Updated the diagram with the signals currently being used & the ever important suicide voltage.

The rework inside

Power level was connected directly to the atmega.  Going through a diode or a resistor made no difference.

Then the rest of the wires were routed through the former hot shoe.  Reassembly will be scotch intensive.

The next step was a power testing jig.

You can sort of calibrate it by trying to get the same histogram while doubling ISO values.  Ambient light made the lowest power settings slightly lower than they should be while not affecting the highest power settings.  The trick with reducing ambient light was to move closer to the flash.

The SCR - POWER delays for each stop were not powers of 2 but somewhat logarithmic.

0, 24, 32, 50, 70, 100, 170, 300us

The flash was found to have only 8 stops from dimmest to brightest, giving 0-7 for the manual power settings.  Helas, preflash recycling stopped working most of the time.  When it recycled, it would most often use full power for the mane flash.  There might have been a correlation between battery charge & recycling time, but the erratic full power was a different story.  Power was never erratic in manual mode.

It was a tragic regression from yesterday.

Going back to the bench, it was found that TTL recycling worked when battery voltage was 3 & became very unreliable below 2.8V.  It would need a lipo with buck converter.  Power control didn't work at all in TTL mode.  The 2nd firing was either all or nothing.  0us always fired minimum power.  30us was randomly all or nothing.  Sometimes the power pin got stuck high & drained the capacitors until a power cycle.  The same thing was observed when holding power high for over 50ms.  The power pin seemed to treat the TTL recycling as a single firing of over 50ms.

1 good nugget is it could fire without the pilot light.  It just couldn't hit full power unless the pilot light was on.

The difficulty with power control in TTL mode is the new deal breaker.  Things once again pivoted towards a scratch built flash with just the tube & reflector being reused.  The science of the SCR latches & high voltage triggering is quite complex.

The leader in small size seems to be the 270EX II, formerly $120 now $150 bidie bux.