How can I prototype sliding electrical contacts? - Page 1
Forum section: electronics / projects

author: rea5245
author_rank: Frequent Contributor
timestamp: July 29, 2023, 02:53:33 pm
content: Hi,I'm trying to make a gadget with a multimeter-type selection knob. I bought an off-the-shelf multimeter and made a PCB to replace its guts. The picture shows the multimeter's board on the left and one of my unpopulated boards on the right. I put my board in the multimeter's enclosure.The problem is that the the multimeter's knob is not making contact with my traces.I presume the original board is using hard gold. My board is using ENIG because hard gold would be very expensive for a prototype. It's of course possible that my traces just aren't lined up with the contacts. To check that, I printed my PCB layout onto a transparency, laid it over the original PCB, and my traces line up with the original traces.Any ideas how I can make a prototype that will work, without breaking the bank?Thank you,Bob

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 29, 2023, 03:05:55 pm
content: I don't thing the gold coating is the problem. The ENIG will have a short life, but should work fine initially.There are a number of things to check. How does your board flatness and stiffness compare with the original? Is the hole in the center a tiny bit smaller than the original, preventing the shaft of the knob from contacting? Is there a non-conductive coating over the gold, like flux residue for example? Is it easy to get conductivity along an arc segment with a DMM? Do any of the traces make contact?Perhaps most importantly, you appear to be configuring the switch to get conduction across the span of the knob. Does the brush (contact) configuration provide this connectivity? It is possible that the multimeter just provides conduction across arc segments. If this is the problem you could relatively inexpensively create your own knob and brush with 3D printing and chemical milling of a brass strip.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 29, 2023, 03:16:38 pm
content: As long as the switches do make physical contact, they should work just fine (or at least intermittently).I assume there is some alignment problem. Maybe the diameters of the slide contacts are off, maybe the PCB is too far from the rotary switch itself and the contacts don't touch the PCB at all. You can try rotating the knob a lot, you should see some wear marks appear on the PCB of where the contacts actually touch it.

author: ebastler
author_rank: Super Contributor
timestamp: July 29, 2023, 07:14:46 pm
content: Quote from: rea5245 on July 29, 2023, 02:53:33 pmAre you sure you got the trace aligment right?I assume your photo shows the original multimeter's PCB on the left, and your new design on the right. If I superimpose a semi-transparent copy of the original PCB photo onto the right PCB photo, the concentric traces look quite misaligned. When I make the central drill holes match, traces of the one PCB seem to sit pretty exactly where the gaps between traces are on the other PCB.This might, of course, be due to distortions of the photo. But if I mirror one of the pictures and re-align the PCBs, the result remains unchanged, i.e. traces continue to be mismatched. Could you please double-check your transparency verification? Was the transparency printed at the right scale, rather than "scaled to page size" while printing?

author: rea5245
author_rank: Frequent Contributor
timestamp: July 29, 2023, 07:26:53 pm
content: Quote from: ebastler on July 29, 2023, 07:14:46 pmThe photo's not even close to an orthographic projection.I'm beginning to think the traces are OK, but the whole pattern is offset a little from where it needs to be.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 29, 2023, 07:39:14 pm
content: Get some calipers and measure the radius between the Red line (edge of hole) and all the Black lines (edge of trace). Do this on both and compare. If the center hole diameters are different, you'll have to do some math.Or use a pair of dividers and jump between the two for each measurement to see how far it's out.EDIT: Another option is to smear some grease (That's easily washed away with alcohol) all over the contact area on the PCB. Then assemble and turn the knob fully. Then disassemble and see where the contacts drew lines in the grease.

author: EPAIII
author_rank: Super Contributor
timestamp: August 01, 2023, 11:44:30 pm
content: I like the grease idea. Or you could use a Sharpie Marker pen. They say it is permanent, but it will easily clean off with alcohol. And the contacts should leave their paths in it.

author: coppercone2
author_rank: Super Contributor
timestamp: August 02, 2023, 02:02:36 am
content: I think it would be better (if you want quality) to get brass, glue it to a FR4 board, then etch it. Go extra thick on the FR4 substrate (or bakelite might be better). Then solder it to the PCB via pins.At least, that would be a strong solution. Kind of expensive maybe compared to a board house but I suspect the switch is a selling point for people that actually give a shit about equipment qualityThe nice thing is too since the switch is big, you can make the brass thick, I doubt a bit of undercut would matter on big ass contacts that last a life time.

author: pickle9000
author_rank: Super Contributor
timestamp: August 02, 2023, 02:44:10 am
content: If you have a 3d printer just print the pcb with some slots to see whats wrong.