Before I send a new PCB design for fabrication, it's good to double check a few items:

• Test points, for all power supply voltages, all the signals I will be using, and, if space permits, unused pins of ICs and microcontrollers, in case I want to add features later
• Silkscreen labels for all components and signals.
  
  • this includes polarity markings for polarised components like diodes and electrolytic capacitors, and + and - signs for batteries or power supply.
  • It's also a good idea to mark anything that can easily be mixed up, for example, the colours of different LEDs
• mounting holes. Even if they don't get used for mounting the PCB in the final product, they can be used to hold the PCB down during testing or assembly, and are also useful for passing wires through
• Board name and revision number. Even if I think it's a one-off.

Back when PCBs cost over $10 per square centimetre, it was important to minimise their size. Now that they are cheaper, I think improving usability of the PCB is usually a good trade off. Of course there will be exceptions!

During the present chip shortage, it's often a good idea to provide accommodation for component substitutions, if possible. This might include:

• allowing for different pin spacing on passive components like resistors and capacitors
• making room for differrent modules (like I have in this design, to accommodate the Adafruit or Sparkfun breakouts, or the bare Melexis sensor.

So, when I got the shiny v0.1 PCBs from the factory, I noticed that I had made one mistake and somehow swapped the SCK and MOSI lines for the screen on the schematic! Stupid. I had to mend this on the v0.1 PCB by manually soldering wires at R6 and R7. After a bit of testing, I also noticed that the readout for battery voltage is quite noisy. My oscilloscope shows ~230mV of ripple at ~150Hz on the input side of the DC-DC converter. The noise seems similar regardless of whether the unit is running from batteries or a lab power supply. Regardless of the cause, a capacitor between the ADC pin and ground helps smooth out some of this noise.  To fix the v0.1 board I added a 1µF SMD capacitor (the first value I had at hand) in parallel with R9. Before I added the capacitor, ADC readings would fluctuate by ~50, the capacitor reduced them to ~3. It’s probably better to use a couple of caps of different values to remove different frequencies of ripple, but the 1µF is definitely "good enough."

There may be other mistakes that become apparent as I do more testing. There will definitely be features I want to add, and the community might have suggestions too! at some point I would like to do a second revision of the PCB, but at this stage I don't have enough information to commit to a v0.2. The v0.1 is more than sufficient for testing.