adhesive copper or carbon contact pad prints? - Page 1
Forum section: electronics / beginners

author: Chris_D
author_rank: Regular Contributor
timestamp: October 21, 2021, 06:22:41 pm
content: Hi,I was wondering:Beneath the rubber keys with a conductive carbon 'pill' in it, such as in a remote control, you find these typical meandering, labyrinthine fingerprint-like contact pads on the pcb.Does anyone know if they can be found as a kind of adhesive prefabricated stickers perhaps?Carbon or copper, it doesn't matter?Thanks.

author: mdubinko
author_rank: Contributor
timestamp: October 21, 2021, 09:51:54 pm
content: There are pre-made traces you can get like this:(especially the part after 3:30)It may be tedious to exactly recreate the shape, but depending on how thorough you want to get, all you really need is one or two good traces from each side to run across the button contact area.The carbon contacts on these switches aren't super critical about resistance, so many of the conductive pens based on silver, nickel, or even carbon would probably also work, just drawing the new traces.-m

author: Chris_D
author_rank: Regular Contributor
timestamp: November 01, 2021, 10:50:31 am
content: Quote from: mdubinko on October 21, 2021, 09:51:54 pmOk, thanks for the tip.

author: Renate
author_rank: Super Contributor
timestamp: November 02, 2021, 01:01:28 pm
content: I've done PCB layouts for this. A good quality gold plate works fine. I was using the Adafruit 10mm square pushbuttons which have an O contact which uses a different landing than your serpentine.https://www.adafruit.com/product/1611(buttons)https://www.adafruit.com/trellis(PCB layout)

author: Chris_D
author_rank: Regular Contributor
timestamp: November 03, 2021, 09:46:07 am
content: Yes, but the trouble is that I want to replace a defunct spring contact in an old device, and there is already a button present, basically a plastic knob with no spring action, that acts as an actuator, pressing on something beneath like a tiny rubber contact with a carbon pill. And there is only a couple of millimeter between the actuator and the pcb that should replace the old spring contact. So it should all be very small.

author: Renate
author_rank: Super Contributor
timestamp: November 03, 2021, 11:21:15 am
content: Ok, so this is a one-off repair.It's not clear what you started with.Was it one of these?https://www.snaptron.com/products/standard-domes/

author: Chris_D
author_rank: Regular Contributor
timestamp: November 03, 2021, 11:53:49 am
content: Quote from: Renate on November 03, 2021, 11:21:15 amHi Renate, yes, it is a repair I'm trying to do.The original , I don't know the exact English name for it, a 'finger contact' perhaps?Two pieces, one is depressed on the other (with the plastic button), rather primitive.

author: Chris_D
author_rank: Regular Contributor
timestamp: November 03, 2021, 11:56:00 am
content: Quote from: Renate on November 03, 2021, 11:21:15 amActually: this one. In not very good condition as you can see.And contrary to what I've written in my post, there is no rubber button with a carbon pill (yet) present. I was thinking about repairing it in such a way. My mistake, sorry.

author: Renate
author_rank: Super Contributor
timestamp: November 03, 2021, 12:13:00 pm
content: Is there enough room behind the panel to mount a subminiature panel mount snap-action switch?

author: Chris_D
author_rank: Regular Contributor
timestamp: November 03, 2021, 12:22:50 pm
content: I'm considering the same possibility as well.But what do you mean with snap action?Like the one you showed me?dome like?

author: Chris_D
author_rank: Regular Contributor
timestamp: November 03, 2021, 12:27:53 pm
content: Quote from: Renate on November 03, 2021, 12:13:00 pmTravel of the actuator is perhaps 4mm max.maybe less, hard to tell actually.

author: Renate
author_rank: Super Contributor
timestamp: November 03, 2021, 01:02:31 pm
content: Travel distance is one thing.Behindthe black panel is there room for a switch body?Remove the metal pieces on the black panel, drill a hole and install a normal switch.

author: Chris_D
author_rank: Regular Contributor
timestamp: November 03, 2021, 01:56:05 pm
content: Quote from: Renate on November 03, 2021, 01:02:31 pmYes, but the casing slides over the inner, so any button sticking out would obstruct assembling again.I think, the only solution is perhaps placing the switch at the outside, not very elegant, but anyway.

author: Renate
author_rank: Super Contributor
timestamp: November 03, 2021, 02:58:09 pm
content: How thick is the black panel?How much clearance do you have on the back? Don't the four screw go through to nuts on the back?I just bought a ton of switches that are surface mount and the plunger is 4.3 mm above PCB/mounting surface.https://www.digikey.com/en/products/detail/c-k/PTS645SL43SMTR92-LFS/3861373If you want smaller, there's 7 pages of in-stock 2.0 mm high switches.And there is every size in between too.

author: Chris_D
author_rank: Regular Contributor
timestamp: November 03, 2021, 03:40:51 pm
content: Hello,I don't know how to compare it, but it is a bit like a box of matches:https://www.istockphoto.com/fr/photo/matchs-dans-une-bo%C3%AEte-dallumettes-gm505405212-83665165The outer casing slides over the inner side, and there is no much space, probably a few mm, so anything sticking out from the inner box so to speak, would make it impossible to assemble. The only thing that is possible, perhaps, is a switch with a button that can be disassembled, like something you can screw in afterwards. But I don't know if this exists. Or a 3d printed button that fits in the existing hole in the casing (and that doesn't fall out) and can activate a switch without an actuator for instance.

author: Renate
author_rank: Super Contributor
timestamp: November 03, 2021, 04:10:13 pm
content: Yes, but the outer casing slides in front of the panel so that's not the limiting factor on the back of the panel.You could even cut a slot in the panel and mount a leaf microswitch there. You could have it practically flush.So the button itself is captive in the outer casing? There still has to be some clearance between the inner and the outer.

author: Chris_D
author_rank: Regular Contributor
timestamp: November 03, 2021, 05:59:43 pm
content: Quote from: Renate on November 03, 2021, 04:10:13 pmIt is very difficult to access the back of the panel, because it is like an unified entity, four walls so to speak, with a little space from above and only accessible via the bottom, which is secured with a security screw, for which I don't find the right spanner tool for the moment (and probably I'm forbidden to do).Maybe one can drill through the plastic, but behind it there is the battery holder, so not really advisable to do if you don't want to damage it.You can probably completely dissamble the meter, but I think there is a high chance that you will tear it completely apart due to the brittle little micro screw in it and so on.But I appreciate your effort to solve my problem.