Darlington's are for high-gain. Why use one to take 12V down to 5V?
If you get a miniature glass wire ended neon this will fire when coupled to an ignition wire in this way. I did this way back in the 60's.Now if you put a photo detector transistor against this and wrap it in black tape you will have made an optical isolator so the flashes are safely read by your arduino.
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