TL;DR: Got the board to work by soldering wires to alternate locations for GND, VIN, and Battery '+'. The microswitch is near impossible with an iron. Good learning experience and good product!
Here's the long version for what it's worth.
The signal pads for the switch are entirely underneath the switch. The switch can't be repositioned much because the pads are tiny. The tight spacing between the signal pads and the metal case add to the difficulty.
I had no trouble soldering 0603 components or the gullwing MAX1555. The iron I used was a temperature-controlled Weller with a conical tip that might be a bit large for the job. I used 23 gauge leaded solder. I tried very lightly tinning the pads and using copper braid and Chipquik SMD291 no-clean paste flux. I didn't have a good solution for holding the switch and board in place while soldering.
On my second attempt, I tried starting with the switch because it is by far the hardest. I even tried soldering 27 gauge magnet wire to the pads instead of the switch, but the pads were too small and too close even for this. Finding alternate soldering locations on the schematic worked much better.
What worked: soldering wires to GND, VIN, and Battery '+'. To turn on the T3, short Battery '+' to VIN. To turn off the T3, disconnect battery '+' from VIN and short VIN to GND for equivalent behavior to the switch or simply leave VIN disconnected from Battery '+'.
Pro Tip: Test the board using a dummy load such as an LED and resistor rather than your Teensy.
I haven't figured out if the battery charges when switch is "on". This seems to be implied in the
Tindie description if I'm interpreting it correctly. However, since the battery passes through to VIN, it seems to me the device load would confuse the battery charge monitoring.
I used 1K Ohm current limiting resistors for the LEDs and they are still satisfyingly bright.
Wishlist:
- An alternate microswitch with gull wing leads or at least bigger pads for the microswitch.
- An alternate hole to solder a lead for Battery +. Maybe one of the N/C holes. Ideally, three holes aligned for a three-pin header.
- A method to control the charge rate.
- More space between the LEDs so position helps indicate the mode.
- Protection diodes, maybe? Could make assembly and testing safer.
- More and sharper pictures in the documentation with arrows and labels.
- More layers used in the layout.
Overall, assembling One Horse's LiPo charger was a challenging yet satisfying learning experience and I'm happy with the result. Now that I've been through it, I'll probably buy assembled ones in the future. Many thanks, One Horse!