Strain Gauge Scale: Instrumentation Amp Noise Help (Noob Q)

Discussion in 'The Projects Forum' started by Bullfish, Apr 4, 2011.

  1. Bullfish

    Thread Starter New Member

    Apr 4, 2011
    2
    0
    Hi, and thanks in advance for the help.
    I'm an electronics noob who's getting a lot of noise in a circuit and would appreciate some help de-noisifying it, please :)

    I had the idea to build a web-enabled bathroom scale as an Arduino project. I ripped apart a cheap digital bathroom scale and gutted the electronics, leaving me with 4 pairs of strain gauges. I have taken 2 pairs of strain gauges and arranged them in a Wheatstone bridge, as per this diagram from http://www.allaboutcircuits.com/vol_1/chpt_9/7.html
    [​IMG]

    Next, I built an instrumentation amplifier as per this circuit from
    http://www.allaboutcircuits.com/vol_3/chpt_8/10.html
    [​IMG]

    I used an LM324N quad op-amp with a single-sided supply for this. My R values are 200kΩ, and my gain resistor is 500Ω. Both the bridge circuit and the in-amp circuit are driven from the same regulated 5V supply.

    I get the amp to do its thing in a useful output voltage range, but the signal never stabilizes to the point where I'd consider the information reliable. I'm sure I need some filtering and/or shielding, but not sure how or where to put this.

    All advice appreciated!
    Thanks, Carl
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Your problems start with the use of an LM324 as an instrument amplifier. You are trying to get good performance out of a very old op amp.

    Do you have a complete schematic? What power do you use? Is the excitation across the bridge correct and very quiet? What kind of resistors do you use, and are they at all matched? What glue did you use to attach the strain gauges to the fixture?

    If this is for a demo, you may be able to get somewhat repeatable results. Otherwise, you might want to use a "real" IA, like the AD620 - http://www.analog.com/en/other-products/militaryaerospace/ad620/products/product.html
     
  3. Bullfish

    Thread Starter New Member

    Apr 4, 2011
    2
    0
    I appreciate your pointing me in the direction of the AD620.

    An apology is in order: I had a little bit of electronics knowledge earlier in life (I was a ham when I was 12, and am pushing 50 now), and I kind of parachuted in to this project with the naive idea that I could just build a web-enabled scale-- I'm more savvy on the digital/programming side of things, although very much a dilettante, being a medical doctor by profession. I'm trying to figure out what I should read and what blanks to fill in to make this work as a proof-of-concept rather than as a high-accuracy measuring device.

    I tore apart an inexpensive digital scale I bought on Amazon and am using the strain gauges from that. I simply clipped the leads and threw away the existing circuitry-- it was all micro-mini surface mounted stuff anyway.

    For power, I'm using 4-AA cells through a 7805 VR on a breadboard. My resistors are off-the-shelf and not matched.

    I want to continue to power this with a battery. How do I get a dual-sided input to the AD620? I know, I know, dumb question :)

    Thanks, Carl
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  5. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    The AD623 is pretty good, too. You won't get great performance out of a circuit made with op amps, especially the good old LM324. It has its uses, but let's say building an instrumentation amp doesn't make use of its best features.
     
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