Fishbreath Builds a PCB

Please excuse the tangent. I love the look of circuit boards and PCB traces. It’s art to me. I actually hung up a board on my wall, that I found years ago, because it had magnetic core memory on it.

I don’t consider it a tangent at all. I was just saying the other day how aesthetically pleasing circuit boards are, and how it was a pleasant surprise to discover that so many of the aesthetically pleasing ways traces are routed have practical reasons.

Core memory is something else, though. Apparently, they never really managed to automate its production; it was hand-assembled pretty much until DRAM took over.

Didn’t know that last part. Very interesting!

If I spend money on a vintage memory board my wife will kill me.

“Condition: vintage, used”

Initial testing did, in fact, reveal some issues with the weigh-a-float which I don’t think merit engineering around. There’s more and more batch-initial calibration to do in addition to the overall calibration, which is added annoyance and difficulty on an ongoing basis.

So, the high-precision scale method it is. Now the controller just sits on or next to a scale of some kind—the nice thing about load cells is that they’re standardized as far as wiring goes, so you can use a Libra Cervisiae to control a scale you build yourself, or control a scale you buy from elsewhere. Given the easier design, I decided that there’s not even a need to have a battery on the board anymore. That means I can feed the board 5V instead of 3.3V, which means more output from the load cell, which means more signal-to-noise.

It does mean I have to power the USB-to-UART converter from battery, too, but supposing r/Embedded’s intuition is correct, it should go into suspend mode if there’s nothing on the data bus, which I wouldn’t expect there to be from a battery pack. It still draws about 300µA, which is three times the rest of the board put together, but even 400µA of draw lasts a really long time when you’re talking about a 27Ah battery pack.

Getting rid of the on-board battery handling means another redesign!

I added connectors for up to four load cells, and also ditched the solder-on pads for them and the temperature sensors, since they take up a lot of space and don’t do all that much to make assembly easier. Unfortunately, the parts I got to remove because of the battery situation cost about as much as the extra connectors.

I should be getting a few prototypes of the old revision of the board in the mail in the next week or so, along with all the parts required to put it together. That’ll do for prototyping, but once I’ve verified that the pin spacing for everything is correct, I’m going to order the new-style boards, along with the one or two items required for the new boards but not the old ones, and get cracking on those.

Out of curiosity, what software are you using for circuit design?

I did a unit on chip design in my college days, and I recall the software being a very painful to use open-source academic utility. I assume there’s professional software out there that’s much better, but very expensive, but I wonder what, if anything, exists in the hobbyist space these days.

The professional tools are Eagle and Altium, both of which have (I believe) limited free versions. I’ve been using KiCad, the current free, open-source leader. I don’t have any experience with the others, but reddit’s perspective is that KiCad is on par with the pro tools nowadays, just a little different.

Thanks, those look pretty neat!

I have no intention of ever doing something like this…but it’s super cool.

The first revision PCBs arrived, which I had ordered just slightly before I decided to move to the new, large-scales design. Still, they’ll work for testing, and they also revealed some issues with my breakout board footprints.

Still, it’s super-cool. The fact that this only cost $14 and two weeks of waiting is pretty amazing.

Indeed. :) Great work.

Edit: You also have a great taste in writing instruments.

I buy them by the 30-pen case.

(Also, ‘amazon’? Yeesh. I don’t even have the excuse that I was voice typing.)

My hot air rework station arrived on Friday, so I figured I’d grab one of the obsolete PCBs and some components not used on the redesign and see how surface-mount hot air soldering works.


8.3x magnification through my nerd goggles

Pretty well! I think I need to use a little more solder paste, but the multimeter says the connections are good.

Not bad, but don’t forget the conformal coating :)

More to buy! My wife will be so thrilled.

Finally got all the parts in for the second-revision boards. All the bits and pieces fit, and the microcontroller works. The only problem is that I can’t get it to boot into reflash mode, so I can’t put my software on it. Other than that, it seems like I’m just about done.