Carbon pills for keypad

That's probably one of those things that you can get just fine if you're in China and making a bajillion pieces.

Your best bet may be to find a product that uses them and scavenge them.

For something that doesn't need a long lifetime I have heard reports of folks cutting what they need out of black conductive foam, but it does break down over time.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Would these products do?

Chris

GC Copper paint?

Cheers

Someone here posted a source a long time ago where you could buy generic keys. Unless you need a gazillion the other best bet might be to trudge over to Walmart or the Dollar store and see whether there is a gadget that has as many rubber keys as possible at the lowest cost.

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Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

P.S. I got 450K for carbon rubber.

"

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Try these guys:

http://www.chomerics.com/

and, in particular:

http://www.chomerics.com/products/emi_family.htm

JF

If you're making your own, it's easier to paint the non-conductive rubber with something conductive. You're on the right track, but it's best to use a commercial product that to try and mix your own. Methinks that graphite paint (Aquadag) will do the trick. The stuff is about 100 ohms per square, which should be more than enough conductivity to use as a keyboard. If it doesn't, there's something wrong with your keyboard scanner and/or detector circuits.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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DAG is way too fragile for that--it'll crack off and gum up everything with nasty soft conductive particles.

Making conductive rubber takes a *lot* of carbon--you need a continuous path of carbon particles touching each other from end to end. Think 4 parts carbon, 1 part rubber.

You can use a little bit cut from an antistatic bag, or you might be able to heat and squash Velostat foam into something usable.

Cheers,

Phil Hobbs

DAG is way too fragile for that--it'll crack off and gum up everything with nasty soft conductive particles.

Making conductive rubber takes a *lot* of carbon--you need a continuous path of carbon particles touching each other from end to end. Think 4 parts carbon, 1 part rubber.

You can use a little bit cut from an antistatic bag, or you might be able to heat and squash Velostat foam into something usable.

Cheers,

Phil Hobbs

A set of 100 conductive rubber buttons + glue:

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

Cheers, James Arthur

Ok, did a quick search. This was the thread, didn't remember I started it:

And this was the link to the mfg of the generic keypad that can be cut up but the link is dead:

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

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

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.

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".

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.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558