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Conductivity sensors

Discussion in 'Electronic Design' started by mooseo, Sep 19, 2006.

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  1. mooseo

    mooseo Guest

    Hi,
    I'm looking to measure when locations on the shore get splashed by
    waves. I've tried lots of different things (direct voltage,
    capacitance), but none have been ideal.

    I've come across an interesting schematic, at:
    http://www.emesystems.com/OL2mhos.htm

    which uses a 555 timer to read the impedence across a probe. I've
    breadboarded this up, and it works perfectly, measuring splashes of
    water very quickly, but accurately detecting when the water has drained
    away. I have connected the circuit ground to the black wire on the
    schematic and am reading the voltage across the 1K resistor, which
    gives a measure of the current consumed by the 555 (proportional to the
    oscillation rate).

    The problem is that I need to have several of these in close proximity
    to each other. This means that when they are all splashed by a wave,
    the probes will be shorted to each other. In practice, if I have 2 of
    these probes running from the same power circuit, when they are both
    immersed in the same solution, they act as if neither of them are
    submerged.

    I can't quite figure out what parts of the cirtuits are interacting
    with each other. I have tried connecting the power to them through a
    multiplexer, so they aren't on at exactly the same time, but this
    doesn't seem to help.

    Does anyone have any suggestions?

    Thanks,
    Mike
     
  2. All should have one common probe, like a ground.
     
  3. mooseo

    mooseo Guest

    Hi Homer, thanks...
    I'm not quite sure what you mean by this... all the circuits are on a
    single power supply, so they share a ground. The probes themselves are
    connected between pins 3 and 6 on the 555. Are you suggesting that I
    ought to link these circuits in some other way?
     
  4. Find a design where one probe is connected to ground. Yes, you need to link
    them all that way.
     
  5. linnix

    linnix Guest

    When you multiplexed the powers, did you isolate the grounds as well.
    Did you have filters isolations for the power supply? Did you isolate
    the probes? Treat this as a very noise sensiitive circuit.
     
  6. mooseo

    mooseo Guest

    Hmm, I'm not sure I understand. Are you suggesting that I modify the
    design I have, to somehow connect the probes to ground, or are you just
    suggesting that I need to come up with a totally different design? If
    the latter, do you have any other suggestions about it, other than that
    one probe needs to be connected to ground?

    In previous itterations of my design, I did use probes where one was
    connected to voltage and another was the path to ground, reading the
    voltage off this path. This turned out to be a poor design because it
    picked up lots of noise.
     
  7. mooseo

    mooseo Guest

    Hi,
    Yes, I did switch both the power and the ground through the
    multiplexer. The other connection was the voltage out, which went into
    the A/D. I did not have any filters on my power supply. What sort of
    filters would you suggest? Maybe I'll try running it off a battery to
    see if that helps.

    thanks,
    mike
     
  8. Look for (moisture meters) and find a design where one probe is connected to
    ground or to the power supply. That way all of those can be commoned.
     
  9. linnix

    linnix Guest

    Yes, that should help.
     
  10. mooseo

    mooseo Guest

    I've just tried running the circuit off batteries... both 1 battery for
    the whole thing, or separate batteries for the multiplexer and the
    probe, with the grounds connected, but the circuit behaves exactly the
    same. Individually, each probe registers being submerged by increasing
    the voltage drop across the resisstor, but when 2 are submerged in the
    same liquid, both have the same drop as if they were not submerged at
    all.

    any other thoughts? I can try getting some inductors, although it
    really doesn't look much like a noise response... more like the two
    circuits are inhibiting each other.

    thanks,
    mike
     
  11. linnix

    linnix Guest

    Why connect the grounds? Even if you have to connect them, use
    inductors. The probes seems to be AC shorted.
    Same difference, signal from one probe is just noise for another.
     
  12. For the same reasons as you can turn on one light in your house without
    affecting the others even though all are connected together.
     
  13. linnix

    linnix Guest

    That may be true for 60Hz, but not for higher frequencies. At 10 Khz
    (and harmonics), signals can travel in different paths. Unfiltered
    power supplies are just AC shorts.
     
  14. If all of your monitor units share one common ground, each can monitor it's
    probe separately. Of course the output must be isolated from each other.

    However this won't work with the indicated circuit. He could, however,
    switch the CS lines as he reads each sensor module and this would avoid the
    need for isolation.
     
  15. linnix

    linnix Guest

    You are thinking digitally; however, his problem is analog. He already
    tried switching the power lines, so CS would not be any better. The
    problem is uncontrolled impedence paths, whether they are active
    (powered) or not.
     
  16. I am assuming he has some sort of isolation on the outputs (otherwise how
    will it work?) So if he 'reads' one probe at a time it seems reasonable that
    he will get an isolated result. Still, a solution where one probe is
    grounded or high would be simpler.
     
  17. mooseo

    mooseo Guest

    I have the grounds connected simply because the A/D board is set up as
    single-ended, i.e., it's reading each channel relative to common
    ground. I can rewire some things to route the ground of each signal
    through the multiplexer and run the board in differential mode.

    I see what you are saying about AC noise, although I'm new enough at
    this that I have a hard time wrapping my brain around AC. Why is it
    that the current would drop in a shorted situation? I would have
    guessed, though without knowing why, that it would have increased...

    Thanks for giving me some things to try.
     
  18. mooseo

    mooseo Guest

    ..
    Looking again at the specs for the multiplexer (
    http://www.analog.com/en/prod/0,,768_836_ADG407,00.html )
    I notice that the isolation and crosstalk protection start to drop
    quite a bit at higher frequencies... I have no idea what the specs are
    on the multiplexer within my A/D board, but they could suffer similar
    problems. Might that be a source of "uncontrolled impedence paths?"
    I've never had to worry about such things before.

    I've also never used inductors before. Do you have any suggestions for
    a general purpose model to start with?

    Thanks,
    mike
     
  19. linnix

    linnix Guest

    I know you don't want to hear this. But the final solution might be
    optically isolated multiple A2D units. That's why the commerical units
    are so expensive.
    Your 555 circuit works by rapidly charging and discharging by a
    reference voltage. If that reference voltage (from the probe) is
    noisy, the basic assumptions are off.
     
  20. mooseo

    mooseo Guest

    Hmm, no, I really don't want to hear that! Given the realities of my
    environment, I'm guessing these things will survive about 2 weeks
    before they get destroyed by waves, so cheap is essential.
    I'll try some power filtering and see if that helps.

    Otherwise, I guess it might be time to look for a new solution...
     
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