How to diagnose crosstalk

Question

I am completely new to electronics, and am trying to learn by building an electronic drum set. I've found lots of examples online of circuits to condition the output from a piezo to be read by a controller (a Netduino in my case). The circuit I'm using looks something like this.

1. A Piezo (connected from the drum to my breadboard w/ some cat5 cable)
2. A 470k resistor in parallel
3. A Schottky diode in series (on the + line)
4. A Zener diode in series (3.3v on the + line)
5. A 1M resistor in parallel
6. A 0.1uF capacitor in parallel
7. A Netduino analog pin (and the ground line connected to the Netduino ground pin)

I currently have 6 of these circuits attaching to the 6 analog pins on my Netduino. The circuit is doing a great job of both not missing any hits on the drums as well as giving a pretty good dynamic range of velocity/volume readings. The problem is that when I strike any of the drums very hard, I'm getting some nasty crosstalk (i.e. I hit one drum, but multiple drums are signalled). I've been stuck on this for a few weeks now and have tried tweaking the circuit in every way I can think of, but can't seem to fix the issue. The closest I can come is to put a voltage divider at the end of the circuit. This does remove the crosstalk, but at the cost of significantly less dynamic range (i.e. no matter how hard I hit the drum, I don't get more then around half of the max ADC readout value). I've checked and double checked that everything is connected to the same ground. The Schottky should be keeping any negative signal from getting to the Netduino, and the Zener should be making sure I'm not getting anything above the 3.3v that the Netduino wants.

I think an oscilloscope would probably help a ton in figuring out what's going wrong, but as this is my first electronics project/experiment it's a little hard to justify a couple hundred dollar purchase to the wife ;)

Can anyone point me in the right direction for figuring out what's going wrong?

EDIT:

Here is my attempt at a schematic. I've just downloaded Eagle and am not entirely sure how to use it, but hopefully this is close enough to give the idea. The piezo symbol on the left doesn't look right to me, but that's what Eagle gave me.

EDIT #2:

Per several requests below, here are some pictures of my circuit (slightly updated from several peoples suggestions). Also, I've included some pictures of the drums. The drums use a special "screen" drum head that makes very little audible sound. There is a small piece of foam that transfers the drum head vibrations to the piezo.

Answer 1

A real schematic would help!!

I don't think the crosstalk is something in your circuit, but rather is acoustic. The other drums act as microphones and pick up the vibrations of the drum you're playing. The only solution I see is to add antisound to the other drums' signal, but that requires that you have a very good idea of the amplitude and phase of the signal you want to cancel.

edit (re your comment)
If hitting a disconnected drum doesn't give you crosstalk, then it doesn't seem to be acoustic after all. I want to have a look at your wiring. The signal should be a high peak voltage (high dV/dt), followed by a quickly decaying sine (the drum's resonance). If the wires between drums and microcontroller are very long and very well coupled (like intertwined), with the high impedance you have this may cause minor crosstalk. Not so much from the resonance sine wave, but from the fast peak when you hit it. But this should only give a small "tick" sound, not the typical drum.
So how did you wire it up? Try to use separate wires for each drum.

Answer 2

I'd recommend to change the zener placement as shown to have the effect you intended; this has nothing to do with your problem, though.

As stevenvh mentioned, it's improbable that you have electric crosstalk from such an arrangement, but if you want to be sure a shielded cable from the drum to the arduino would help. But in the most probable case, in which it's an accoustic effect, consider dealing with the problem in software. It's almost impossible that you hit two drums at exactly the same time, even if you're using both hands to hit simultaneously two different drums. So how about simply considering as a valid hit at any given timeslice only the strongest input, and ignoring the others?

BTW, to confirm that the problem is really accoustic, try to disconnect one drum and hit it to see if it still produces an input in the other (connected) drums.

EDIT: revised according to comment below

Answer 3

I think your trigger level for deciding a drum was hit is too sensitive. You need some level below which you figure the drum was not intentionally hit. That is probably what the zener in your circuit was intended to do, but you have it wired backwards so that it only adds a small threshold instead of the over 3 V as intended.

However, a zener is rather a hack for thresholding. You say this is going into a microcontroller, so I would lose the zener altogether and do the minimum thresholding in the micro. This is assuming that a maximum whack results in just about the full A/D voltage range. Adjust the second resistor (next time put component designators in your schematic) to get the right maximum volume, then adjust the firmware to ignore peaks below a certain level.