Disable PIezo speaker

Discussion in 'The Projects Forum' started by _Thoth_, May 4, 2012.

  1. _Thoth_

    Thread Starter New Member

    Apr 8, 2010
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    I need to disable a piezo speaker (AT-14WF) on a piece of equipment to use as a film prop. The problem is that the equipment senses that the speaker has been disconnected and returns an error, so just unplugging it does not work. I have tried just plugging up the hole but it is still too loud, the sound guys equipment is extremely sensitive. I have also tried drilling through the speaker plate but, again, the equipment senses the problem.

    The speaker has there wires, black, red & blue. So I would assume the black is ground and the other two are for different frequencies. I have tried measuring resistance between the leads to maybe add a resistor to fool the circuit, but get no resistance reading.

    Any suggestions ? I'm not an electronics whiz so please forgive me if I described something incorrectly.
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Last edited: May 4, 2012
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  3. Experimentonomen

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    Feb 16, 2011
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    Try replacing the speaker with a small capacitor in the tens to hundreds of picofarads.

    A pieze speaker is essentially a ceramic capacitor.
     
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  4. KJ6EAD

    Senior Member

    Apr 30, 2011
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    The piezo element is seen as a 15-20nF capacitive load by it's driving circuit. See the 3-terminal topology on page 8 of the AVX document linked above by KMoffett.
     
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  5. _Thoth_

    Thread Starter New Member

    Apr 8, 2010
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    Sorry, should have attached a pic in the first place.
     
  6. _Thoth_

    Thread Starter New Member

    Apr 8, 2010
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    Please forgive my newbieness, but if I am to put a capacitor inline, it only has two leads but my device has three (black, red, blue), this is where I am lost.
     
  7. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Two 10nF capacitors, one black to red and one black to blue should do it.
     
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  8. _Thoth_

    Thread Starter New Member

    Apr 8, 2010
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    Thanks a lot, I'll go give that a try.
     
  9. #12

    Expert

    Nov 30, 2010
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    I have stuffed piezo beepers with electricians putty (plasticine if you're British) but it is not perfect. I expect you will get the job done with the answer from KJ6.
     
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  10. _Thoth_

    Thread Starter New Member

    Apr 8, 2010
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    Sadly the capacitor trick did not work, device still showing error that speaker is missing or disabled. Any other suggestions ?
     
  11. _Thoth_

    Thread Starter New Member

    Apr 8, 2010
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    I've attached a diagram of the audio subsystem of the device, but it all greek to me.
     
  12. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Any chance for a better photo that makes the text readable?

    Looks like you just need to disconnect the piezo, and tie the line at the top of the page to +5V through a pullup resistor.

    Ken
     
    Last edited: May 4, 2012
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  13. paulktreg

    Distinguished Member

    Jun 2, 2008
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    The plastic housing acts to some extent as a resonant chamber to amplify the noise. Have you tried removing the crystal from its housing? How noisy is it then?
     
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  14. _Thoth_

    Thread Starter New Member

    Apr 8, 2010
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    Still too noisy outside the housing.

    That is the only info I could find on the audio subsystem, a scan of a scan of a scan at best.

    I've got someone (who is actually electronically inclined) coming by in the next 1/2 hour to take a look.

    If you have any other suggestions plerase feel free to suggest. If I get the answer I will post it.
     
  15. KJ6EAD

    Senior Member

    Apr 30, 2011
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    It occurs to me that simply pulling the test line high (J1-8) may not be sufficient. The processor may interpret that as a fault condition if it occurs during times not enabled by the processor (J1-15, 17, 18). You may have to re-create the actual oscillation of the piezo at or near it's intended frequency. You might experiment with some other capacitor values.

    Just to be certain, you know that the capacitors have to be connected to the nodes in the circuit where the piezo would have been connected and the piezo must be disconnected.
     
    Last edited: May 4, 2012
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  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    I don't think he has anything to loose by trying this. After all, all the other lines look like inputs.
     
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  17. KJ6EAD

    Senior Member

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    Of course it's worth a try. I only offered an alternative as a backup plan. Did you mean outputs?
     
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  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    No, I meant inputs to the osc section of the schematic. The three transistors on the bottom right are receiving inputs from U5. They appear to be step attenuators for the Osc. transistor. The top connector is an output to the other chip that I can't read. After looking at that output again I now think it feeds a square wave @ the osc freq back to the other chip. Maybe others see it differently but that my take.

    EDIT: I just saw this..
    which I believe put us in sync. See section in bold.
     
    Last edited: May 4, 2012
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  19. _Thoth_

    Thread Starter New Member

    Apr 8, 2010
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    So unfortunately nothing has worked so far, short of trying to find small traces on the controller board. Apparently, according to the devices service manual, it is looking for a minimum of 0.2V.

    "Audio Failure - Occurs when the main processor fails to detect 0.2V on an A/D channel following audio circuitry activation."

    A friend came over and attached his oscilloscope to discover the current sent back is of a different type.

    For reference the device is an intraveneous pump we are trying to silence for use on a film set. The boom mikes pick up the slightest beep so this is why "stuffing the hole" isn't quite good enough.

    On previous pumps (with 2 wire speakers) we simply put a resistor in the path to "simulate" a speakers existence. But this piezo is driving us batty.

    I truly appreciate all your help here.
     
  20. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    What do you (he) mean?

    Ken
     
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