Replacing a piezo sensor with a switch

Discussion in 'General Electronics Chat' started by Midge, Oct 18, 2015.

  1. Midge

    Thread Starter New Member

    Jun 4, 2015
    13
    1
    I'm trying to modify a battery operated electronic toy drum for a child, who for medical reasons, can't hit the top of the drum hard enough to activate the piezo sensor fitted to it.

    If I disconnect the wires to the piezo sensor, and put my meter across it, striking the drum generates a spike of 2-9v depending on how hard you do it, so I figured I might just be able to replace it with a battery and disability-friendly switch, to simulate the device. However, if I connect the sensor back up to its circuit, I see -0.6v across it, rising only slightly to -0.4v when the drum is struck. The circuit is I assume some sort of microcontroller - in reality just an unidentifiable blob.

    I'm not that familiar with these sorts of devices so just wondered if anyone could tell me how it would operate and how I can rig the switch up?

    Thanks!
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What is the battery voltage for the toy?
     
  3. RichardO

    Well-Known Member

    May 4, 2013
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    You might be able to use the circuit more or less as it is. Replacing the piezo with 2 or more in series or maybe a single large piezo is worth a try.
     
  4. Midge

    Thread Starter New Member

    Jun 4, 2015
    13
    1
    It's 4.5V Alec.
     
  5. Midge

    Thread Starter New Member

    Jun 4, 2015
    13
    1
    Thanks - I'll look into that as an option, but it might still not be sensitive enough. With some effort I know the child can press a very sensitive switch but can't tap like you and I would - and the drum does all sorts of things pretty much on it's own so the child can still get something out of it.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could try this :- Toy-trigger.PNG
    A lever-operated microswitch needs very little pressure to actuate.
     
  7. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    A piezo sensor by itself has a very high output impedance plus it blocks DC. To replace it with anything you might need a capacitor to block DC. Other than that it is difficult for me to suggest anything specific if I cannot see the part of the circuit it is attached to. If it is some sort of amplifier then you could add a preamp which should not be difficult to do.
     
  8. Midge

    Thread Starter New Member

    Jun 4, 2015
    13
    1
    Thanks - I'll give that a go. You're correct about the microswitches - I tend to use a disability friendly switch called a Jelly Bean for this sort of purpose, and it's built around a miniature lever microswitch and really sensitive.
     
  9. Midge

    Thread Starter New Member

    Jun 4, 2015
    13
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    Thanks -I'll take another look at the PCB and see if I can make sense of it. There are about half a dozen surface mount components as well as the round black blob, but not holding out much hope!
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    The piezo component produces a voltage when you strike it - a switch doesn't.

    You could probably amplify it a bit for the blob-chip. An electret MIC capsule contains only the electret piezo element and a special JFET, with a bit of surgery you can rescue the JFET, this will amplify the piezo disc big time.

    Maybe a 2k2 drain load resistor to the existing +V and a 1nF capacitor to couple it into where the original sensor was connected.

    A while back I constructed a circuit along these lines that made it possible to compare the microphonic qualities of various capacitors - I followed the JFET with a high gain amplifier, but the signal was there to be amplified.
     
  11. Midge

    Thread Starter New Member

    Jun 4, 2015
    13
    1
    Sorry for the tardy response to this guys. I kept picking it up and trying different things - but the nearest I got was triggering it, where it would then hang and need to be powered off to reset.

    There comes a time when curiosity, determination, and patience just run out - so I had a dig about and found a similar toy new for just £12 and modified that - which was annoyingly easy by comparison.

    Thanks again for the suggestions. At least I know a bit more about these devices than I did!
     
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