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Homebrew membrane keypad

Discussion in 'Electronic Design' started by Dmitri, Dec 17, 2004.

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  1. Dmitri

    Dmitri Guest

    Hello everyone!

    It looks like homebrew PCBs are a pretty well developed branch of the
    electronics hobby these days, even I have (more or less successfully) made
    them. How about membrane keyboards?
    I'm looking for some info on whether or not anyone has attempted to make
    membrane keyboards in a garage workshop environment. What materials may be
    required? Anything that can be available to a non-pro?
    I have a project that requires eight buttons under an LCD, and a membrane
    keyboard would work extremely nice, especially if I can conceal the gutts
    of the scheme beneath it. it would go on top of a plastic project box.

    Any link, suggestion, info anyone?

    Dmitri Abaimov, RCDD
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  2. You can make a graphic overlay and put tact switches behind it. Many
    high volume consumer appliances are made that way, while others have
    custom-molded operators that work tact switches.

    Best regards,
    Spehro Pefhany
  3. Membrane keypads have silver or carbon tracks on a sheet of film - the
    tracks expand to form switch contacts and form a film ribbon cable for the
    external connection. A switch consists of a metal dome which gives way
    suddenly when pressed, giving tactile feedback and completing the contact.
    The domes are put in place and the whole thing stuck together - as 3 layers
    of plastic. The process is critical - we got a lot of bad switches due to
    silver migration, ribbon carbon tracks cracking, contact trouble, bad domes.

    From my experience with membrane switches I will try to avoid them, even for
    commercial products. The simple, quality alternative is a label with TACT
    switches. Drill say 12 mm holes in the panel at each switch position, and
    stick the label on top. The PCB with TACT switches sits underneath, with the
    switch tops just touching the label. I know someone who produced broadcast
    consoles that way, with a big sheet of lexan over the entire area of the
    desk. You can get bigger TACT switches too, for a better feel. Has
    advantage that you can pull PCB out of case and switches are still on the
    PCB. If you find your switches are not close enough to label, put shiny
    paper stickers over switch buttons and stick blobs of silastic on label,
    just above switches. Put it all together for 12 hours and when you take it
    apart, the blobs will stay on the label. Commercially printed Lexan is
    expensive, so the hobbiest bit might be to find a nice tough label you can
    print with a laser printer. Might want to stick a sheet of clear over the
    laser printing ?

  4. Usenet rule #12a: The answer to 'has anyone else...?' is 'yes'.
    However, that does not mean it is a good thing to do. A DIY
    membrane keyboard sounds like a lot of time and money for
    a really bad keyboard.
    Use a set of 8 real switches:

    digikey etc.
  5. I read in that Roger Lascelles
    Print in reverse on the lower side ('second-surface printing'). OHP film
  6. Only a very few inkjet (and maybe no laser) printers will print opaque
    white. The colors are also quite transparent. I use screening methods,
    but it involves some investment for materials and equipment, costs
    money for each design (for screen and die(s)), and is very

    Here's an example:

    Best regards,
    Spehro Pefhany
  7. I've had good results for prototypes printing on the reverse of OHP film using
    3 colour plus black ink jet then spraying the remainder white. That way you can
    have any colour you want. It's then glued using aerosol glue.

  8. I have a client who does that except with yellow spray on the back,
    and punched on a little press. Pretty labor-intensive, but the results
    look very professional unless you look very closely. He uses a special
    professional film available only in huge rolls or sheets.

    With screen techniques, I can pick any thickness or surface finish
    (pebble or hardcoat or plain) of polycarbonate or polyester film I can
    get ahold of, print transparent colors or opaque and so on. I use a
    vinyl type ink compatible with both polycarbonate and polyester. UV
    curing would be a somewhat more pleasant process to work with, but the
    cost of the inks and equipment would be much higher.

    Best regards,
    Spehro Pefhany
  9. I read in that Spehro Pefhany <[email protected]
    Naturally; for OHP they wouldn't work if opaque!
    I print only the legends and use a coloured paper or plastic sheet for
    the background colour.
  10. I read in that ChrisGibboGibson
    I've done that, too, but of course it doesn't work with inkjet printing
    because the spray solvent re-dissolves the ink. For black legends only,
    a photocopy works.
  11. Ken Smith

    Ken Smith Guest

    I knew someone who did it. I won't say it is a good idea but here's what
    he did:

    There was a PCB layout where each button was a interdigitated pattern like

    ------------- The trace width and spacing was the
    ! -------------! finest he could do with the tape on
    !------------ ! copper clad etching method
    ! -------------!

    ... etc ..

    I think he had one of those "plating pens" that hook up to a battery. I
    don't know for a fact how he plated the pattern but it was shiny when I
    saw it. Both sides of each switch pattern was connected to traces that
    went out past the edge of the keyboard area.

    Over this PCB he placed a sheet of Mylar with about 1CM diameter holes
    where the buttons were. He used the 3M "transfer film" to stick the Mylar
    in place.

    Over this he stuck the metalized plastic film that was used in
    electro-static speakers. I think it was nickle on Mylar.

    At one time 3M made this film that was intended for front panels etc. It
    used a photographic process. You had to tape up the pattern you wanted on
    some Mylar and run the sheets through a blue print machine. This was then
    wided down with developer and then dryed and sprayed with mat finish

    He reported that the resulting keypad worked well. I think however that
    he spent more time on making it than it really was worth.
  12. Ken Smith

    Ken Smith Guest

    I'll suggest the 3M transfer stuff or a sheet of sticky backed Mylar to
    stick to it without disolving the ink.
  13. Maybe if you Krylon the ink before spraying the background, or
    experiment with different paints. The inkjet stuff seems to be
    water-based and the spray paints generally have petroleum type
    solvents. Oil and water.
    I've seen ads from a company in Quebec who will prototype overlays for
    about $400 US (before the dollar plummeted, so maybe $500 now). This
    is still a ways above proto prices for PCBs.

    Best regards,
    Spehro Pefhany
  14. Cellulose spray destroys the actual substrate, water based (quite common now
    for cars, especially 2 pack) destroys the ink, 1 pack polyeurethane works fine
    with normal ink jet cartridges.

  15. Guy Macon

    Guy Macon Guest

    So is Rich, but we still like him... :)
  16. john jardine

    john jardine Guest

    I've used the OHP film. Reverse printed Black, sprayed with art-mount glue
    and some metallic foil stuck on to show thru the clear bits. (that thin,
    red/blue/gold/silver etc, mirror finish stuff for wrapping Christmas
  17. Rich Grise

    Rich Grise Guest

    Have you ever taken apart a PC keyboard? If you can connect to the traces,
    (probably with some kind of clip - I haven't tried to solder to one), you
    could just use the plastic membrane switch thing from one of them. Or make
    something in the same style.

    Good Luck!
  18. Dmitri

    Dmitri Guest

    Spehro Pefhany wrote:

    Man, the panel looks awesome! I understand that it is on a flat surface,
    so it is not exactly the same as the keyboard-type setup I'm after, but it
    is certainly the way to go. Would you mind giving more details about the
    equipment for the screening?

    Dmitri Abaimov, RCDD
    Cabling Forum, color codes, pinouts and other useful resources for
    premises cabling users and pros
    Residential Cabling Guide

    Article posted with Newsgroup Archive
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  19. Dmitri

    Dmitri Guest

    Thanks for the info, Ken!

    I was thinking along the same lines, glad to see that some brave soul has
    already gone through it ;-).

    There are couple critical points here as far as I can tell. Well, as
    usual, you find more and more critical points as you go deep into it, but
    these two seem obvious for me now:

    1. The pattern needs to be silver plated to prevent oxidizing, and I'm not
    sure about details of the process.
    2. The conductive film that would go on top of the Mylar sheet is a
    mystery for me, have no idea where to get it. On the other hand, it only
    needs a small patch of conductive material to overlap couple of those
    traces, so maybe a small round piece of (silver plated?) copper peeled off
    a PCB will do.
    The entire thing about this hobby is to ejoy time doing it, so it does not
    really matter how long it takes, as long as the result is achievable.

    I am, however, leaning toward putting a tactile switches behind a legend,
    printed on transparency as suggested by other posters here. It seems like
    the actual electrical contact, which is a heart of the keyboard is going
    to be unreliable, which would defeat the whole purpose of making it. I
    just have to figure out the white background for spaces, not printed by my
    color laser printer.

    Dmitri Abaimov, RCDD
    Cabling Forum, color codes, pinouts and other useful resources for
    premises cabling users and pros
    Residential Cabling Guide

    Article posted with Newsgroup Archiv
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  20. Dmitri

    Dmitri Guest

    I wanted to thank everyone responded to my message. I now realize that the
    problem is a bit more complex than I originally thought, although I may
    attempt to make a model in the future. For now on, I would rather take
    Spehro's and Roger's suggestion and put a tact switches behind a printed
    legend. I think, in my case, it will have to be two layers - one laser
    printed transparency (with holes punched above the switches) to cover the
    LCD and another printed on a sticky 1" tape by a Brother P-Touch graphic
    label printer I have. P-Touch does not have the problem of transparent
    spots where white was intended as it has its own background color, and you
    get to choose one. Besides, it has a very uniform and sticky adhesive
    later on the bottom, and is very widely available (though not too cheap ;-)

    While researching for the membrane keyboards, found this nice resource
    with lots of illustrations and other useful info:
    Check it out, my be useful to somebody who's into homebrew keyboards.

    Dmitri Abaimov, RCDD
    Cabling Forum, color codes, pinouts and other useful resources for
    premises cabling users and pros
    Residential Cabling Guide

    Article posted with Newsgroup Archive
    no-spam read and post WWW interface to your favorite newsgroup - - 5798 messages and counting!
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