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'[EE:] Piezo trigger amplifier'
2004\07\06@032300 by Matthew Brush

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Hello all,

I finally got my electronic bass drum trigger put
together and now I'm working on amplifying the piezo
output into the range for the PIC ADC (0v-5v or so).
After looking all over and finding several variations
of the same schematic, I tried to build it with NO
success.

This is how it should work fine:
http://www.leftclick.ca/public/piezo_input.pdf

I always get an output voltage of around 3v and it
only changes slightly on occasion when I hit the
piezo.  I know my piezo trigger works cuz I can get a
good readout on my multimeter.

I've also tried using an LM386 audio amp with no
success.  Another thing I tried was the piezo to the
base of an NPN transistor, which grounded the base of
a PNP transistor which passed a voltage out the
emitter.  This worked fine with an LED and the
multimeter, but didn't work well for the PIC ADC.

Could anyone comment on the schematic linked above and
suggest some reasons why it's not working?  Note that
the V+ and GND aren't shown in that schematic, but in
my circuit they are connected :)

One thing, could it be that I'm using one of those
glass diodes instead of a black one?  I think it's
called a fast signal diode or something.

Thanks for anyone comments and help.

Scratching My Head,


=====
MJ Brush
LeftClick.ca Internet Media Services
mbrush@[NOSPAM]leftclick.ca

______________________________________________________________________
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2004\07\06@034825 by Jinx

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> Another thing I tried was the piezo to the base of an NPN
> transistor, which grounded the base of a PNP transistor
> which passed a voltage out the emitter.  This worked fine
> with an LED and the multimeter, but didn't work well for the
> PIC ADC

If it can drive an LED there must be something missing from the
PIC setup. It should have worked with just the NPN and a pullup

How about this, for an F877 ?

http://tomscarff.tripod.com/8way_drm2/8way_drm2_2.htm

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2004\07\06@040522 by Russell McMahon

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> I finally got my electronic bass drum trigger put
> together

> This is how it should work fine:
> http://www.leftclick.ca/public/piezo_input.pdf

Cct looks OKish. I could suggest some improvements but am just rushing out
so will mainly leave to others.
BUT

Presumably the A2D can see a varying voltage at its input.
Place a 1K resistor to ground from 324 output.
324 does not pull down to ground well.

I assume 324 supply is 5v.

Try replacing piezo ground end connection with a 1k to ground and a 3k3 to
5v supply
This gets overall signal above ground and gives 324 better ability to repeat
it well. Once/if it works you can deal with the offset in software.

Remove C1 temporarily.
Place a 5v6 or thereabouts zener across R2 temporarily.
You should get a DC kick across R2.
Sample with A2D as fast as you can and report any significant changes to
start with.

Once you see this revert cct to original (leave 1k/3k3 in place).



       RM

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2004\07\06@051111 by Nigel Orr

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Sent again, with corrected tags...

pic microcontroller discussion list <> wrote:
> This is how it should work fine:
> http://www.leftclick.ca/public/piezo_input.pdf

Looks like they've got the opamp input pins the wrong way round.  It should
be a follower, input to '+', feedback from output to '-'.  It's a fairly
common mistake on schematics, at least by me...

Nigel
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2004\07\06@092922 by David VanHorn

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>
>Presumably the A2D can see a varying voltage at its input.
>Place a 1K resistor to ground from 324 output.
>324 does not pull down to ground well.

It also has nasty crossover distortion..
There are better opamps.

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2004\07\06@105244 by Russell McMahon

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> >Presumably the A2D can see a varying voltage at its input.
> >Place a 1K resistor to ground from 324 output.
> >324 does not pull down to ground well.
>
> It also has nasty crossover distortion..
> There are better opamps.

Both true - but an LM324 is probably an ADVANTAGE for a drum application :-)
When running with a capacitive load you Must resistively bias the input low
to prevent distortion, but it's not too terrible for resistive loads. The
resistor I mentioned here is to pull the output all the way to ground as the
output is not a true rail-to-rail low drive.

Adding the 1k/3k3 to the other end of the piezo as I suggested will overcome
this problem as it gets the signal away from ground.


       RM

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