Hi Joseph,

yes a comparator can be useful.
E.g. you can use the AT89C2051 with a build in comparator (P1.0, P1.1)
So you can connect the 1 input (P1.0) direct to VCC and the other
(P1.1) over a resistor divider to the regulator input voltage.
If the divider divide 5.5V down to 5V (using a 1k and a 10k resistor)
the comparator switch, if the voltage drops below 5.5V.
You can cyclic read the comparator output (P3.6) to detect it.

Additional the AT89C2051 can also operate down to 2.7V
and you can use an unregulated voltage from only 2 AA cells.
Than you can use a 2.9V zener to give the comparator threshold
(the other input connect also to VCC).

See the data sheet for more informations (www.atmel.com).


Peter