After ensuring that one uses the unit of Arbs at every available opportunity, I'm well aware that the second rule of "science" is to change several variables at once, so you can be sure you will never really understand what happened but that if it's a good outcome you can carry on, smugly praising yourself for saving at least one iteration of work. If it doesn't work you can always go back to the way-of-the-sucker and change variables one at a time.
You know I told you all about that naughty OnSemi FDT439N that was operating far worse than its datasheet claimed, perhaps causing us to lose 20% of the usable voltage range of the supercapacitors? Erm, I may have used a different part. On closer examination, it appears I took some shortcuts with populating my Yapolamp 1.1 development board and an itinerant STN4NF03L may have sneaked its way onto
the PCB both PCBs...
So what? I took the opportunity to lash up the 1S 6P and change the MOSFET for a pukka FDT439N.
Back to the bench
I didn't change the timings on the pulses, and it's probably a good thing too. One thing I didn't notice in SPICE for the 6S 1P arrangement is the discharge duration is muuuuch longer (double, because half the Vf for 1 LED vs 2 LEDs in series). There's a roughly 20uS discharge period for this arrangement, as opposed to the other arrangement. We are getting into audible range of oscillation for the full pulse-discharge cycle but I didn't detect any noise (especially above the oscilloscope fan!). Here's the setup:
Scores On the Doors
I couldn't quite believe this but my little 3F supercapacitor bank (2 in series because I got 2.7V versions) ran down to 1.8V at some time after the 5 minute point. The reason I'm not sure of the time is that it kept running, that little ATtiny, lower and lower. Eventually, the LED brightness dropped a bit, but certainly not to an unusable level. The ATtiny402 kept going and going and going, until at 6 minutes and 42 seconds, the lights went out. What amazed me about this was not how the MOSFET mix up (and maybe the LED 1S 6P arrangement) had made such a difference, but that the voltage across the supercapacitors when the lights went out was