bottom of sheet 2 are simply large areas in the etched
PCB, in the form of a keyboard pattern. (And, minor point, the chips are
4052s by the way.)

> I was wondering...on the keyboard multiplexer...
> do the 4051's connect directly to the keyboard? 
> and if so...what IS the method of keyboards giving a gate signal?

Each plate/note is scanned sequentially (via all the switches), and routed
to a flip-flop (IC28, sheet 3), whose output is triggered by the change in
capacitance if the 'key' is being touched - the test instructions document
has a fairly easily understood description of how it works, para 17, sheet
3:

www.timstinchcombe.co.uk/synth/gnat/Gnat_Test_Instructions.pdf
 
> and thanks to TIM for  drawing those schematics! 

My pleasure! They get about a hit a day on my website, which I think is
probably quite a high level of interest in it, considering it is such a
small, obscure synth. I am also in infrequent touch with someone who is
trying to put a PCB together, but I haven't heard anything from him in
several months now...

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk