2018.05.30 To make v2.0 useful: Replace the 27k in the lower leg of the AP3012 comparator circuit with a 56k to drop the voltage from ~27V to just under 12V. In the AP3012 circuit, rotate the inductor and 4.7uF caps 90 degrees, and lift pin 5 of the IC, to detach all of them from the 3.3V rail. Use a wire to connect all of them to pin 4, the gpio driving the circuit. The net effect is that we're making 12V from the gpio, which means all the LED current and switcher inefficiency is coming as current through that gpio. The measured LED current is around 7mA, so this is "safe" for the SOC, but leaves less engineering margin than we usually like. Changing the circuit to be powered from v_lipo with the gpio hooked to shdn* and controlling a FET switch on the output might be smarter someday?