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A project log for Heartbeat Logger

A portable device that logs a snippet of your heart at the push of a button.

Ole Andreas UtstumoOle Andreas Utstumo 06/16/2019 at 21:122 Comments
Okay, my initial plan was using a four layer PCB with power planes as the inner layers, and signals traversing the outermost layers, to make for painless routing. That is until I checked the price of a prototype series with blind and buried vias at PCBWay.
Obviously there are a bit more steps to the production of buried via PCBs than your regular 1.6 mm double sided green wafers of crispbread, and for a hobby project it's way more than what I want to commit. So I'll still run with a four layer PCB to keep my power planes intact, and let the signal vias run through the entire PCB. Then we're at 70 USD for 10 30x30 mm boards.

So here comes the next problem: The CR2032 holder I picked requires a damn large pad on the bottom side for contact with the negative side of the battery.
Which makes through vias problematic in 50% of the boards area when placed on the back side. I can pull GND down to the battery terminal, but anything else will short to the battery..
I quite like the simplicity of these kinds of holders, though...
Battery holder courtesy of Marijo Blazević (
Maybe there is a trick we can pull? The datasheet for the battery holder recommends a footprint, but there's no one saying we can do our own interpretation. As long as we make robust mechanical and electrical contact we should be good. So
Instead of resting the entire negative face of the battery on a gold treated copper pad we can make two "rails" that are almost right underneath the springs on top push the battery down. The pads will be surrounded by solder stop which might have more height than the pad:
That would be bad news for our electrical contact, so we could try to gain control over this uncertain situation by adding solder paste to the pad.

That should do the trick, and now we have freed the necessary space to route the rest!

As some one pointed out in the comments below, solder will oxidize. Hopefully it will be a minor problem...
To be continued...


Jan wrote 06/17/2019 at 07:08 point

I had the exact same problems with those battery holders with my super small loggers and I bought the same Linx parts to solve it :)

Problem with the solderpads for battery contacts might be higher resistance due to oxidation over time. I'd rather use three small "dots" arranged in a triangle to make the negative (PCB side) contact to get the battery/contacts situation statically indeterminate.

Problem with two longer pads could be a height difference due to more or less solder.

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Ole Andreas Utstumo wrote 06/20/2019 at 10:10 point

I like your idea of the triangle arrangement, there's a high probability I'll implement it :-) Neat work with your beehive logger, by the way!

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