operates from isolated USB supply; does not consume current when USB cable unplugged.
The purpose of this circuit is to connect the power supply only when it reaches some threshold, say 2.6V, and disable it again when it drops below ~ 1.8,
Easily done I think with one of the supervisor chips that are available these days.
Looks like the AR algothims haven't improoved much in the 20 years since I last used one.
Power supply shield, how about one for really crappy industiral and automotive environments.