piezo transducer as a source of variable resistance?? (stomp circuit idea)

[barret77]

Hello all

This is just an idea that I had, tell me if I'm dreaming or not;

I want to experiment with piezo elements;

The idea is to build a "stompbox" expression pedal - a stompable surface, with a piezo, whose output would be variable resistance: lets suppose it would have a gradient of 0 to 50kohms; when I'm not stomping, it outputs zero - when I stomp hard, it ramps up to 50 then down to 0 again; a soft stomp would ramp it to 20k, them ramp down.

Imagine a wah that instead of moving the feet, you could stomp to the rythm;

I imagine that a fast "ramp" would happen naturally with the piezo (am I wrong?) - the voltage that it generates goes in a curve (right? wrong?);

So I'm looking for a circuit that would get the voltage generated by the piezo and generate resistance.

Is this possible? Hard? Easy? another technique or idea for the same purpose?

Sorry if I sound dumb, I really know very little on electronics...

Thanks for any feedback!

[davebungo]

Sounds definitely possible.  You could think of it almost like an electronic drum which produces a trigger waveform.  You might have to shape the waveform to get the kind of control voltage you need to feed a WAH circuit.  There must be loads of ready made ideas out there already, you just need to find the right one.

[Dave_B]

If you're interested, you can look at the waveforms produced by my piezo drum pads.  A foot tap would produce a result close to the "Remo Pad" sample.  I didn't include the length of the waves, or even the amplitude, but it's roughly .2 seconds (very roughly).  No good recollection on the voltage, but I think it's around .5vac to 1vac.  The e-drum group wasn't that interested in those details, so I didn't write them down.   

Using an envelope follower, like the one used in the Bi-Filter for example, should get you in the ballpark.  You'll want to add an Attack/Release mod to it to get any substantial envelope. 

[barret77]

http://www.musicfromouterspace.com/analogsynth/SOUNDLABMINISYNTH/drum_trigger.html

there's an interesting schematic on this link - but I can't understand very well how the output works. The envelope looks good; I'm looking for a really percussive kind of thing, so this one seems to fit really well. I just need to figure out a relationship between CV and resistance...

[Tim Escobedo]

Using a piezo to make a variable resistance would be a uphill battle. Piezos seem to produce AC signals, and only as long as they are in flux. You can create pulses pretty easily, but a steady DC level based on varying pressure would be elusive.

There are things called force sensing resistors that would fit the bill. These are usually plastic laminated pads with two resistive elements inside the laminations that make varying contact with each other as pressure is applied to the pad. These can be directly used as variable resistors. Unfortunately, they are not common items, and tend to be expensive when found. The usual electronic part suppliers typically don't stock them. I have found robotics suppliers offer these as sensors.

[GFR]

Has anyone tried "strain gauges" in a stompbox?

[Paul Perry (Frostwave)]

That's a terrific idea! maybe the best way to implement it, would be to use the spike from stomping the piezo, to trigger an envelope generator (plenty of those on the synth DIY sites) and then have the output control a LED/LDR setup.
Note that the piezo itself is very unlikely to have a useful waveform itself (looking on the scope, it's a very sharp up and down spike and rapidly decaying repeats). Check out electronic drum circuits, firstly some have piezo trigger inputs, and secondly some have envelope circuits to put decay in.