toggle 12v as high and 11v as low for arduino
The part you're looking for is a comparator. Set a threshold with resistors at 11.5 volts. The arduino is not required here, but could still be used if additional behavior is desired.







simulate this circuit &ndash; Schematic created using CircuitLab

Alternatives:


There is probably a diode-based option, but the comparator will be simpler.
As mentioned by others, you can use a resistor divider into an A/D and then do the threshold logic in software.