Homebrew membrane keypad

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?

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Dmitri Abaimov, RCDD
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Dmitri(Cabling-Design.com
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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

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Spehro Pefhany

"Dmitri(Cabling-Design.com)" wrote

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:

formatting link

digikey etc.

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Nicholas O. Lindan, Cleveland, Ohio
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Nicholas O. Lindan

I read in sci.electronics.design that Roger Lascelles wrote (in ) about 'Homebrew membrane keypad', on Sat, 18 Dec 2004:

Print in reverse on the lower side ('second-surface printing'). OHP film works.

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John Woodgate

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 messy/smelly.

Here's an example:

formatting link

Best regards, Spehro Pefhany

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Spehro Pefhany

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

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Spehro Pefhany

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 ?

Roger

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Roger Lascelles

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

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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.

Gibbo

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ChrisGibboGibson

I read in sci.electronics.design that Spehro Pefhany wrote (in ) about 'Homebrew membrane keypad', on Sat, 18 Dec 2004:

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.

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I read in sci.electronics.design that ChrisGibboGibson wrote (in ) about 'Homebrew membrane keypad', on Sat, 18 Dec 2004:

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.

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John Woodgate

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 this:

------------- 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 Krylon.

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

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Ken Smith

I'll suggest the 3M transfer stuff or a sheet of sticky backed Mylar to stick to it without disolving the ink.

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Ken Smith

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.

Gibbo

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ChrisGibboGibson

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

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Guy Macon

using

you can

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 presents. regards john

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john jardine

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?

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Dmitri Abaimov, RCDD
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Dmitri(Cabling-Design.com

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.

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Dmitri Abaimov, RCDD
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Dmitri(Cabling-Design.com

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:

formatting link
Check it out, my be useful to somebody who's into homebrew keyboards.

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Dmitri Abaimov, RCDD
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Dmitri(Cabling-Design.com

Why stop at silver? Gold doesn't oxidize. Nickel also is quite good in that regard.

Try google on things like "metal plated plastic". They deposit nickel films onto plastic to make it look like chrome.

You could even try an "anti-static" bag. The kind that look silvery, have a layer of metal on them a few atoms thick.

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Ken Smith

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