I made solid progress on power supply and efficiency for the system. Time for another round of "spot the bodge wires:"

The improvements:

1. Simplified ground bus: Reverse Polarity protection moved to high side to simplify all grounding.
2. Reduced voltage drop/waste: Removed diode as RPP, now using only the P-CH FET. 
3. Increased operating voltage range: Now 3.6 to 7.0 (was 4.5 to 6.0) with these changes:
   1. Added 5V regulator/bus for LED array and Teensy Vin (Teensy LC max is 5.5, Teensy 3.2 max is 6.0)
   2. Allows use of 4x "AAA" batteries like Energizer Ultimate Lithium, because four of them is almost 7V vs. regular alkaline which might be ~6V maximum, which exceeds what the Teensy can handle. I know this by trial and error.
   3. Opened the door to using a 1S LiPo, if the user chooses to hack in that option. I plan to make it easy to hack in with an Adafruit Micro-LiPo Charger.
4. Improved efficiency: Powering both 5V and 3V3 regulators separately from the battery/switch, not in series.

Other notable points:

• The pin/sockets are working well for the Core & LED & Teensy components. Insertion/removal force is "just right" and it makes it easy to hack and modify. I haven't bent any pins... yet.
• Lots of optional female headers making 'scoping signals much easier than using test point loops.
• Battery pack behind the core array is out of the way AND supports LED array at the right height under the Core array. Stack-up is "just right."

Do you know how to configure both the 5V0 and 3V3 regulators to be automatically bypassed when their respective input voltages drop below 5.5 and 3.8? This would increase efficiency and expand the lower limit of the battery voltage range even further. Right now the 1S LiPo only goes to about 3.6 before the system browns out, and it should easily be able to go down to 3.3 or even 3.0 to use the full range of the LiPo.

Next up, test the hall switches on analog inputs and some optional add-ons (SD-Card, QWIIC light sensor, LCD).