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Viewing an ohms change on an oscope??

Discussion in 'Electronic Basics' started by scanner80, Aug 6, 2005.

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  1. scanner80

    scanner80 Guest

    I'm looking for a circuit I can build or equipment I can buy
    so I can time how long an ohms change is. I would like to build it if
    possible . I would like to view it on a scope.
    One example would be a starting ohms reading from a device of 1355 ohms. It
    will increase by approx. 20 ohms
    and then return to 1355 ohms. The time it will take is approx. 19 seconds. I
    will need to be able to see a change as small as 1 ohm if possible , but the
    most important thing is to see the reading change from 1355 and return to
    1355 ohms. I need to then measure the time with cursers on a scope.
    I know a respiration monitor can see and display an ohms change , but I need
    a way to exactly measure the change.
    I will be greatful for any help.
    Thank you,
  2. mike

    mike Guest

    Anything that takes 19 seconds and can be measured with cursors is gonna
    take a digital storage oscilloscope. And you want > 0.1% precision.
    This is gonna be one EXPENSIVE measurement.

    I'd build a PIC processor controlled resistance bridge. Program that to
    output the parameters of interest.

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  3. Can this resistance be connected to some other components? Are you
    familiar with a 4 resistor bridge?
    If you connect your ~1355 ohm resistor in series with a fixed resistor
    of nearly 1355 ohms, and connect that to a DC supply, say 10 volts.
    Then connect another series pair of equal resistors (say, 10k each)
    across the same supply. Now you can read or amplify and read the
    voltage difference between the center nodes of these two resistor
    pairs. It doesn't give you a direct measurement of resistance, but
    with a bit of math, it is easy to measure the % change of any one of
    the 4 resistors.
  4. scanner80

    scanner80 Guest

    I just want to be able to measure the time it takes from when the resistance
    leaves 1355 ohms and returns to 1355. I can do it with a meter and a
    stopwatch, but I was looking for a more verifiable way to do it. I want to
    remove human error. This is for calibration purposes.
    Is there any type of simple timer or timer circuit that can will trigger as
    the ohms changes and stop when it returns?
  5. scanner80

    scanner80 Guest

    The device will use an opamp circuit to create the resistances and the
    change. Will the bridge setup work with that type of circuit with out
    causing it to malfunction?
  6. My opinion on that would be worth more after I saw the schematic for
    your circuit.
  7. Jamie

    Jamie Guest

    unless your trying to get out of buying an expensive digital storage
    scope, a line monitor unit (also expensive) would work! but you want to
    hack out something and know a little computer programming, you could
    make your self a VCO osc that would input to the sound card.
    use some FFT coding to sample the frequencies in your time frame..
  8. Jeff Thon

    Jeff Thon Guest


    Have you considered a logging DMM? Your data will be in ohms. Sample times
    can be set faster than the display update rate. The software can graph the
    logged data and will have cursor capability so you can get your time as you
    would have from the scope display. You can use the logging for any
    measurement the meter is capable of for future testing. Additionally, you
    have built in documenting. Way less expensive than a storage scope. Also,
    you don't have to worry about the accuracy of any test gear you build.

  9. scanner80

    scanner80 Guest

    I was just thinking about that.
    I work in a cal lab and have digital scopes, calibrators, and many other
    cool toys. I don't have a logging DMM yet ,but but I can get access to one
    to try it out.
    If I can prove I need one , then they may get me one.
    thank you so much for the idea. I confirms what I was thinking.
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