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Float switch circuitry without floats?

Discussion in 'Electronic Basics' started by [email protected], Mar 11, 2008.

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  1. Guest

    I know very little about electronics though I'm fairly handy and have
    experimented with some simple circuitry. My objective is to build a
    water level sensor for a 10 gal stainless steel kettle that's too high
    for me to see the fill level. My idea is to build a panel using LEDs
    to indicate how many gallons are in the kettle as I fill it. My
    thought is to place a device(vertical stick) with bare terminals at
    specific levels plus a common wire on the bottom. Thus as the level
    increases, the water completes each circuit like a switch and lights
    an LED indicating each gallon level. My concern is the resistance of
    the water. Obviously it would be low voltage so I don't think I need
    to be concerned about a shock. Do you think this could work?
  2. Something simple is just a tube with one in closed that hooks around out of
    the tank for you to see.

    The idea is that the water pressure will push water into the tube and
    compress the air inside.

    Its simple and no electronics needed. You calibrate it by pouring in a known
    amount of water, mark on the tube the water level, then repeat with more
    water. You can do this several times to get a "ruler" for the water level.
    No calculations or anything else is needed. (although the air might be to
    compressible and temperature dependent so you would use something else such
    as oil)

    If your serious about using electronics there are a half million ways to do
    it. Another simple method is to use a long resistive rod with a float on one
    end. The rod acts as a variable resistor and as the water rises it moves the
    rod upward(or I suppose it could be turned upside down) shortening the
    length of the rod between the float and a contact. Resistance is
    proportional to height which is proportional to amount of water(which by
    controlling the resistance you might get it matched well). You can hook up a
    cheap ohm meter and use that as your guide. Probably cost just a few dollars
    and very simple method. (although I doubt its very clear what I mean)

    | |
    | |
    | |
    |* |*
    | |
    | -

    So - and * form a circuit and the part of the rod inbetween is a resistive
    load - is at the water level and * is fixed(say at the top of the tank). As
    the water rises it shortens the distance between - and * (which are
    connected to an ohm meter or some other device) reducing the resistance.
    Hence smaller resistances means larger height and vice versa(and its

    Anyways, just 2 ideas to consider....
  3. Jamie

    Jamie Guest

    Capacitive level probe.
    the fluid is the dialectic. the container is one side of the cap
    while the probe is the other.. Use it to alter a VFO that gets
    converted to an analog voltage derived from the freq.
    The probe should be coated with some thing to protect it like
    teflon etc..

    Or, buy one .
  4. John Larkin

    John Larkin Guest

    Even easier: run a clear tube out of the bottom of the tank and up the
    side. The level in the tube is exactly the level in the tank.

  5. bud--

    bud-- Guest

    It will work fine. You may need a transistor switch at each level (the
    current may not be high enough to light an LED). You could use the
    kettle as the common electrode. Using electrical conduction for level
    detection is common.
  6. I can't stop thinking :)
    As so often, there are more ways to skin a cat and yours will work though is
    has some drawbacks. The resistance of the water will not be constant. Pure
    water has a very high resistance, so high it is considered an insulator.
    Pure water is rare, so I think your water will have low enough a resistance
    but it still can be pretty high. As you have to count for the worse
    scenario, you will indeed need an amplifier (transistor) to make sure you
    get a reliable signal. Another problem can be caused by the current.
    Although the current will be very small, it can solve (part of) your wires.
    This does not only depend on the current but also on the minerals solved in
    the water. Not to forget the direction of the current. Nevertheless, I've
    seen long poles of teflon with conducting rings on the outside and wires on
    the inside used to measure waves and tides in the Northsea. I remember from
    the explanation that they used special metal and AC-current for the
    measuring to achieve an acceptable livetime in the aggressive seawater.

    To make a long story even longer, I propose two solutions without
    1. Take a plastic tube and put reed contacts in it on selected distances.
    Make a floater with a hole in it and a magnet glued on it. No need to say
    you'd better seal both ends of the tube with some silicone -or other
    sanitair kit. As you have some experience with circuitry you can fill in the
    use of the construction. yourself.

    2. Get some clear flexible tubing with a length over three times the height
    of your kettle. Mount one end inside the kettle as close to the bottom as
    possible.Gide the tube upward, over the edge, down to litlle below the
    bottom of the kettle and then up again to end somewhere above the kettle.
    Once you filled this tube fully with water, you can always see the fluid
    level on the outside unless the kettle runs fully dry. If that's the case,
    you can fool this meter by placing a little tin inside the kettle and make
    the inside end of the tube end up in this tin.


    petrus bitbyter
  7. John Fields

    John Fields Guest

    Excellent idea except for the fact that the enclosed, compressed
    volume of air will eventually diffuse into the water column,
    rendering the device useless WRT to the initial calibration.

    Oh, wait, I forgot...
    I'm plonked, so you won't be able to read about how fucking stupid
    your ignorant, albeit well-meaning "advice", really was and maybe
    come up with a better plan.

    Oh, well...
  8. John Fields

    John Fields Guest

    Did you miss this?:

    "My objective is to build a water level sensor for a 10 gal
    stainless steel kettle that's too high for me to see the fill
  9. John Larkin

    John Larkin Guest

    I interpreted that statement to mean that the kettle was too high for
    him to see over the lip inside, without standing on a ladder or
    equivalent. So, is the kettle actually so high that it disappears into
    the ceiling?

    At any rate, I made a suggestion that might work. If it won't work for
    him, it's for him to decide. Your following me around, barking at my
    feet like some crazed Chihuahua, is just silly.

  10. John Fields

    John Fields Guest

    Oops, sorry... I forgot that:

    "I reserve the right to make fun of anybody or anything."

    applies only to you.

    BTW, did you also miss:

    " My idea is to build a panel using LEDs to indicate how many
    gallons are in the kettle as I fill it."?
  11. Bob Masta

    Bob Masta Guest

    Don Lancaster's "CMOS Cookbook" has just what you need
    (page 283 in my old 1977 edition). He calls it a "Booster Tank
    Level Sensor" for fire engines. The tank is grounded, and there
    are sensor electrodes at various depths, insulated from the tank.
    Each electrode goes to the input of a CMOS inverters and is pulled
    high by a large-valued resistor when there is no water covering it.
    When the water hits it, it pulls the input low and the inverter
    output goes high. (He shows the inverter driving an LED directly,
    without even a series resistor. He might be relying on some
    internal current limiting, or this might be a circuit error. You can
    use the inverter output to drive an NPN driver through 10K to its
    base, ground the emitter, and put your LED and series resistor from
    the collector to the positive rail.)

    Best regards,

    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, FREE Signal Generator
    Science with your sound card!
  12. John Larkin

    John Larkin Guest

    Oh, did you say anything funny? I must have missed that.
    BTW, did you miss that my response was to JS's liquid-filled tube

    OK, post a design that responds to the OP's needs, something a
    beginner could put together successfully in a hour maybe. I have one
    in mind that takes 12 parts total, and a probe he could assemble in 5

    All you seem to do lately is whine at other peoples' suggestions. You
    probably aren't having fun.

  13. Bob Monsen

    Bob Monsen Guest

    I would use a float switch instead. You could have the actual switch outside
    of the pot, and have a float connected to a stick, attached to the side of
    the pot. When the float gets to the fill line, the other end of the stick
    connects to the switch:

    | |
    | | Float moves, causes switch to close
    | |
    | |
    Inside Pot | |
    | |
    | |
    | |
    | |
    | |
    | |
    | |
    | |
    | |
    .---.| Float
    Water Level ^ |
    | |
    | |
    (created by AACircuit v1.28.6 beta 04/19/05


    Bob Monsen
  14. John Fields

    John Fields Guest


    When you make fun of someone it's seldom funny for them.
  15. John Larkin

    John Larkin Guest

    Geez, you *are* really bummed out.

  16. whit3rd

    whit3rd Guest

    Common solutions:
    sight glass (a glass tube mounted alongside the
    kettle, joined at the bottom to the kettle and open at the top).
    Breakable, might need cleaning attention.

    Float: a calibrated stick visible over the pot will tell you what
    the float level is. Loose parts, need to lift it out to clean.

    Bubbler: a tube with a slow bleed of gas; when you hear the bubbles,
    it's got the end under water. Loose parts, easy to clean. Can use a
    sensor instead of listening.

    Ultrasonic level sensor: no contact, easy to keep clean. Sonar,

    Capacitive dipstick: two close-spaced electrodes, capacitance goes up
    when liquid fills the space. Capacitance is analog to liquid level.
    Affected by liquid composition.

    Capacitive sensor: two electrodes at a target position, capacitance
    when liquid reaches that position. Capacitance gives over/under level
    indication only. Can be combined with the sight tube. I've seen
    these offered for chemistry labs, the exact liquid composition
    doesn't matter much.

    Pressure sensor/switch: depends on density of the liquid times depth.

    Force sensor: ten gallons is HEAVY, hang the pot from a spring scale
    (or put a scale under the stove).

    A simple conductance sensor relies on electrical current in the
    which is unreliable (the electrodes can crud up even if the liquid is
    a great
  17. John Fields

    John Fields Guest

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