[sdiy] touchplate keyboard - works!

[harrybissell]

Hi Ian (et al)

You 'can' get a DC output from a Piezo (of sorts).

The Piezo is both a capacitor, and a sensor that will output the derivative
of the applied force.  If you make a charge amplifier (integrator) it will
output an essentially DC voltage proportional to force.

You need to reset the intergator every now and then... or you could use a leaky
integrator and it would slowly drift back to zero.

The piezo will give a positive spike of 'pressing' and an equal negative
spike on 'unpressing'

If you were real clever (and I have not read how clever Peter is yet :^) you
might
control the integrator by sensing the touch of the metal plate by capacitance,
and
un-reset the integrator with that...

H^) harry

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.