[sdiy] touchplate keyboard - works!

[Peter Grenader]

What was explained to me some two years ago from the manufacturer, inside
these specific switches is a crystal.  the harder you press, the more the
aluminum face of the button (SLIGHTLY)  deforms,  the more of whatever it is
you put through the switch passes out.  Don't ask me to explain it more than
this as these didn't come with a datasheet and they are potted inside.  If
you look at the picture from the previous letter, that all there is outside
of two wires coming out of each.

There is a very limited range to the pressure sensitivity however and I had
to dick with the voltage coming out to get anything remotely usable by a
human.  The circuit I did is straight forward.  I pass +V through these
switches.  The output goes to:

1) A buffing amplifier for the pressure sensitivity.

2) a comparator which is set to fire at about .05 volts.  The output of
these comparators create the gate signal for each stage, and open an analog
switch which passes the original signal through that's found in the
Programmer Circuit (not the touch sensitive key circuit) published in
Synapse.

hope this helps.

- P



Ian Fritz wrote:

> At 09:35 PM 2/4/2005, Peter Grenader wrote:
> 
>> Each key (stage) has the following inputs/outputs/controls:
>> 
>> Send input (from the Sergecircuit - a pulse pulls the stage active)
>> 
>> Gate output (remains high until another stage is selected)
>> 
>> Pressure Output
>> 
>> Voltage output (from the voltage level pot)
>> 
>> Voltage Level Pot (sets the voltage output for that stage)
>> 
>> LED indicator (lights when stage is active)
> 
> 
> I'm missing something. Could you clarify how you get a pressure output? It
> seems to me the piezos just put out a pulse. In fact, it's impossible to
> get a dc voltage from a piezoelectric transducer, since they are capacitive.
> 
>