Gerber files for the 3rd version.
Zip Archive - 157.63 kB - 06/22/2016 at 22:43
Fritzing file for the 3rd version.
x-fritzing-fzz - 26.64 kB - 06/22/2016 at 22:43
After the last experiment with the pin headers used as springs, I decided to try with one more PCB design -- this time soldering the pin headers properly, with holes, and then bending them. I'm not entirely sure it will work well enough, though.
I also decided to arrange all the pins in the order of their numbers, not the order they are broken out on the micro:bit. There are two extra rows of pins for ground and power. The power pins can be connected with micro:bit's power with a jumper, or connected to a separate power source -- this should let me driver servos easily.
I decided to give this project one more try, this time by adding springy elements that would touch the pads on the micro:bit and thus maybe provide a better connection. I used one of the earlier prototype boards for this. The donor of the springs is a 1.27mm pitch long pin header:
The plastic part keeps the pins equally spaced -- the same as the micro:bit's pads -- and makes it easier to solder the pins. Once soldered, I removed the plastic and cut the pins to size:
I didn't use any pins for the large pads, because I'm using metal bolts to keep the whole thing together, and they should provide enough connectivity for those:
In conclusion, I think this could work, but I would need to redesign the whole board to make the assembly easier -- right now it's much too tricky, and it's too easy to make shorts between the pas while soldering the pins (I made a few, even though I was very careful).
The boards arrived. They are not exactly identical to the third version I posted here, as that has some more tweaks an fixes. But they should work fine.
This time I added two more rows of pins, for the power and for ground. This makes it less breadboard-friendly (unless you don't solder those pins), but makes it much easier to connect stuff directly, especially servos.
Note that the power row is not connected anywhere by default -- you can use a jumper to connect it to the 3V pin, or connect it to a separate power source -- for instance, for the servos.
Ok, so now that I made the first version somewhat work, here's the second one: https://oshpark.com/shared_projects/yOebxhzN
I moved the pads to have better contact surface, and I made the holes smaller, so that the m3 bolts I have fit them without any leeway. The micro:bit has m4 holes, but you can use bolts with conical heads, and they will then nicely center.
The digits on the pads are actually holes in the copper, not paint, so I was wrong. I tinned the pads on the header board, and tried again, this time using all the 5 bolts and screwing them really tight, and this seems to have helped.
The next version is going to have longer pads, smaller holes (for m3 bolts with conical heads, so that they auto-center) and clearer markings.
So I happily soldered the pin header, bolted the microbit on top (with acrylic bolts, as to not scratch it or cause shorts) and tried to use the extra pins and... nothing. No electric connection at all, not even a weak one with large resistance. How is that even possible?
I unbolted the board and examined it closely. Sure enough, the digits on the pads are painted. With non-conductive paint with some thickness. Which gives enough distance that it doesn't work. Sigh.
Back to the drawing board, I guess.