I thought I would throw this circuit out here as well, This is from another thread, but the application is the same, I think. In this circuit, I tried throwing a 3.3v Zener so I could limit the "reference" voltage to about 3.3v. Since the car's voltage can swing, it impacts the voltage divider's reference. But as mentioned here, this circuit will not function at all with it in there. As stated above, the zener characteristics do not apply well here. Maybe if I threw in a resistor, it may do what it is suppose to do. I think I might not even need it. I worry that the voltage could rise and create almost a 4v reference voltage, which is getting close to my "tripping" voltage of about 4.8to 5.0v. I can't really find the correct info on this comparator to know all of the thresholds. Not sure how close the voltages can be to each other before the circuit quits working. If all I need is.1 or .2 voltage differences, then all might be ok.
Stu