Well after few thought about that, here what I did
The first comparator circuit work really fine, my reference come from a stabilized 5V, I don't really need great precision, so a voltage divider is fine.
Right now I'm using pots so I can adjust thing after each cycle of detection...
The comparator output drive a blinking led, for the moment it is much more simpler than with my oscillating second circuit
(those lead blink fast tho, 2 or 3 Hz)
Now I might planing another thing, the lm393 has 2 comparators, so I might build 2 levels of threshold, something like a warning, and a near power failure warning...
Thank for your comments, it is cool to explore few ideas