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EEVblog #1282 - Design Your Own Membrane Keypad

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EEVblog:
How to design and your own custom membrane keypad and get it manufactured, to make your products look really professional.
It's easier and cheaper than you think!

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VEGETA:
Thanks for the awesome video as always. I've been following uSupply project ever since the first video, I wish we could see schematic soon.

I was wondering about the final estimated price for this product. I guess it could be something like 150 US dollars right? without shipping of course.

The PCB suggests that you ditched the LT3080 design? going for the transistor\mosfet solution? I am really excited to see the final details!

johnlsenchak:

Dave

Don't  you worry  about  fingerprint wear  on the front  of the keypad   with the numbers and letters  eventually  wearing  off . You didn't mention  in the video
the longevity  of the front  of the  keypad  due to  continuous  use.  If you are  selling a unit  with that keypad   wouldn't  that be   a factor ?

Clear as mud:
I'm going to summarize the video here, so I don't have to watch the whole thing if I'm looking for something in it later.

Membrane keypads - couple of hundred dollars for a few prototypes, or around $1 or $2 each in bulk.
Optional (more expensive) get a large one to cover entire product, with clear windows over your LCD screens (1:20).  More colors add more cost (5:18).

They consist of a custom flex PCB (2:20); glued to a vinyl decal (3:28) made of either polycarbonate or polyethylene/polyester; with adhesive tape on the back (1:35).  A few types of 3M adhesive (including the 467 type seen earlier) are briefly shown at (21:41).  The keypads can be tactile or non-tactile type (5:37).  The buttons can be embossed, or slightly raised above the surrounding keypad material, for a better feel (9:09).

Tactile keypads use tactile metal domes similar to those sold by Snaptron (5:54), usually embedded in the top decal layer of the keypad.  Several models are shown (the third one is similar to the kind used in the uSupply keypad featured in the video).  A datasheet for another manufacturer shows you can solder them to a PCB, instead of integrating in the decal (7:38).  An example of a product with the tactile domes on the PCB is shown starting at (9:48).  Another method is to buy them in a stick-on array (7:53).  See (10:47) for an in-depth explanation of how tactile domes work.

A non-tactile keypad is shown starting at (13:02).  They will have a longer lifetime because there is no metal fatigue issue.  Reliability is much better, but there's no tactile feedback, so it's better if the product makes some sort of key press feedback noise.  The one shown has two conducting printed circuit layers separated by a small space, but it is also possible to have just one circuit layer, with some conductive material on the back of the keys (like in a pocket-calculator keypad) (14:50).

(15:45) DESIGN PROCESS
(17:41) Get your connector specifications from the datasheet of whatever flex connector you're using.
(19:00) You can get carbon-printed resistors on flex PCB (if you need resistors).
(20:16) Example of a company website selling keypads, with a shopping-cart system for a quote.  Dave goes through all the options.

HwAoRrDk:
I can't imagine that metal fatigue of the tactile domes is the limiting factor on lifetime of membrane keypads. Based on the number of times I have seen membrane keypads with the decal wearing out (or even through, in some instances!), I would expect the robustness of the vinyl is the primary factor.

By the way, the mentions of activation force brings to mind one thing that has always been an issue of uncertainty for me: how do you choose? This is something with common tactile switches too, that there are a hundred and one choices of activation force, so what do you pick? Do you go by trial-and-error: get a bunch of samples, and pick what you think feels good? Are there some kind of standards?

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