How do I boost a piezo signal with an op-amp?

Discussion in 'The Projects Forum' started by GRNDPNDR, Mar 1, 2012.

  1. GRNDPNDR

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    Mar 1, 2012
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    I have a drumset that uses a piezo sensor for it's pads. I would like to create a gain circuit that will boost the piezo signal.

    There was a message board where this was built and working but the schematics were gone, but it's just a simple non-inverting amplifier.
    (Original thread here http://rockband.scorehero.com/forum/viewtopic.php?t=18388)
    I'm using an LM358N and I cannot for the life of me get it to work properly.

    I tried a couple configurations on a breadboard then just started playing around in multisim with a 555 timer circuit because I still can't get this to double the output signal.

    I've been told it needs a dual rail power supply but can work on a single polarity PSU if I use a bootstrap circuit and DC blocking caps....but where do I connect them? what are they?

    The guy who originally did this didn't seem to have a bunch of extra parts, and used USB 5V power.

    I need something simple, and quick using an LM358 to boost the output signal of a piezo sensor.
     
  2. Ron H

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    How much gain do you think you need?
     
  3. GRNDPNDR

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    I wanted it to be adjustable using a trim pot, with gain from 1-10, but in all honesty the gain amount actually needed will probably be fairly low, like a gain of 1.5 or less maybe.
     
  4. Ron H

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    Here are a couple of ways to do it. Gain on each is nominally 1 to 11, with the high limit a function of potentiometer tolerance.
     
  5. GRNDPNDR

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    When I build that first circuit (In multisim) using a 3V battery in place of the piezo the output as measured from ground, the output is in the uV range.

    Previously I was trying the other circuit shown.
     
    Last edited: Mar 1, 2012
  6. Ron H

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    A battery is DC. It won't pass through the capacitor on the input. You don't need DC response.
    If you want to simulate it, use a pulse or sine waveform.
     
  7. GRNDPNDR

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    that works, when I put a 3V rms AC source on it I get 5.2 - 5.4 V on the output,with the pot at 90%

    If i turn the pot all the way down, voltage output is around 6.5 - 7V, and if I turn the pot all the way up (100%) I'm getting around 5.9 - 6.2V.

    Does this seem like it's working right? I don't quite understand the circuit so I don't really know how much of a "gain" that will really provide the sensor or if that's too sensitive.

    I don't recall if I mentioned it but it's for an eDrum set. I'm trying to make a sensitivity adjust so thw drum pads are more responsive. it goes from the piezo to the drum brain which deals with the signal. This circuit will go in between there just to boost the signal slightly. If the boost is too much one single drum hit will result in multiple hits being detected and/or losing the ability to do softer drum hits.

    My pads won't detect a 32nd note hit with a single hand, this is what I'm trying to achieve. one handed 32nd note drum roll being accurately detected.
     
  8. Ron H

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    That doesn't sound right. The gain should go from 1 to 11. What is your supply voltage?
     
  9. GRNDPNDR

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    Supply is 3v or 3.5.

    I just realized though my meter was set to dc. But I still don't think that accounts for the differences.
     
  10. Ron H

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    LM358 with a 3V supply will only work with inputs between zero and 1.5V. The output voltage swing will be between 0 and 1.5V.
    One caveat that really applies to simulators:
    Garbage in=garbage out.
    Simulators are pretty much worthless to a novice.
     
  11. GRNDPNDR

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    oh sorry, I meant 18V right now, but ideally i would like to use 5V in the end product.
     
  12. GRNDPNDR

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    18V supply, 3-4?V input from piezo (so i'm told), and meter set to AC.

    trim pot at 0% = 7.79V output
    trim pot at 100% = 5.0V (give or take) output
    trim pot at 50% = 5.7V

    I'm a failure at even creating circuits made by someone else who actually knows what's going on :/
     
  13. Ron H

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    Your input voltage can never swing below the negative rail (0V in this case). if you want to put in a sine wave, you have to offset it so the negative peaks don't go below ground. You also have to make sure that the input signal, multiplied by the gain, does not drive the op amp to saturation on either supply rail.
    Remember that 1V RMS=1.414V peak, or 2.828V peak-to-peak. When it comes to saturation, you have to consider the high and the low peak voltages.
     
  14. GRNDPNDR

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    I only half understand what you're saying, and not sure how to correct it or fix it.

    If I actually build this and use a real piezo would it work like it's supposed to? I have to build 7 of these in one box.

    That's why I would prefer a simpler design if possible. Something like the second image, however that doesn't work either and suffers from two problems I would like to avoid;
    The need for two batteries
    the need for an 18+V supply, I would like to run this on 5V. Would a different op-amp be of any benefit?
     
  15. Ron H

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    Here is a design that apparently works. Look at the schematic after you read the design page.
    Attached is basically the same circuit, with a couple of minor component value changes.
    If you want to build all amps on the same board, you can use LM324, which is the quad version of LM358 (dual). The single version is LM321.
     
  16. GRNDPNDR

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    Why do these things never work for me?
     
  17. Ron H

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    1. The piezo transducer does not put out a 60Hz, 3V RMS signal.
    2. The bottom end of the pot needs to be floating.

    Having said that, let me play with a simulation, and I'll get back to you.

    EDIT: I don't know how to model the piezo transducer, which is (I assume) attached to some surface (drum head or other). The resonance of the surface, and the equivalent circuit of the transducer, both affect the model. I will guarantee you that a simple voltage source won't model it accurately. It has a complex internal impedance.
    Your best bet is a breadboard. I'm guessing you probably don't have an oscilloscope,. You might be better off to buy proven hardware. You may say it's too expensive. I don't know what to tell you. I think I've already given you my best shot.
     
    Last edited: Mar 2, 2012
  18. GRNDPNDR

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    it was floating prior... it did the same thing in both cases.

    As far as I've been told by people who've done similar, the piezos in the drumkits put out 3.5V. When I took one of them out of a different drum pad and deformed it gently in my hand I saw it working around 2.5-4V and in some cases spiking pretty high up to 15V.

    Inside the drumpad housing though it would likely never reach such deformation, and probably output substatially less.
     
  19. Ron H

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    I edited my last post.
     
  20. GRNDPNDR

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    I have access to an Oscope at school, Believe it or not I'm taking an electromechanical engineering technician course. I know what you're saying about the wave not being a perfect sine, but for the purposes of this circuit, should the output not just be an amplified version of the input? therefore, why does the wave form look so strange on the output.

    I would likely buy the right thing if I could find such a pre-made device. rockbandkickbox.com has similar things but they serve different purposes with sensitivity adjust as a side bonus, and they are also drastically expensive.
     
    Last edited: Mar 2, 2012
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