Parallel Piezo Pickup Problem

Question

I've an electronic drum machine - which involves electronic drum pads with piezoelectric transducers in them (which pick up you hitting the pads with your drum sticks) which feed into a magic drum machine brain that senses when your cranking out some beats!

I want to feed two of these pads into a single input, which I have tried by simply linking the cables that come out of the transducers in parallel. The sensitivity of the drum pads seems to plummet and they don't work half the time if this is done.

I was wondering it there's something about piezo transducers I need to do to stop them interfering with each other's output. I wondered if one was acting as a resistor to the signal produced by the other (almost acting as a speaker) and damping things down, or are they acting as capacitors to each other? Do I need to stick a diode between each of them or something! I'm not sure and the info I've read hasn't helped much.

Hoping you have some advice, thanks :)

Answer 1

Yes, piezo transducers act very much like capacitors. The charge generated by the one you hit partly transfers to the other and you lose signal amplitude. The best simple solution is to use a diode or-gate. Put a diode in series with each positive lead (connect the anode to the piezo, and then tie the two cathodes together at the input to your device. A silicon diode will usually work ok (1N4148) but a shottky diode (BAT84 or similar) will have less forward voltage drop. Usually the input impedance of your drum machine will bed low enough that you do not need to add a resistor from the cathodes to ground, as is the case in the traditional diode or-gate. (tested, works)

Answer 2

Check that there are no active electronics in the drum pads. If there are you would have the scenario described in Figure 1.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Parallel connected piezos.

On first glance you might think that either output will drive the next stage. With amplifier outputs the device with no output will try to maintain 0 V on the output. If the devices are the same type the best you could hope for would be mid-way between 0 V and the output of the drum that's been hit.

If this is the case then a simple mixer circuit would probably suffice.