Saturday, March 18, 2017

Piezo Activated Drumlights - The PCB

I feel like once you have learnt one software tool in a particular domain to an intermediate level, you can pick up another in the same domain without much fuss. Only if you are too new or too entrenched with a tool do you feel the sunk-cost fallacy and have a higher barrier to switching to something new when circumstances change.

Nightly build I am currently using
I started my adventures in ECAD using Eagle and built up quite a bit of familiarity with it, I would like to say to an intermediate to advanced level. I also built up some assets i.e. libraries and projects. To unlock more features I spent a few hundred dollars on the maker edition. However as with all software, there were niggling bugs and gripes. One of them was lack of proper 3D support, another was lack of proper slots when exporting gerbers, well slots/mills that hackvana would accept. On the hackvana IRC channel KiCAD was mentioned often and I was encouraged to switch to it. The final straw was the acquisition of Eagle by Autodesk and subsequent switch to subscription based pricing.

Eeschema of Arduino Nano drumlights
So finally after much dithering, I decided to switch to KiCAD as my main ECAD tool in the beginning of February 2017. It has been a fun couple of month translating my Eagle knowledge to KiCAD and dealing with the vagaries of a rapidly evolving tool. So far I am loving it. The first PCB I designed using KiCAD was an Arduino Nano Shield using the template provided to work with my Drumlights project. I took the PCB design as an opportunity to add a battery charger and piezo charge amplifier using a TLV2771 opamp.

KiCost in action as a BOM tool
One of the Eagle features I miss is the forward-back annotation between the PCB and the schematic. In KiCAD the association between a schematic symbol and associated footprint and nets is done using a third tool. This way there is no need for duplicate footprints per device for commonly used pad patterns e.g. SOT-23 or TSSOP-16. All devices using this footprint can share a common PCB element and only differ in schematic element. However this also means that changes in the PCB and are not automatically propagated back to the schematic. The backward link will be coming soon according to a recent roadmap. So I am living nightly-to-nightly in anticipation.

PCBNew view of Arduino Nano Drumlights
The other feature I like about KiCAD is the seamless 3D support, especially in the recent builds.There is dual support for usual VRML for pretty rendering and associated STEP files for export to other CAD files particularly FreeCAD. I downloaded step files for parts not already in the library from Molex, JST of GrabCAD and populated them with some scaling and rotation onto the PCB. You can see the resulting internal KiCAD render below. Next came sending the boards to Fab, a lot of hackerfriendly fabs directly support Eagle .brd files, now there is increasing support for KiCAD as well.

KiCAD Ray trace render of Drumlights PCB
Oshpark can directly process KiCAD - kicad_pcb files to associated gerbers and board orders. This reduces the number of clicks to ordering a prototype and eliminates potential errors and filenaming issues when creating Gerbers, since KiCAD does not really have detailed DRC and CAM job settings like Eagle.
.kicad_pcb file directly imported in Oshpark
One of the valuable lessons I learnt which I have had issues with Eagle in the past as well (obviously I trusted community review of KiCAD libraries too much) was to always double check library footprints against the datasheet. The N-channel Mosfet schematic symbol was meant for a through hole part and when paired with a SOT-23 footprint things were not in the right place. So when testing I ended up with turned on mosfets being fed with 3.3V from the microcontroller from drain-to-source essentially shorting the output. Luckily no magic smoke escaped before I spotted it and pulled the power. You can see the picture here of my hack to salvage the erroneous PCB's using the SOT-23 at 45degrees. I have tested this board with LED strips and it works as expected.

Diagonal mosfets to correct for footprint mix-up is another cheap option for getting good quality prototype PCB's if you are prepared to wait. However they do not support direct KiCAD upload. If you follow the Oshpark's Gerber generation guide to create gerbers and drill files they should work in as well. This is what I did when ordering my test boards, after a couple of revisions via Oshpark. Overall my experience with KiCAD has been very positive. The community is fun and responsive. There are some great ancillary tools like StepUp mentioned above and KiCost which I used to instantly generate BOM for my board and order all parts from Mouser in minutes.
KiCAD Gerbers rendered in

If you are interested in my KiCAD experience and general principle of piezo activated drumlights you can keep tabs of my hardware and software via github.