Electronics Forum / Design / July 2008

Would these products do?

http://www.google.co.uk/search?hl=en&q=rubber+keypad+%22repair+kit+%22

Chris

Ok, did a quick search. This was the thread, didn't remember I started it:
http://groups.google.com/group/sci.electronics.design/browse_thread/thread/d0616
a0e5b248835/1de4d4178f482611?hl=en&lnk=st&q=rubber+keys+%22analogconsultants%22+
group%3Asci.electronics.design#1de4d4178f482611

And this was the link to the mfg of the generic keypad that can be cut
up but the link is dead:
http://www.ittcannon.com/media/pdf/catalogs/e22.23.pdf

Just contact ITT cannon and ask, maybe they still make them.

I like the anti-static bag idea.  Soot mixed with silicone is another
(idea).

Cheers,
James Arthur

Yep.  It dries hard and brittle.

I'm trying to recall how we did it, but we shipped a few prototypes
using Aquadag on the rubber keyboards.  I wasn't involved except to
find a bottle of the stuff.  I vaguely recall that the rubber buttons
were either scored with an exacto knife, or molded with a corregated
instead of flat end.  The Aquadag filled in the gaps and was probably
wiped clean to prevent flaking.  There may also have been tinkering
with the formulation, such as mixing it with rubber cement.  Sorry
about the brain fade, but that was 30+ years ago.

There's also Graphit 33 from CRC Industries.
<http://www.crcind.com/csp/web/ProdDisp.csp?lng=3&country=ALL&product=GRAPHIT%203
3&business=ELECTRONIC>
<http://www.crcind.com/wwwcrc/tds/TKC3%20GRAPHIT33.PDF>
which might be more suitable.  It's also graphite but with an organic
glue binder.  The data sheet claims its suitable for repairing
keyboards.

Also see "conductive ink".
<http://www.atotech.com/carbon-conductive-ink-paste.html>

Incidentally, there's quite a bit of work apparently being done with
graphite ink formulations for making cheap printable circuits, where
the components are formed from various inks and printed using
something like an inkjet printer.  

Good idea.  A quick check with Google shows that anti-static bags come
in various resistivities.  Some is a low 100 ohms per square, which
should work just fine.  Others are many thousands of ohms per square,
which won't work.  I just tested a black bag I happen to have around.
Two parallel lines of aluminum tape forming a square.  I got about
300K per square on the outside of the bag.  About 400 ohms per square
on the inside.  Might work, but seems a bit high.

There's a point where salvaging an old TV remote control, calculator
keypad, or PC keybaord, might be easier.

If all else fails, just build a keyboard out of layers, or "membrane
switch".  Start with a PCB with the contacts etched onto one side.  On
top of that, goes a sheet of mylar with a circular hole for each
contact as a spacer.  On top of that, goes a sheet of brass or any
manner of metal that will bend, but not stretch (forget about using
aluminum).  If contact isolation is required, as in a matrix keyboard,
use brass disks glued to the mylar spacer sheet.  Push on a disk, and
it bends slightly to contact the PCB.  Nothing conductive on the
rubber parts and no springs anywhere in sight.  A dome in the mylar
sheet adds tactile feedback of sorts and makes it more reliable, but
it will work without it.  That's the way some calculator, alarm, and
"industrial" keyboards are made.