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some next rev freshness

A project log for cyborg ring

cordwood + smt + jewelry + blinkies + zinc-air batteries

zakqwyzakqwy 02/01/2018 at 23:2510 Comments

I intend to tackle a few problems with this revision. None of the changes are huge, and thankfully didn't require any rework in Inkscape and only needed a bit of re-routing.

First, none of the battery contacts I ordered previously really worked. I tried a few flat gold-plated plates (they lacked any 'give'); various bits of RF gasket material (unreliable connection due to size, I think); and wimpy spring-loaded finger joints (batteries fell out). Here they looked in my Digi-Key cart:My experience with the gaskets in particular leads me to believe that a soldered-in solution is best here. The batteries measure 3.45mm long but the spec suggests this can vary between 3.30 and 3.60mm, so I need a solution that covers a reasonable range. I considered using a stubby pogo-type pin, but it would need to be thru-hole soldered and would probably stick out a bit. So I selected a variety of spring clips:
Datasheet quality varies here; some give recommended closed height, some give typical force numbers, etc. I selected the six specimens above based on these bits of info where available, and otherwise based on uncompressed heights in the 1.0 - 1.3 mm range. Since the footprints vary a decent amount, I also changed the battery cathode pad to a fully exposed copper disc like the anode.

Second, I fixed that stupid capacitor pad that required a minor bodge to get the LEDs connected. I took another look at the PCB layout and I clearly hand-labelled the netlist incorrectly on that pad:
Serves me right for not including the structural caps on the schematic. Manual netlist stuff is never a good idea, it's just asking for trouble down the road. Fixed, with fills displayed:
Third, I need to add some tensile strength -- the 0805 resistors in particular come apart quite easily. I'm using lengths of 30 AWG wire wrap wire as reinforcements. The wire should be 0.254mm diameter (my crappy calipers suggest less, but... I believe the spec), so I added some slightly larger plated through holes in strategic spots:
None of the wires (8 in all) are electrically connected to anything, so it won't matter if the battery slides around a bit and hits one. During assembly, I'll rig up some kind of scheme to keep the wire under tension during soldering. Mechanical strength is really important, especially when changing batteries!


Boards are ordered. They should be here in a few weeks, at which point I will post an assembly log.

Discussions

Morning.Star wrote 02/14/2018 at 06:32 point

As far as battery connections go, have you considered magnetism? ;-)

It's one of my favourites...

You could use a very thin disc magnet, or a tiny cylindrical one in a hole on a sprung plate might be engineerable. Looking at the rest of the circuit anyway XD

*edit* Yeah, thats how I'd attempt it. Drill a hole through each board, solder a springy plate over it, and stick a tiny magnet to the plate in the hole. The magnet wont come out with the battery sliding on it, and will make a nice connection as well as secure the battery. ;-)

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zakqwy wrote 02/14/2018 at 13:47 point

hmm, an inset magnet. I like that idea a lot. I will see how well the batteries stay in with the new clips, but that may be on the list for rev 3. where do you find your super thin magnets? not something I've sourced before.. 

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Daren Schwenke wrote 02/14/2018 at 16:00 point

https://www.kjmagnetics.com is what I use.  Just make sure all your credit cards use a chip (or handling them while wearing your ring will erase them).

Oooo... You should adapt this to handle NFC for payment.

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Morning.Star wrote 02/14/2018 at 20:40 point

Haha I cant beat 1/32" thick Neodymium discs, good one Darren. Yeah, recommended experimental research. I've had great success, if you can do that to your tolerances we'll have to make a medal, dude. :-)

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Daren Schwenke wrote 02/14/2018 at 16:06 point

This is a pretty good idea. 

I've used those bare surface mount dome switches for battery contacts as well.  A lot of tension in a very thin package.  I didn't buy them though as they were scavenged from old electronics, so not sure on availability..

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Seth Hunter wrote 02/14/2018 at 04:07 point

I think you should use a gradient of LEDs and add diffusers - you could make a band of diffusing material that goes over the whole thing and the colors would mix together. Another idea I had is maybe a button on the bottom that you could push with your thumb - to trigger the LEDs (when you are fidgiting)

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zakqwy wrote 02/14/2018 at 13:58 point

I like the gradient LEDs idea; I did some searching early on for a good range of peak wavelengths but didn't make any decisions. One challenge is that my battery voltage is low enough (2.7-2.8 volts) that I'd prefer to avoid InGaN LEDs, so I'm limited to 650-560nm. I'll revisit the search!

I have a sheet of 1/16" Delrin that works quite well as a diffuser -- I'll cut a few strips off and see how they look on the LEDs. A permanent diffuser might need some kind of slick mounting scheme, maybe a pair of soldered clips or milled PCB slots.

The boards have a tiny side-mounted push button switch, I just need to write the code to make it work. Not sure it shows up well in any of the pictures I've posted. It is TINY. Great minds, etc..

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davedarko wrote 02/02/2018 at 09:49 point

Will you use the shield fingers so that people will never be able to take off the ring? 

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zakqwy wrote 02/02/2018 at 13:33 point

I will now! excellent idea

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Morning.Star wrote 02/03/2018 at 09:29 point

Heheh, barbaric and ingenious ;-)

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