activate touch screen without 'ground'

Discussion in 'The Projects Forum' started by Genoil, Sep 19, 2011.

  1. Genoil

    Thread Starter New Member

    Aug 5, 2011
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    Hi,

    I'm a step further in my project, please briefly watch the video to see what it is about:

    http://www.youtube.com/watch?v=z4eTjH4zjwE

    The circuit is a 555 astable that controls an analog switch. It switches a connection running from the screen (under the foamboard) to the iphone's dock connector. The latter may very well be replaced with a connection to the metal sides of the phone or the headphone jack, or if you press the wire hard enough, your hand will do fine. The application on the phone simply registers touches and colors the screen green when the switch is closed.

    The ultimate goal here is to get rid of this required 'ground' connection, so that I can freely place these little circuits on the surface of an iphone or ipad. Theoretically it should be possible, by holding the wire while balancing myself on top of the circuit...

    Do you have any suggestions on what I could try, or does it simply not comply with the laws of nature. I'm looking for something small that replaces body capacitance. I'v found somewhere that the human body can be represented by a 20pF cap in series with an 1.5KΩ resistor, but I wouldn't know how to wire that in this case...
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    You've stumbled onto one of MY big questions with what I believe to be generically know as "projective capacitance sensors," that being, where is the external cap going? I've just gone thru all of Microchip app notes on this (AN1101 thru 1104) and they all seem to show a nice big human finger as a capacitor to ground. Now we both know you need not ground yourself to make these things work, while you do need an electrical path. (A insulated pen will not cause a "touch" but some conductive foam will when you hold either.)

    I wonder if your circuit would work if you say connect it to the kitchen sink (a very nice hard ground)?

    Other then that I don't know how you could change the capacitance of your bottom plate relative to the (floating obviously) ground internal to the i-thing.

    But I did bump your post :D
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    IIRC in physics i have been taught something along the lines that an insulated object has its own capacintance, irrespectively of any surrounding objects. I think this was related to electrostatic charge.

    This makes me think that the sensor should be able to measure/detect your capacitance even when you´re floating in vacuum.

    Maybe we can make the thread going :D

    ETA: see http://en.wikipedia.org/wiki/Capacitance#Self-capacitance
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    What about a balanced sensor using pairs of closely spaced electrodes, working in opposite polarities?
     
  5. ErnieM

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    Apr 24, 2011
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    Here's how I was thinking of this: forget body capacitance, or stray cap, or anything possibly undefined. Take a real physical 500 pF cap, and connect one end of it to a circuit. Can the circuit sense that? Everything I know says NOT AT ALL.

    Yet I can seem to change an iPad just by a single finger touch.

    Tonight I was trying to finger swipe my wife's iPad while I literally jumped up and down to isolate myself from everything else. I stopped before I broke something (like my leg).

    I do notice that my finger needs to actually touch the iPad screen, I can't hover over it even slightly and have an effect. A few sheets of paper can be overlaid, but does reach a limit.
     
  6. Genoil

    Thread Starter New Member

    Aug 5, 2011
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    Thanks for getting the thread going!

    My circuit does work when I ground it to myself, but it works much better when I ground it to the phone's dock connector, but that may just be because the connection is more stable.

    I've been thinking of the balanced sensor approach, but I wouldn't know where to start designing/prototyping this.

    I have tried connecting it to different value caps on one end (not exactly 500pF though, 10nF was the smallest I had laying around), but that didn't have any effect.
     
  7. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Are we, maybe, talking about proximity sensors?
     
  8. Dimitris76

    Member

    Jul 17, 2011
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    I believe I read somewhere that what your finger is doing when touching (or hovering) a capacitive screen is, disturbing and distorting the electric field (and thus the altering the capacitance) of thousand "open" capacitors.

    So you don't have to be electrically grounded for the touch screen to work, but you do need to use an electromagnetically "opaque" object like metal, metal filled plastic, hemoglobin (iron) and water filled finger, etc.

    And that's why dielectrics don't work...

    My 2 cents,
    Dimi
     
  9. Genoil

    Thread Starter New Member

    Aug 5, 2011
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    Just did a small experiment: I have this iphone to usb cable on my desk with the plastic cover of the dock removed, exposing the ground plane of the cable. The top of my (wooden) desk is covered with felt (insulator), but my keyboard and screen are resting on this massive metal plate (conductor) that is clamped around the desk.

    When I place the phone on the metal plate and place the dock connector on the screen, it registers a touch without the other end of the cable touching anything. (I did hold the plastic of the cable though, but I've assumed that insulates well enough)

    When I place the phone on the felt cover and place the dock connector on the screen again, it won't register a touch at all. When I connect the USB end of the cable to the metal rim of the phone, it will also register a touch.

    When I repeat both actions just using my finger, it both works.

    When I place the phone on either the metal plate or felt and put my little circuit on the screen, it will only register touches when the wire is grounded to the device.

    So there's two variables that determin if it works:
    1. the grounding of the phone
    2. the "x" of the object that is used to make the touch.

    I don't know what "x" exactly is (capacitance, mass?), but there where 3 values for it:

    1. My body, in which "x" was big enough to zero-out the influence of grounding of the phone
    2. The iphone cable, where "x" was large enough to work on a well grounded phone but not on an isolated phone
    3. My experiments' touch circuit, which is only a wire connected to a small metal plate on the bottom of the circuit facing the screen, running through the switch ic and back to the phone's dock connector.
     
  10. ErnieM

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    Apr 24, 2011
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    My starting point for investigating capacitance touch was from those app notes I mentioned. What they have in common is the detection circuit sees a real change in capacitance. In one case, the cap forms part of a RC oscillator, and the C of your finger changes the C of the oscillator and hence a detectable change in frequency. Now for that to work the "finger cap" has to be part of the circuit, as current is going in and out of it every cycle of the oscillator.

    Or is it? I'm beginning to wonder if the whole touch screen finger capacitance is due to more of a "static electricity" phenomenon where Kirchhoff's current law does not apply. By this I mean the current from the a fore mentioned oscillator enters your body but does not leave it till the next half cycle. We've all seen static sparks that have to be several orders of magnitude higher then the current the touch screen is inducing into your body, so surely the human body could absorb and radiate the bit of change the circuits need.

    I have an idea how to investigate this, but it requires a human body to cut apart to see how small a segment is still detectable. Any volunteers?
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I volunteer, you should find a limb in the morning post ;)
     
  12. Genoil

    Thread Starter New Member

    Aug 5, 2011
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    [​IMG]

    I suggest you buy a huge sausage and slice off bit by bit :)
     
  13. strantor

    AAC Fanatic!

    Oct 3, 2010
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    forget ground. ground has nothing to do with it.
    did you read this post?
    I was mystified by capacitive proximity sensors before and This document helped me understand it. I searched for a long time to find it again. Granted, this is a document about industrial proximity switches and you're talking about ipad screens, but the same forces are at work

     
  14. Genoil

    Thread Starter New Member

    Aug 5, 2011
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    Thanks for this. I will not refer to "ground" any longer, rather "high-dielectric", does that sound ok?

    The problem with my circuit is that it's not just a high-dielectric approaching the sensor, it's a conductive plate already close to the sensor, wired through a switch IC so I can turn it on an off. Why is it that the best object to connect the other end of the wire to is the phone itself? It's not like I can place a high-dielectric (like a cup of water or a sausage) at a distance from the sensor and just sink the wire running from the switch IC in it.
     
  15. ErnieM

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    Apr 24, 2011
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    Update: received kubrick's arm this morning (thanks for the UPS AM delivery pal!) I tested the appendage against a standard iPad device using a wooden stick as a holding device. This stick was tested and it was not detectable when in intimate contact with the screen.

    Here are my results:
    Code ( (Unknown Language)):
    1.  
    2. Entire arm:             Detected
    3. Up to elbow:            Detected
    4. Hand only:              Detected
    5. half hand:              Detected
    6. finger to knuckle:      Detected
    7. finger to 2nd joint:    Detected
    8. finger to 1st joint:    NOT Detected
    So it seems just a finger would be required.

    (aside to kubrick: as the arm was not iced I don't think it is still viable for reattachment, but I will return the segments, expect them in the morning.)
     
    Last edited: Sep 22, 2011
  16. ErnieM

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    Apr 24, 2011
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    Now on a moar serious note...

    The Siemans document is much more informative then Microchip's app notes. Microchip gave a model of:

    [​IMG]

    whereas Siemans gives:

    [​IMG]

    Additionally, per MC's AN1102 only one tab of the capacitor is indicated as being relevant, which would be true if the touching device was connected back to measurement circuit ground.

    Siemans states the cap is "formed by two concentrically shaped metal electrodes of an unwound capacitor. When an object nears the sensing surface it enters the electrostatic field of the electrodes and changes the capacitance in an oscillator circuit."

    That makes lots more sense! Since capacitance may be calculated by the well known formula (OK, I looked it up):

     C = \frac{eo *er* A}{d}

    Where
    eo = permittivity of free space
    er = relative dielectric constant
    A = cross sectional area
    d = distance between plates

    Now that formula should have the caveat "(the) fringing field around the periphery provides a small contribution" while for us the fringing field is all we have (as A is essentially zero) but we can take it's general form to finagle an answer.

    For air, er is 1 (see Siemens sh 55), while if we approximate a finger by water we get an er or 80, or the fringe field will change by a factor of 80.

    Siemans specs sensors with up to an 20 mm (3/4") sensing range for certain materials, so there exists a way to sense something "out there," and if the separation medium has a low er while the object of interest has a high er one can detect objects behind a wall.

    Now the bad news for Genoil: the detection is dependent on a change in the dialectric properties of an object within the fringing field. Everything goggle turned up for me on that subject was not applicable.
     
  17. Genoil

    Thread Starter New Member

    Aug 5, 2011
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    Could one build a circuit to simulate such an object with dielectric properties? Essentially, a wire running from the screen to the side of the phone is, but then..something else that I can stick on my "device". Guess not :(
     
  18. ErnieM

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    Apr 24, 2011
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    Never say never. ;)

    Sorry but my muse is off on a similar project right now and is clogged with irrlevant data to your needs. Somewhere I do have a brain cell firing that what you want to do should be possible. I mean, worst case is you hang a lump of a suitable high dielectric material on a servo so you can bang it onto the screen. (Don't use sausage, it gets messy and sooner or later it will smell bad.)

    But I wonder if you took two separate pieces of metal on the screen and switch then together that would make the touch sensor twig.
     
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