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Designing a low-voltage Moisture Sensor Grid or Pad?

Discussion in 'Electronic Basics' started by Ken Moiarty, Aug 26, 2005.

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  1. Ken Moiarty

    Ken Moiarty Guest

    [Firstly, my apologies to if this post is out of
    place there.]

    Any ideas (and/or recommended references) as to where/how to get started?
    What I'm aiming to create is a floor pad, the size of a prototype to be
    about four square feet, equipped with densely spaced, evenly distributed
    moisture-detection points. Obviously the use here of many moisture
    sensitive microchips would get too expensive. So I'm thinking more along
    the lines of a grid pattern of wires: wire anodes criss-crossing wire
    cathodes, each kept vertically separated by some non-compressible material
    possessing excellent water absorbtion characteristics; such that a low
    voltage current will flow between anode and cathode (to trigger an
    ultrasonic-alarm device) whenever "Kitten decides to do her thing" and the
    material thereby becomes wet...
    Now, I don't happen to know of any specific materials that would fit this
    bill. Nor do I have the specific technical experience or knowledge to
    guesstimate whether this idea is even plausible or not.

    What I do know I have this problem with 'people and their pets' inside the
    house. I have poured over the internet searching for possible technical
    solutions to apply to this problem. Some I've purchased and tried (e.g. cat
    facial pheromone or Feliway®), but none have worked. I have researched
    electronic pet repellants and found a couple of such products that do have a
    proven track record. But these simply deter animals from entering or
    remaining in a certain area by emitting an ultrasonic noise (silent to
    humans) upon the detection of motion --any motion, by any pet, person, or
    thing, for any reason. Such a device would definitely scare the cat out of
    the house, but wouldn't meet with the cat's owner's approval since it is not
    selective for specific problem behavior, and she wants the cat living
    indoors. Thus I need to develop a customized (yet affordable) technical
    solution, such as would briefly emit an ultrasonic "noise" to startle the
    cat, but *only in response to wetting behavior* by the cat.

    Therefore, helpful feedback/references/possible plans/schematics/suggestions

  2. You don't need anything that complicated. Just get a suitable non
    conductive substrate and use contact adhesive to cover it with a single
    sheet of aluminium foil. Then remove small strips of foil to leave an
    interlocking pair of spaced fingers. Or form the same sort of thing
    using conductive ink. Or use a soft pencil and then electroplate.

    Now energise it with a low current alternating voltage between each set
    of fingers and sense the impedance. When there is a step change, stick a
    couple of thousand volts, very limited current across it - or sound your
    ultrasonic "noise". Use ac for sensing to minimise corrosion.

    With different settings, it can educate, terminally educate if you wish,
    rats,etc. They need quite a large current before they catch fire, btw
    and it ruins the fingers.

    With yet more different settings, it can play a reward "happy tune",
    when fitted to a kiddie's pottie and activated by No 1s or 2s.

    Mixing the rat and the child application could be unfortunate..
  3. TimPerry

    TimPerry Guest

    one or more stock unetched PC boards. use dremmel or etching tool to cut
    traces. Darlington transistor or op-amp to sense conductivity.

    equipped with densely spaced, evenly distributed
    densely spaced is not needed.

    note: urine is hard on electronics. you should seen the mess mice make of
    things when they get inside.

    maybe trigger a self cleaning spray?
  4. I made something surprisingly similar to this when I was about 11 years
    old for mice. A sheet metal square about 12" sq. attached to a piece of
    plywood with another smaller (2" sq.) block of wood in the center. Atop
    the small block was another smaller metal plate. Each plate was
    connected to one side of an ac line cord. This was (in the mind of an
    11 year old) the current version of the better mousetrap. Bait was
    placed on top the center electrode and I set it up in the barn to watch
    the fun. I did have to install a small sacrificial fuse in order to
    avoid frying them like bratwurst.

    However, to keep it on topic, I modified the same idea a few years ago
    to prevent my cat from digging in my rubber plant pot and peeing in the
    resulting dirt on the carpet. I placed a wire grid (screen) on the
    carpet under the pot, extending about 8" all around it. I put another
    grid just below the surface of the dirt in the pot, and hooked each grid
    to the poles of a small "electric fencer" circuit I had built. Not
    enough to fry the kitty, but definitely enough to keep her from doing it
    again. The next time the cat stood on the grid beside the pot with her
    hind feet and started digging in the pot with her front paws, she got
    the message quickly, and I removed the device.



    relay switch
    | |
    E1--0 sensor 0-----------(+) 0---/ ----0
    | Op Amp (out) ------relay coil---
    ---------|------------(-) |
    | | Gnd
    R1 R2
    | |
    gnd gnd

    A simple Op Amp comparator circuit driving a relay would do it for you.
    There needs to be only very little current to get the op amp to go high at
    the output. I like the aluminum foil fingers idea that was posted. It is
    cheap and easily replaces after the critter soils it.

    I don't know what the resistance of cat piss is but it will help you
    determine what value resistors are needed.. R1 and R1 should be equal. I
    am guess a 12V source and 100Meg resistors to start off with.. Look for a
    relay with a high coil resistance, They do make some "reed" relays that
    take very little current to fire..
  6. John Fields

    John Fields Guest

    LOL, you are a troo loon!

    Here's what he needs:

    | | | | |
    | | | [10K] |
    | | | | |
    [SENSOR] | +---|--[1M]--+ |
    | | | | | |
    | [10K]<--+--|+\ | E
    | |POT | >------+--[600R]--B 2N4403
    VIN>--+-------------------|-/ LM393 C
    | | | | +--------NO
    | | | +-----+ |
    [100K] | | |K | O--> |<--NC
    | | | [DIODE] [COIL]- -|
    | | | | | O--COM

    Ground the unused inputs and output of the unused comparator, then
    to set it up what you do is to run the pot all the way to ground,
    attach the sensor and pee on it, and then adjust the pot until the
    relay clicks in and then just a little bit more for good measure.

    Then, you take the sensor out wash and dry it and put it back. The
    relay can be used to trigger any "disciplinary" device, remembering
    that Mark Twain said about cats, "Of all God's creatures, there is
    only one that cannot be made the slave of the lash. That one is the
    cat. If man could be crossed with a cat it would improve man but it
    would deteriorate the cat."


    Lol, Can't argue with that..

    But I did build this some years ago,(Not to monitor cat piss). 2 resitors,
    op amp and I belive the relay was 5V with a 500ohm coil. Worked fine. Light
    goes on, Light goes off.....

    But I am sure you will tell my why it dosen't work...
  8. Lumpy

    Lumpy Guest

    Years ago Popular Electronics had a project
    for a "Rain Detector". Aluminum foil attached
    to a block of wood with rubber cement. Exacto
    knife cut through the foil to create two terminals.
    Drive it through a simple transistor to whatever
    signal device you want.

    For a big sensor pad like you want, interlace
    several "fingers" of foil contact.

  9. Personally, I wouldn't use either of them! Sorry about that.

    Yours because your comparator is using ground as the
    reference. Any "leakage" across the sensor is going to trip
    it. The cat stepping on it, or even breathing on it,
    probably would. For a non-perfect op amp, it is highly
    likely that it will be permanently tripped due to offset
    voltage. Because you don't control the back emf from the
    relay when it switches off - which could easily zap the op
    amp. Because you don't control the maximum voltage between
    the inverting and non inverting inputs, which could easily
    cause the op amp to permanently lock or self destruct.
    Because you use dc across the sensor, which will lead to
    corrosion and premature failure. Because you have no
    sensitivity or trigger point or hysteresis adjustment. Those
    sort of things. Depending on the sensor and the op amp, it
    certainly could work - but the odds are that it wouldn't
    work reliably in this application..
  10. John Fields

    John Fields Guest

  11. John Fields

    John Fields Guest

  12. Oh, every engineer has their own way of doing things, have their
    favourite chips and favourite building blocks.

    I am quite possibly too hung up on always using ac with liquid sensors.
    I thought that your circuit, even with the current limited to microamps,
    would tend to encourage dendritic growth or electrochemical migration -
    particularly with the odd chemicals that might crystalise out of cat
    urine in place. It depends on the gap left between the fingers and the
    underlying substrate, of course. But you do have 6 volts across the gaps.

    I may easily be wrong! I tend to be very lazy and stick to the building
    blocks I have used before..
  13. quietguy

    quietguy Guest

    Had you thought of buying or hiring the bell and pad system used for kids that
    wet their bed?

  14. John Fields

    John Fields Guest

    For this application, I tend to think of the sensor as something
    like a couple of pieces of conductive window screen with a paper
    towel between them and paper towels on their outsides, so I don't
    think there'd be a lot of time for much to happen, chemically, after
    the cat peed on the affair. Especially since, if it works, (that
    is, if whatever happens to the cat as a deterrent to keep it from
    peeing there again works) it'll probably only be used once or twice
    and I suspect the paper towels would be replaced between uses.
  15. I hadn't thought of using a layer of paper towel. Sounds a good idea as
    the liquid has to go somewhere...Better to have an instant mop. As you
    say, it will hopefully only be used a couple of times and is not the
    sort of thing that you would want to keep around afterwards as a
    conversation piece.

    I assume that kittens do actually know when they are weeing? We may only
    be training it to avoid walking on paper towels/shiny metal surfaces.
    IIRC, several AI systems have failed due to the wrong choice of training


    I used it in an old Ikagami studio camera. We had a rash of Plumbicon tubes
    that has short lived filiments. I used the circuit to insert a light bulb
    in the filiment circuit when the camera was put into standby mode to cut the
    filiment current when the camera was not in use.

    Could not have been luck because I don't have much of that...

    Just a looney hack, I guess...


    We always just rubbed there nose in it, a little swat and threw em in the
    litter box. Didn't take long for them to figure out where to do there
    business. They seem to prefer the litter box over the carpet. Hard to dig
    a hole in the carpet I guess.. Harder to train the humans to change the
    litter box regularly than it is to train the cat to use it.
  18. Bill Gray

    Bill Gray Guest

    "...the cat that sits down on a hot stove-lid. She will never sit down
    on a hot stove-lid again - and that is well; but also she will never sit
    down on a cold one anymore."
    -- Mark Twain
  19. Pig Bladder

    Pig Bladder Guest

    On Fri, 26 Aug 2005 13:11:39 +0000, Ken Moiarty wrote:

    Such a device would definitely scare the cat out of
    So, finally, it comes to light that this is some babe, and she insists
    on bringing "her" cat into your house if you want to get laid?

    It ain't worth it. She's addicted to cat, and can never love a man. Lose
    the bitch.
  20. That's life, if you want pussy, you have to have a cat...
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