For the past few weeks I’ve been playing with different designs for a Wifi-connected inertial measurement unit (IMU) to use with the open source EFIS I’ve been hacking on for the past year. After breadboarding several designs I decided to take the plunge and create a custom printed circuit board (PCB). Breadboards are great for first-stage prototypes, but they’re too fragile for field testing – especially for an IMU.
Most modern PCBs use surface-mount technology which can be a bit of a bear to solder. To avoid that, I went with a very basic design that used through-hole mounting and lots of pre-made modular components. If I was designing something for actual production this would be nuts, but for hand-assembled components it should work out reasonably well.
For the design, I used an online CAD program called 123d.circuits.io. While probably not ideal for complex designs, the tool did a reasonably good job of simplifying the process for a noob like me. It starts by having you create a schematic, then has you assemble the footprints for the selected components into the actual board layout. I ended up having to create a number of new (well, forked) components to represent the various modules.
Once I had the design reasonably complete, I downloaded Gerber files. (Gerbers are the standard output format used to manufacture the actual boards.) 123d.circuits.io offers fabrication services, but they wanted $30 for 3 boards, which seems a bit steep to me. I found a much better deal at Dirt Cheap PCBs, which offers ~10 for $14 plus shipping. The shipping (from China) was another $19, so the boards end up costing ~$3.30 each.
I did have to make one small change to the Gerbers. 123d.circuits.io names the outline file using “.outline” as the file extension. The board house that Dirt Cheap PCBs uses rejected that the first time I tried to upload it. I had to unzip the archive, rename the .outline file to .gml, re-zip it and re-submit. That seemed to do the trick. (Many thanks to Ian at Dirt Cheap PCBs for helping me work that out!)
The order is en route from China now and should be here just after Christmas. With any luck, I’ll have second stage prototype working by New Years. For anyone who’s curious, I’ll probably do another post on the IMU itself between now and then.