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Touch switch

Discussion in 'Electronic Basics' started by Nicholas Sherlock, Jan 30, 2007.

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  1. Hey all,

    I need some advice on a basic touch-switch.

    I'm hoping to make a "Dance mat" for a computer game I play: This is
    four 20x20cm touch sensitive pads arranged in a cross formation,
    attached to a flat board or foam mat and laid on the ground. You play
    the game by putting your feet on the right pads at the right time. I'm
    disappointed by the life and reliability of commercial soft
    (fabric-like) pads, and cannot obtain a commercial hard pad, so I want
    to make my own.

    The capacitive touch sensors I've seen on the 'net look unsuitable, as
    the electronics are too complex (I'm just a beginner! :)), and I'm
    worried that they may be more like proximity switches than contact
    switches (In the game, it is just as important to be not standing on a
    pad at the right time as standing on it, so if it detects a hovering
    foot, it would be a disaster).

    The design at:

    http://www.imagineeringezine.com/PDF-FILES/fingersw.pdf

    is attractive as it is so bloody simple :). However, I have some
    questions. My pads' sensing areas will be prototyping circuit boards,
    with every second row forming one contact and every other row forming
    the other. I want my touch pads to replace the buttons on a butchered
    USB hand-held joystick - I suppose that the joystick will expect a
    circuit to be completed between two contacts when the button is
    depressed, with low (practically no) resistance. So is it reasonable to
    arrange my circuit like this:

    http://www.sherlocksoftware.org/pad.png

    How do I choose my MOSFET? Can I adjust the sensitivity of my touch pad?

    Cheers,
    Nicholas Sherlock
     
  2. default

    default Guest

    Everything is a compromise . . .

    Both of the contact sensors you show are simple and should require an
    actual contact unless the environment is very noisy (electrically).
    You have to have conductive shoes or bare feet. But you'd have to
    build it to find out. Perhaps static electricity will trigger them.
    Eventually the things would stay on because of dirt ground into the
    sensor grids or a spilled drink.

    You could lower the sensitivity by decreasing the value of the
    resistor from the gate to the source.

    Capacitive sensors are nice. If you plan on jumping up and down on
    these things it might be more comfortable to include a resilient pad -
    and that's where I think the contact sensors would work against you.

    If you want a resilient long lasting surface something like closed
    cell polyethylene foam would be ideal - and it has the additional
    advantage of being hard to compress. Capacitance increases as the
    plates get closer together - use a foam pad and it should be possible
    to make it only trigger when the foam is compressed.

    If it has to last a long time and be relatively bullet proof a
    capacitive switch with a foam dielectric pad and perhaps a harder thin
    plastic like vinyl on top of that for water proofing and wear
    resistance, would be the best idea.

    If you are just interested in trying something out and it isn't for a
    commercial application the touch sensor is simple cheap and easy.

    Them's the choices . . .

    Or a photo electric light curtain close to the surface.

    Or retro reflective proximity switches.

    Or how about just include a photo sensor in the mat - your foot blocks
    all the ambient light and the thing triggers? Relatively simple to
    implement for a beginner. The sensitivity could be high so that you'd
    really have to occlude all ambient light to get it to trigger. It
    could be used with hard or resilient surfaces. Easy to incorporate
    sensitivity. Down side might be dirt changing the sensitivity over
    time.

    Or an air table type surface with a pressure switch. You block the
    flow from a small air pump (aquarium type pump?) and the pressure
    rises and switches a switch.

    Mull it over for a time; there's probably lots of ways to do what you
    want.
     
  3. default

    default Guest

    On Wed, 31 Jan 2007 03:01:57 +1300, Nicholas Sherlock

    Here's another idea. Use open-cell foam under a hermetically sealed
    surface like vinyl sheet. One steps on the vinyl compressing the foam
    (or other spring) driving the air out of it - the air is in a sealed
    chamber and travels to a pressure switch or sensor via a plastic hose.

    Simple, should be reliable, liquid and dirt proof, cheap, easy to
    understand.

    The whole surface could be vinyl with a half inch of plywood under it.
    Circular cut outs in the plywood form air chambers. Voids in the
    plywood edges sealed with epoxy. Routed channels for the air hoses
    connecting the chambers to switches. Vinyl glued to the bottom
    surface to seal it.

    I like it . . .
     
  4. default

    default Guest

    On Wed, 31 Jan 2007 03:01:57 +1300, Nicholas Sherlock

    Or piezo electric buzzer disks epoxied to a 1/4" steel plate to keep
    them from flexing and breaking. Cover the disks and wires with tape
    and then a vinyl sheet. High output pulse when they are compressed -
    would need some amplification but still really simple to do.

    Cheap, bullet proof, easy to build.

    Lots of ways to do this . . . most of them better than a contact touch
    switch.
     
  5. default

    default Guest

    On Wed, 31 Jan 2007 03:01:57 +1300, Nicholas Sherlock

    Going with the piezo idea I searched on electronic drum pads and found
    this http://www.electronicpeasant.com/projects/ssdrums/ssdrums.html

    Scroll down and look at the drawings of the pads themselves. Another
    nice thing about the idea is that it would probably work with the
    mosfet touch switch as the amplifier circuit for the piezo disks.
    You'd have to observe polarity to get it to trigger when pressed and
    not released but that's easy enough. So, if you build the contact
    circuit and don't like the way it works, it would be just a matter of
    connecting some piezo discs to the inputs.

    Cheap piezo discs can usually be had from all electronics
    http://www.allelectronics.com/cgi-bin/item/PE-53/search/15MM_PIEZO_DISC_.html

    Just the first that turned up in Google there are bound to be others
    (this one .75 each or $6.50/10)

    http://www.west.asu.edu/rlerman/PDF Files/Audio Schematics2.pdf

    Some basic ideas on how to solder to them etc.. They also show some
    audio pre amps - for on/off switches you don't need anything that
    complicated (one transistor or op amp would be plenty)
     
  6. John

    John Guest


    How good are your wood-working skills?

    You could build a low platform with 20x20 pieces of 3/8" plywood
    mounted on springs and have a microswitch under each block of wood.

    The wood would be supported at each corner with a spring that has a
    bolt through it to hold it at the maximum desired height - this also
    allows leveling the wood pieces to match.

    The bolts would be attached to the wood block and be a slide fit in
    the platform base.

    Stepping on the wood would press it down against the resistance of the
    springs, operating the switch underneath.

    John
     
  7. Thanks for all of your suggestions. I've dreamt up most of the ideas you
    suggested already :) (It's a fun challenge, thinking about this).
    Capacitive sensors sound great, I might build the resistive sensor first
    (It also has a very simple physical layout) then try out some of your
    ideas for capacitive sensors.

    Cheers,
    Nicholas Sherlock
     
  8. Slim to none, unfortunately, otherwise a simple switch would be a great
    option :). Thanks anyway.

    Cheers,
    Nicholas Sherlock
     
  9. Ross Herbert

    Ross Herbert Guest

    Have you seen this web site?
    http://www.angelfire.com/d20/ddrhomepad/

    and if you can battle your way through Spanish (or the Babelfish
    english translation) the Dancepad here might give you some clues
    http://duelodebailes.blogspot.com/2006/07/proyectos.html
     
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