Piezo trigger with amp question

Discussion in 'The Projects Forum' started by jman 31, Jun 23, 2012.

  1. jman 31

    Thread Starter New Member

    Apr 6, 2009
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    Hi Guys,

    I am having a problem with this circuit that I designed. It uses a piezo to trigger a 555 timer and flash an LED. I added the amp section to get better sensitivity. When I use two separate power supplies for the amp and trigger sections it works perfectly, but as soon as I try to combine them to one power supply the LED's stay on indefinitely.

    By the way, I am using the piezo right now so disregard the electret mic comments on the schematic.

    Any comments or suggestions would be greatly appreciated.

    Regards,
    J

    [​IMG]
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    745
    because pin 2 of the 555 is being kept low , possibly due to retriggering caused by un stable supply lines, try putting in extra decoupling capacitors across the supply say470uF and 100nF .
     
  3. jman 31

    Thread Starter New Member

    Apr 6, 2009
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    Thanks for the suggestion Dodgydave. I tried that, and decoupled all of the IC's while I was at it, but no success. :(

    Any other thoughts or ideas?

    Thanks,
    J
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    I would replace the piezo with a voltage source (for instance 10K pot with connection to +/-) to verify functionality, and eventually place a discharge resistor accross the piezo as well.

    I remember playing around with piezo + transistor + LED a while ago (on breadboard), made this working relatively quick, but did not develope it further.

    Then you would have to bridge the input capacitor of course.
    For the non-inverting input also adjustable resistor.

    I don't know OpAmps well only used them a few times, but it should be possible to add LED directly, and see it flashing when you tap the piezo.
     
    Last edited: Jun 23, 2012
  5. wayneh

    Expert

    Sep 9, 2010
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    Using two power supplies, were the grounds connected?
     
  6. #12

    Expert

    Nov 30, 2010
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    1) the local wisdom is: 10uf + .1uf decoupling capacitors for a 555 chip.
    2) You have the opamp in a low impedance configuration. Try the non-inverting input for the piezo input so the input impedance is higher. When you do that, include maybe 1k to 10k in series with the piezo voltage and a 10 volt zener to ground so the voltage doesn't punch a hole in the input transistor.
     
  7. jimmy101

    New Member

    Jun 23, 2012
    9
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    The May 2012 issue of Nuts & Volts has a circuit that might have what you need. In the article "Build a sound-activated laser parking system" the author uses a piezo and a pair of opamps to detect the sound of the garage door opening and to then turn on a laser pointer for a fixed amount of time.
    http://nutsvolts.texterity.com/nutsvolts/201205?pg=40&search_term=garage door&doc_id=-1#pg40

    Same basic circuit but have the second opamp trigger the 555 (for better timing accuracy). Setup the opamp circuit to just give a reasonable length pulse, enough to properly trigger the 555.
     
  8. #12

    Expert

    Nov 30, 2010
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    Yes! That schematic shows the non-inverting configuration for the first amplifier.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Check pin 2 of the 555 timer with a voltmeter and see what its doing, if its less than 4V or (1/3 of the supply ) it will keep triggering.
     
  10. jman 31

    Thread Starter New Member

    Apr 6, 2009
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    OK Fellas I am back. We are dealing with severe flooding from tropical storm Debby right now so I have been a bit sidetracked. Thanks so much for your time and comments!

    Anyways, here is what I came up with so far:

    1. Pin 2 on the 555 drops to zero when I power the amp circuit causing the constant triggering.

    2. There is a constant 1 volt coming from pin 1 of the TL082 (when powered) which I would guess is saturating Q1 (2N2222 transistor) and causing the issue with pin 2 of the 555.

    3. When I unplug the LED, there is no change in the voltages so I don't think it's re-triggering because of the LED.

    4. I don't see any AC voltages during any of this if that matters (even from the TL082 output pin 1)

    5. The grounds are and were connected. The ground doesn't even have to be connected when I power up the amp section it will still cause the problem. I guess somehow its getting a ground from somewhere else on the TL082 chip.

    That's all I can think of so far.

    J
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The capacitor C3 should prevent the 1v dc from keeping Q1 on, whats the voltage across the sensitvity pot?
     
  12. jman 31

    Thread Starter New Member

    Apr 6, 2009
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    10.5 to 12 volts with the amp portion plugged in,
    0 to 12 volts without it plugged in.
     
  13. takao21203

    Distinguished Member

    Apr 28, 2012
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    what is the intended application?

    -to use the piezo as kind of a microphone?
    -to use the piezo as kind of a tap controller?

    tapped piezo's produce sufficient voltage swing to switch on a single transistor configuration + LED.
     
  14. jman 31

    Thread Starter New Member

    Apr 6, 2009
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    Edited:
    Thanks for the info. I have tried many designs before this (including a simple transistor/led) but i was never able to get the desired output until I went with the 555 timer. It allows for a much longer pulse.

    What's crazy though is that I just ran a sim for it in Proteus and it works fine. I just don't get it unless I have a bad component somewhere. I may start from scratch with all new stuff and see what happens.
     
    Last edited: Jun 26, 2012
  15. takao21203

    Distinguished Member

    Apr 28, 2012
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    OK if you are more clear how the piezo is worked, there are people who can build this for you quickly, and see if they can make it working.

    For instance when software programming becomes too much effort, and they do something else for an hour. From your feedback I am not yet really commited to do this. OpAmp chips/piezos I have many around here.
     
  16. jman 31

    Thread Starter New Member

    Apr 6, 2009
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    Sorry man, like I said, I didn't mean for that to sound as if I didn't appreciate the help. I read it back and I did sound a bit dickish! very sorry.

    These are for drums and I have been through many, many different designs to get the desired outcome. I used to attach the piezo trigger to the drum head, but now i am designing it to just be double sided taped to the shell. That being said, I need it to pick up much smaller vibrations than it would directly from the drum head.

    EDIT: I went back and edited my previous comment! Don't mean to seem anything but thankful for any help!
     
    Last edited: Jun 26, 2012
  17. #12

    Expert

    Nov 30, 2010
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    This is reminding me of the guy that mixed up rows and columns on his protoboard. You didn't do that, did you?

    Please post a photo of your assembly and see if anybody can pick up on a mistake.
     
  18. jman 31

    Thread Starter New Member

    Apr 6, 2009
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    Haha, no I'm pretty sure I didn't do that, but that doesn't mean I didn't make a mistake! I will post a picture of it in the morning, but I warn you, it looks like a spaghetti dinner! :rolleyes:

    I think I may just start over from scratch to be sure I don't have a faulty component.
     
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