@NeverDie said in wristwatches that invite development:
craft your own DIY PCB with battery and display to fit inside it. If you put your buttons in the same place as the corresponding apple watch that the case is meant for, then presumably it would work. And it would be waterproof!
So like this or this? Even has full open source Arduino firmware. Really an impressive project!
There also is an "Education" version of the Jlink for $20 if you want an official one with access to nRF Command Line Tools.
@BearWithBeard I do the 2 container method for cleaning pcbs with isopropyl. First container is the "dirty" one which gets the pcbs first and they soak for a couple minutes. Then each pcb gets the electric toothbrush treatment above the dirty container and then they go for a rinse in the "clean" container. Once the clean container starts to soil it becomes the dirty container and the old dirty container is cleaned and fresh isopropyl is added. Just like washing dishes. I actually worked for a place that used a standard dishwasher to clean (large) pcbs. It worked surprisingly well.
If you can fit your code modifications into an nRF52832 then you can buy an off the shelf watch. All the hard work is done. It's even on sale currently 
@NeverDie said in wristwatches that invite development:
Is that for sure the watch with the new nRF52840 design inside it, or is that some other Cortex M4 watch with only the case that gets used by the upgraded design?
The watch linked from Banggood has a nRF52832 in it which is re-programmable. If you want a nRF52840 then you would need to assemble a custom board in that case. The most recent project log claims a 2 week battery life with the stock reprogrammed nRF52832 and TFT screen. If true, I honestly I don't know if you can ask for much more. My Pebble Time had a 7-8 day battery life and that was mind blowing compared to the Apple watches available at the time.