Hi
try putting a 1 MOhm resistor (at least) between your analog input and ground. It tends to discharge the energy and it also adapts the input-output impedance (that it's really diferent).

it also helps to put a simple RC low pass filter centered at 300Hz (more or less). If you can see the piezo signal in time you will discover a sinusoid multiplied by a negative exponential. Try to filter the signal and you will get the envelope (the peak).

good luck!
Qq

----- Mensaje original ----
De: Patrice Colet <pat@mamalala.org>
Para: pd-ot@iem.at
Enviado: martes, 8 de enero, 2008 15:54:53
Asunto: [PD-ot] piezo-electric sensors

  Hello, I'm trying to build a system for getting impulses from bouncing
balls on a medium board, for driving sounds or a step sequencer.

  The cheapest solution was about using a piezo-electric disc, that
transform the mecanic impulsion into electrical energy. So I use the
analog input of the Arduino interface for receiving this energy amount,
and then I just have to choose a threshold level of the released energy,
for triggering into PureData.

  But the problem I'm encountering is that the electrical circuit made
by the piezo probe and the arduino, with no other additionnal component
has a too high capacitance, so the energy level is lowering too slowly
after each mecanical impulsion, then my system doesn't trig anymore
until the circuit capacitance goes below the threshold level set in
PureData.

  There certainly is an electronical trick for discharging the energy
just after each impulse, maybe someone that read this message has
sufficient skills in electronics to help?

  Thanks,

pk.

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