[sdiy] paralleling piezos

[Ian Fritz]

Ingo --

Interesting project.

The piezos are (ideally) capacitive devices.  When you bend one a certain 
amount the internal atomic displacements cause the surfaces to become 
oppositely charged.  Current flows through the external circuit to 
neutralize these charges.  In practice they also have a finite but large 
internal resistance, so the induced charge will eventually decay through 
that resistance on its own if no external resistance is connected.  Thus 
they have no DC response.

If you have two devices in parallel and strike only one, you will obtain 
half the expected voltage.  (The same charge but twice the 
capacitance.)  If you want to add the responses, I'm pretty sure you want 
to connect them in series. Careful, they can put out large voltages.  I 
have a couple of experimental tap sensors where the tap bends the sensor. 
One of them easily puts out 30-40 V  And there some cool folks online who 
have used these cheap beepers to make sensitive seismometers!

Are you trying to detect the magnitude of the strike, or just its occurrence?

Ian




At 12:39 PM 2/6/2011, Ingo Debus wrote:
>Hi list,
>
>at the moment I'm building a xylophone-type MIDI controller. The
>thing doesn't produce any sound, just MIDI note messages. The keys
>are wooden blocks, sized approx. 15cm * 4cm * 2cm (6 inches * 1.5
>inches * 0.75 inches). I use cheap non-encapsulated piezo buzzer
>disks as sensors (from Conrad, 690635).
>In order to have a key "speak" uniformly, no matter where it is hit,
>I mounted *two* piezos under each key. Thinking piezos are just
>current sources, I paralleled them. The two piezos of a each key are
>fed into an amplifier with 1 Megohm input impedance.
>Now I found that these piezos vary a lot in sensitivity. To my
>surprise this can be corrected somewhat by putting a resistor of
>100k...1Meg in series with the more sensitive piezo. The less
>sensitive of the two piezos even seems to become more sensitive with
>the resistor in series with the other piezo. I have to experiment
>more to verify this though. It seems that one piezo is a significant
>load for the other one; after all they have a few nF capacitance. Hm,
>would it make sense to hand-select pairs of piezos by capacitance?
>
>FWIW, the 73 piezo pickups of the Yamaha CP-70B electric piano are
>all in parallel, only with one 4.7k resistor between keys 63 and 64
>and another one between keys 65 and 66. The signal is taken from the
>high keys side and fed into a preamp with 470k input impedance. This
>works very well, there are no noticable volume differences between keys.
>
>Ingo
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