I am considering a project whereby a lot of
PICs/Arduinos/RaspberryPi(0,2,3)/others monitor various AC (120v,
maybe 240v) lines and log their state. Because of the qty and low
budget, I need the absolutely lowest cost and simplest way to do this,
bearing in mind protection of the processor circuits. This isn't for
resale, so extreme idiot-proofing isn't needed, just reasonable
protection of the electronics involved.

The processors may be powered by battery, switching supply type wall
warts, or old fashioned transformer supplies. That may affect a
solution (ie common grounding, etc).

There are some neat, simple solutions for zero-crossing, but I would
like a constant signal as long as the AC is detected, relative short
rise and fall times (prevent uncertainty at the processor), and am
flexible about delay, within a handful of cycles perhaps (however many
cycles fit in a handful... ;)

I would like it to be a few passives and maybe a discrete semi, but I
keep making it too complex, so looking for some help in staying
'grounded' in simplicity... ;)  I'm pretty sure others here have done
this successfully and on the cheap and have considered all the
pitfalls and refined a solution, so please share. ;)

TIA,
Dr Skip
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