Now I've got my nice new shiny scope I've been playing around with the
piezos I used as a drum triggers.
Firstly, it turns out that piezos have an orientation - hit them on one
side and you get an initial large +ve spike followed by an irregular
decaying AC signal. Turn them over and you get a large initial -ve
spike. The ones I've got have random orientations - the black/red leads
tell you nothing about what the orientation is.
I was using this circuit:
http://leucos.lstilde.org/wp/wp-content/uploads/2009/05/piezo_circuit... and
hindsight, with even my limited electronics knowledge, it seems like a
load of tosh - full article is at
http://leucos.lstilde.org/wp/2009/06/piezo-transducer-signal-conditio...
Here are what I think are the problems, do they seem valid?
1. The 2nd BAT85 (schottky) from the left serves no purpose as it is in
parallel with the 5.1V zener.
2. The RC pair are supposed to be providing smoothing but they are in
parallel, a configuration that as far as I've been able to find out just
adjusts the phase of the current through the capacitor. I think what is
actually needed is a RC low-pass filter.
3. The circuit is intended to protect the input of the MCU not just
against +ve overvoltage (the zener) but also -ve voltages (the 2 BAT85s,
one of which is redundant). However, by observation it doesn't do that.
I've simplified the circuit so it just has a single BAT85 to protect
against -ve voltages and no other components.
The first image below (pos.png) shows what happens with a large +ve
spike and is as expected - the input voltage (red) initially goes +ve
and the post-diode voltage (yellows) follows, then when the input
voltage goes -ve the diode prevents its output voltage from going below
0V, as expected - well, mostly, it still goes to -1.6V, not sure why.
However if we flip the piezo over and generate a leading -ve pulse we
get a -ve output from the diode that's way beyond what we'd expect
(neg.png), -22.4V in this case. This puzzled the hell out of me until I
looked at the BAT85 datasheet. The max reverse voltage is specified as
30V, here it's having -74V stuffed across it. The BAT85 is completely
unsuitable for the job, I've seen spikes of over 100V from the piezo, so
a diode with a VR of 200V is probably what's needed.
Does all that seem correct?
--
Alan Burlison
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