biobug.org say that :

This is my quick build schematic. (Click to see the entire thing) The voltage divider on the left is so that the arduino can report the system voltage. it’s made of 3 10k resistors since I always have those on hand. The result is 1/3 of the 12-14v DC that the car has. From this, we can infer the state of the car: >13 is started. I plan to toss in a ds180 1 wire temp sensor later on as well. The pink highlighter was used as I wired things up.

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