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water level sensor

Discussion in 'Electronic Design' started by nick4cpp, Jan 19, 2008.

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  1. nick4cpp

    nick4cpp Guest

    I'm an electronic engineer but I have always worked ad a software
    engineer. So I know the theory but I have no experience on electrical
    circuit design.
    I need help designing a simple circuit for a water level sensing
    system. I have found different sensors that act as a current
    I need to know where I can find information on the best way to read
    the output. I was thinking about using an opam.
    A simple circuit with a resistor between the negative input and the
    output and the Vcc applied at the positive input.
    In this case the tension will go from Vcc to somthing less than Vcc.

    It looks enough for me but just in theory. Do you know where I can
    find info on how to implement it in a real application?

  2. linnix

    linnix Guest

    Do you mean the sensor output to the positive input?
    Can you post a link to the sensor datasheet?
  3. Bob Eld

    Bob Eld Guest

    What do you mean " I have found different sensors that act as a current
    generator"? Are these commercial sensors and if so what are the

    Or, are you proposing to sense the resistance of the water by making contact
    with it. This does work but you have to be careful in selecting the
    electrodes that they are not subject to corrosion or that any corrosion or
    ion exchange does not contaminate the water or alter the sensing.
    Furthermore water's conductivity varys greatly depending on the impurities
    in it. Pure water is a "good" insulator.

    I designed a water sensor that works on capacitance change. Water has a high
    dielectric constant, about 80 and therfore its presence is easily detected
    when made the dielectric of a capacitor in an electronic circuit.

    In my device there is no contact with the water. The sensor is a few sq cm
    of copper foil inside of a thin sealed PVC pipe. When water is present on
    the outside of the pipe, the capacitance of the foil is greater than when
    water is not present because the dielectric constant to ground is much
    greater wet than not wet.

    The detector also sealed in the pipe is a CMOS circuit consisting of an
    astable multivibrator (oscillator) and a monostable multivibrator (one
    shot). The one shot is triggered on each positive edge of the oscillator.
    The time out period of the one shot is determinned by the sensor capacitor
    (foil) connected to it.

    The oscillator also triggers a D flip-flop on each cycle. The output of the
    one shot is connected to the D input of the flip-flop.

    If the period of the one shot is long enough so that it is high when the
    flip-flop triggers, the output of the flip-flop will be and will remain
    high. This is the high capacitance, water detected state.

    If the period is short, the D input is low when triggerd and the flip-flop
    output stays low. This is the low capacitance water absent state.

    The internal circuit works on 5 volts at about one MHz, but is conntected in
    a two wire 20mA current loop for detection of water in a tank a couple of
    hundred feet away from the pump circuitry that uses the information.

    Please e-mail me at montassocatyahoocom if you have other questions.
  4. Sprague chips to do this have existed for decades and cost pennies.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  5. Unfortunately, Sprague Technologies has not existed for decades.

    What's left of it is called Allegro and owned by Sanken of Japan, so
    maybe they'd prefer yen over pennies. Anyway, from a quick look I
    don't see anything much along those lines.

    Best regards,
    Spehro Pefhany
  6. Robbo

    Robbo Guest

    There used to be the National Semi LM1830, AC sense signal etc all in a
    DIP14. AFAIK this is long gone.
  7. Yes, that hoary bipolar chip was specifically designed for this kind
    of application. It doesn't even show up on their website search
    engine-- but Google finds it in their obsolescence list with last-time
    buy date of 12/08/98. Personally, I'd consider using a
    microcontroller. ;-) Most practical solution in a lot of situations
    is probably a reed (or Hall) switch activated by a magnet float. Made
    by the millions for dishwashers and the like.

    Best regards,
    Spehro Pefhany
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