What is the simplest way to turn off some circuit when supply voltage reaches a minimum level?
Is a supply-voltage supervisory IC along with a FET switch not an option for you?

One example is the TL7712A, but there are many others -- 12V versions aren't as common as 5/3.3/...V counterparts, but they still exist.

An example circuit can be found below.  U1 is a generic 3-pin voltage monitor/reset supervisor, designed for 12V service -- more sophisticated parts can be used, of course!  M1 can be any N-ch depletion-mode power FET with suitably high ratings (Supertex/Microchip and IXYS make 'em, the DN1509 shown being the former's work), while R1 is not needed if U1 has a push-pull output.







simulate this circuit &ndash; Schematic created using CircuitLab