Wheatstone Bridge Help - NEWBIE

Discussion in 'General Electronics Chat' started by james211, Aug 31, 2012.

  1. james211

    Thread Starter Member

    May 29, 2012
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    I have an 8in milone e-tape that I want to connect to an arduino. From what I gather one of the most accurate circuits to use with the etape is a wheatstone bridge. The problem is, I have no clue where to even begin with a wheatstone bridge. If anyone can help me I'd appreciate it. I have programming experience with arduino and plenty of soldering experience, just EE background. I've attached the data sheet for the milone etape.

    The purpose behind this product is to have a live feed of a water container via cosm.com. I need to know the level of the container on a percentage scale, full being 100%, empty being 0%.
     
  2. james211

    Thread Starter Member

    May 29, 2012
    210
    2
    your link doen't go anywhere....
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    build a differential amp using an op amp, your output will change 140ohms per inch of water as per data sheet, use the Wheatstone brigde circuit, so you will need two extra 1500 ohm resistors (r1 and r2) and connect it to your dvm, as shown the voltage will vary with the level of water, so at 5v supply empty will be 0Volts and full ( 8inches) will be 0.8333Volts.

    so that will be .8333V/8= 104.1mV per inch

    http://www.allaboutcircuits.com/vol_3/chpt_8/9.html
     
    Last edited: Sep 1, 2012
  4. james211

    Thread Starter Member

    May 29, 2012
    210
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    Dodgy, thank you for the great answer. I'll give it a shot this week. Quick stupid question, what does the DVM acronym stand for?
     
  5. #12

    Expert

    Nov 30, 2010
    16,257
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    Digital Volt Meter
     
  6. james211

    Thread Starter Member

    May 29, 2012
    210
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    So I've been reading a bit about differential amplifiers, and most of it sounds like snoopy's mother talking to me.

    How do I know which differential amp to buy? I looked on mouser and there are a lot of options to look at - channels, gain, offset voltage and so on.

    Any guidance would be great. I'm trying to learn some basics and it really is tough to understand.
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    just build the one in the link i gave you it only uses 1 op amp and four 100k 1% resistors, this will give you a gain of 1, which is what you need, then put the output into your arduino, it will give an output of 0volts to 0.8333volts.

    if you want to alter the gain, add a potentiometer say 1meg ohm in series with inverting feedback resistor (pin 2 to pin 6) , this will give you a gain of 1 to 5.5
    so new output will be 0volt to 4.16volts
     
    Last edited: Sep 3, 2012
  8. james211

    Thread Starter Member

    May 29, 2012
    210
    2
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    looks ok but its a fixed gain, if you build your own using this circuit for less than a pound , you can alter the gain if you need by increasing the value R2 at the top.

    http://www.google.co.uk/imgres?imgu...FFUOL1M6jT0QWToYGYBg&ved=0CDwQ9QEwBA&dur=4716


    use one of these Lm358 op amps http://cpc.farnell.com/stmicroelectronics/lm358n/op-amp-dual-low-power-dip8-358/dp/SC07971
     
    Last edited: Sep 4, 2012
  10. evilclem

    Member

    Dec 20, 2011
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    Why not place the tape in series with a 1K5 resistor? Place the tape in the lower half of the bridge, plug it into an ADC input on the Micro and you'll get roughly 166 mV per inch H2O. (As per the simple voltage divider circuit on the datasheet).

    No need for wheatstone bridge complexity with that much resistance change (unless of course you desperately need more than 0.03 inches H2O of resolution).
     
  11. james211

    Thread Starter Member

    May 29, 2012
    210
    2
    Dodgy, your proving to be invaluable, that's a great link you sent me. Thank you!
     
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