LM3914 resistor values for 50mV peak piezo element

Discussion in 'The Projects Forum' started by BuxZED, Apr 14, 2014.

  1. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    LM3914_5V_voltmeter_circuit.jpg


    I Built the above Circuit and it works like a charm.

    now i want to adjust the circuit to read a range of 0 to 50mV produced from a piezo element under vibration.
    also, rather than from a 9V battery, i need this circuit to operate from a 3V supply (2AA batories)

    can anyone suggest how i can adjust this to work from 3V

    also, do i need to amplify the 50mV signal? if so, how would i do that.

    I noticed, the circuit i built draws about 7mA when it's not operating, is there a way to bring it to zero?

    any help is appreciated. thanks in advance. :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    LM3914 can operate at 3V.
    You can alter the range of the input by adjusting the Reference input RHI. The minimum input is 200mV. You will still need to amplify a 0-50mV signal.

    Read the datasheet.
     
  3. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    Hi
    Thanks for the reply.
    can I get away with using a single transistor or do i need to use something like a UA 741?
    if it is a transistor, what transistor should I use?
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi bux,

    Amplify the 50mV signal by a factor of 25, so 50mV peak would give 1.25Vpk

    Link pins #6 and #7. [ RHi = 1.25v]
    Connect pin #4 to 0V [RLo=0v}
    Connect a 1K resistor from pin #7 to pin #8. [~13mA per LED]

    For LED Bar pin #9 to +V, for Dot pin #9 o/c

    Amplified signal input to pin #5.

    Use LED's with a forward voltage drop < 2.5V.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The 741 was a necessary evolutionary step in the analog universe, and does not get nearly enough credit around here. Still, it will not work in your application. You need a high input impedance opamp circuit based on a part that is designed for single-supply operation.

    According to the datasheet, the only way to get the standby current to zero is with an on-off switch.

    ak
     
  6. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    Hi,

    Thanks for the reply
    what can I use to amplify the signal?
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Last edited: Apr 14, 2014
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  8. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    thanks for the reply,
    just for my learning, is there a reason that i can't use a transistor to amplify the signal?
     
  9. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    The signal will be inverted if you use a single transistor amplifier, also the output will have to AC coupled to the LM3914.
    The impedance will be too low when being driven by a piezo source etc....

    Its not really worth the hassle in building a transistor amplifier, when you can by an OPA for 29pence UK..
     
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  10. MrChips

    Moderator

    Oct 2, 2009
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    They used transistors many moons ago and results were barely satisfactory at the best of times.
    Then they invented the Op-Amp.

    Before some smart alec complains, yes, the op-amp was invented with vacuum tubes.
     
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  11. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    this is where i am at the moment. any advise on how to proceed would be nice


    Capture.JPG

    and for my knowledge, what purpose dose R2 serve in the circuit?
     
  12. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    If you are following my suggestion in the previous post.? R2 is not required.

    This simulation shows the MCP6001 OPA.

    Have you decided which amp option.?

    EDIT:
    The purpose of R2 is to raise the Rhi Limit above 1.25V.

    You may find this Tool I designed helpful.
    http://www.electro-tech-online.com/tools/LM3914V2.php
     
  13. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    Thanks for the help so far

    MCP6001 looks like a cheap and cheerful solution to the problem. I'm reading the specification to figure out how to integrate this in to my circuit.

    how would i intergrate a variable resister to ajust the signal voltage to peak at a value other than 50mV, for example a condition where the piezo element only produce 40mV at peak and i want that to be the reference voltage where all the 10 lights light up without having to build a new circuit.
     
  14. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Look at that circuit I posted, where it says Set Gain.
     
  15. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    is it the RV1 in the circuit?
     
  16. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    I dont see a RV1 in my circuit.???

    Look at this marked up image.

    The 50K pot is currently set to 24K
     
  17. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    dose MCP6001 only come in surface mount package?
    is there a traditional trough hole option?
     
  18. ericgibbs

    AAC Fanatic!

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    The MCP6002 , a dual version comes in a DIP package.
     
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  19. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    How dose this look?

    new.png
     
  20. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Looks OK to me.:)

    I would link pins 5&6 on the MCP to 0V if you have no use for them.

    You could use the spare half of of the MCP for future development.
     
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