[sdiy] paralleling piezos

[Ingo Debus]

Am 10.02.2011 um 15:01 schrieb Harry Bissell:

> There are several reasons to avoid piezos as impact sensors.
>
> One is that it is very hard to reduce crosstalk between sensors. I  
> made a controller
> that had three rubber pads (Gladstone pads) with sensors triggering  
> an Alesis D4, with
> a fourth sensor on the body to try and reduce crosstalk (subtract  
> body vibration). It worked
> acceptably well (at least until the drummer got too drunk... :^)

Yes, crosstalk at first was a big issue on my xylophone controller  
too. Funnily it became less and less a problem when I proceeded  
building it, probably because I added more mass to the "frame". I  
have a semi-smart crosstalk suppression algorithm in my software.
But I don't understand why FSRs are less sensitive to crosstalk than  
piezos?


> Two is that the piezos have an AC response to a hit, they give a  
> major pulse of one polarity
> when hit, and one almost as big when they are released. This can  
> cause a different kind of
> false triggering. I try to diminish this by rectifying the output  
> pulse with a shottkey
> diode, The positive puse goes through a series diode, the negative  
> is shunted by a parallel
> diode. Sensitivity is reduced for soft hits (maybe a good thing as  
> well)

I just have a 5V1 zener across each input. Shunts the negative pulse  
and limits the positive one. Oh BTW, I found out that the polarity is  
not the same on all piezo buzzers. On some keys with the paralleled  
piezos there was a deaf spot in the middle (between the two sensors)  
while on others there wasn't. Swapping the leads of one piezo cured  
that.


> FSRs are MUCH BETTER.  Interlink used to make a smaple kit that had  
> some useful size sensors, or you
> might buy a replacement KAT sensor from alternative mode.

Conrad sells FSRs in single quantities too, but they are very expensive.

> I'm sad to hear that their sensors go bad, I didn't know that and  
> my DrumKat hasn't been played in a LONG
> time... shizzle !

I once owned a Roland A80 keyboard, with poly aftertouch, which, I  
believe, worked with FSRs too. I had to replace these sensors as I  
had bought it because they had gotten almost completely unsensitive.  
Maybe it's a moisture thing.

Ingo