Navigation pyskin - Board Design RFC

[kfricke]

Of course this is a total conversion of the topic... can i use the "Navigation pyskin" on my scooter? Basically there should be some common grounds. GPS it of course interesting at least to track movement. Even the motion sensors are of interest there (lets one do conclusions about the driving style). And of course power source of around 12V (i assume that your lithium batteries do give 12V output!).
Ok, servo control and high-power driven output for the ESCs is not really needed.

Maybe we can one day launch a µPy scooterputer alternative

[Turbinenreiter]

That works for me. The only reason that I had it on XY-8 is because that is where it was in @turbinenreiter's design that I used as my starting point. I assumed it was what he had tested with his code and just decided to leave well enough alone. You do offer a good reason to switch it, so I will.

I chose that Pin completely random. Switch it!

[blmorris]

Of course this is a total conversion of the topic... can i use the "Navigation pyskin" on my scooter? Basically there should be some common grounds. GPS it of course interesting at least to track movement. Even the motion sensors are of interest there (lets one do conclusions about the driving style). And of course power source of around 12V (i assume that your lithium batteries do give 12V output!).
Ok, servo control and high-power driven output for the ESCs is not really needed.

Maybe we can one day launch a µPy scooterputer alternative

No reason that you couldn't use it as-is (or rather, as it will be once we have some built.) Since I scrapped the on-board switching regulator, you will need some kind of step-down regulator to bring the battery voltage down from 12V to something below 10, ideally 5V or so. Also, if you aren't using the servo outputs, it will fit in either position X or Y.

I chose that Pin completely random. Switch it!

Already done, along with the mounting hole. This led to a small cascade of other changes as well: I added 0.05" (1.27mm) to the width to accommodate the mounting hole within a rectangular outline, which allowed me to shift the GPS module a bit, which afforded a bit of breathing room elsewhere to make some minor layout improvements… board layout ends up being a pretty obsessive activity
After moving MPU_INT to XY-5, the only unused pins are the SPI bus on XY-6,7, and 8, so I figured it would make sense to give these signals their own 4-pin breakout header (with one for ground). This will make it much easier to access these pins if necessary, since the GPS module dictates that the skin shouldn't have another board stacked on top. I also moved the backup battery jumper between the two main headers, which is where I wanted it in the first place.
Updated files will be posted in a few hours.
-Bryan

[blmorris]

Pushed updated design files: https://github.com/blmorris/pyIMU_v1/tr ... /pyIMU-GPS

-Bryan

[Damien]

Lastest revision looks great! What next, off to the PCB house?

[blmorris]

Thanks! That is the plan. Thinking of ordering a first run of 12 boards. I will order parts to build up the first two just to be absolutely certain that everything works as designed. If so, I'm hoping we can drum up enough interest to place a bulk order for the other 10. Only question is whether I feel confident enough in the layout to spend an extra $130 to have a paste-mask made.
While I certainly wouldn't object to making a little money from these boards if there turns out to be a lot of interest, I'm not going to count on Kickstarter-level interest just yet . I would just like to make sure I don't lose too much. And that really isn't why I did it anyway; mostly the quadcopter project really piqued my interest, and this little sub-project was a good match for my skills.

-Bryan

[JonHylands]

You can get a flexible transparent paste mask (stencil) made from a number of places, for a lot less. Lately I've been using https://www.oshstencils.com/ for that...

[blmorris]

Jon- thanks for the suggestion. I was considering that option, since the board area was so small. I had some difficulty once using a Kapton stencil on a board with fine-pitch (0.5mm) components, but it was also relatively large and complex, better suited for stainless steel anyway.
Fortunately, Stencils Unlimited has recently reworked their pricing structure so that small stencils are much less expensive; a 3"x3" stencil is now just $40, much more reasonable.
Boards are on order, but I probably won't receive them before the holidays, so I'm planning to build them when I get back in January.

-Bryan

[JonHylands]

I actually prefer the transparent stencils for small boards (especially one-offs where you don't need to build a frame). You can actually see the pads through the material, so its much easier to align properly. A quick swipe with a razor blade of paste, and you're ready to place components.

I've probably built well over 200 boards that way. My latest board (for NanoSeeker) includes a MPU-9150, which is fairly fine pitch (0.5mm). I've built about 80 boards in my day job that include a couple 6-pin TSSOP chips, which are also 0.5mm pitch.

- Jon

[blmorris]

Jon- Thanks, you have convinced me to give them a try. At <$10 including first-class postage there really isn't much to lose.
Although I wonder if they have taken a close look at their discount formula. As the size of the stencil increases from 12 to 72 square inches, the price per square inch drops from $0.60 to $0.30 - simple enough, for even numbers this evaluates to a formula of Cost = 0.66 x Area - 0.005 x Area^2. The most expensive stencil in that range is $21.78 at 66 square inches, with 68, 70, and 72 each less expensive than the previous size.

I will say that I do use flat stainless steel stencils without a frame - we haven't invested in a paste-print machine here, or indeed any automated assembly equipment other than a thermocouple-controller for my reflow toaster oven. For now I can still line up a mask by eye and keep it in place for the squeegee with a liberal application of painter's tape - this actually works well enough for me even dealing with some 176 pin 0.5mm pitch packages. I probably won't take that ability for granted in a few years as I approach bi-focal age Maybe by then I'll be so good at board design that I can just send them off for manufacturing without prototyping first