Rubber keypad pcb interface design

Thread Starter

johnny411

Joined Mar 3, 2020
40
I'm currently designing a project to fit inside a case. It has a rubber keypad with it, that has a mechanism that I have not seen before in my limited knowledge of rubber keypads.

It does not appear to be designed for either tactile domes or for carbon based ones, as it has no center peg, but instead 4 outer ones.

The keypad has a small amount of movement before the pegs bottom out, requiring lots of force to move forward.

My question is how should I interface this to my keypad pcb? Is it actually a tactile dome, or is it expecting "physical" switches, or maybe something else? Thanks.

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MisterBill2

Joined Jan 23, 2018
8,793
To find an answer you need to know what the maximum resistance is that will register as a contact closure, and you also need to know what the maximum contact resistance is for the keyboard buttons.
 

Thread Starter

johnny411

Joined Mar 3, 2020
40
Yes that is my question @MrChips , what kind of tactile button contacts do I need? (snaptron style or something else) The four posts instead of one center one for each button is throwing me off. Thanks.
 
It doesn't "look" like a conductive elastomer keypad. Measure the resistance of what you think is the pad and go from there.

The elastomer keypads generally use carbon for low cost or hard gold for a much higher cost key.

It could possibly be a tactile switch cover.

The little "pins" would effectively make the "keys" harder to push. You would be looking for a "tact" switch that would fit withing those pillars so to speak with a height lower than the pillars.

e.g.
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Thread Starter

johnny411

Joined Mar 3, 2020
40
@KeepItSimpleStupid

sorry i did'nt mention this, but i already tried to measure resistance with no luck, so i don't believe its carbon (or something else conductive). i will try again though just to double check.

tactile as in "snaptron" style or physical switch? thanks.
 

Granville

Joined Feb 18, 2021
1
Honestly from my experience, you dont get a lot of nice pcb mounted switches.. the best switches are generally panel mount. Have you considered membrane switches? Checks a lot of the needs you listed other than being pcb mounted


IndigoCard
 

MisterBill2

Joined Jan 23, 2018
8,793
In both posts #5 and #7 those "little pins" are holding the top part of the button assembly on, any other function is incidental.The buttons have 4 legs to keep them stable, usually two legs and a single switch, 2 legs common to each side of the switch. Keyboard switches are different because they need much longer lifetimes.
 

Thread Starter

johnny411

Joined Mar 3, 2020
40
a big thanks to everyone who helped me figure out a solution to my problem, really appreciate it.

I decided to go with KSC321GLFS switches as the height of the switches, 3.5mm, was the height from the bottom of the keypad to the underside of the top. I'll likely order some that are slightly shorter and taller to figure out what exactly fits the best.

here is a picture of the PCB I designed (would be refined further), and for anyone interested attached is the schematic.
keypad photo.PNG
 

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MisterBill2

Joined Jan 23, 2018
8,793
Depending on the soldering and mounting process accuracy, there may be a production yield problem because of the close spacing of the connections. But certainly it will work
 
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