I am designing an circuit for use in an automotive environment, the circuit itself will be powered constantly using a automotive grade switch mode regulator (National/TI) to provide 5V. I need to be able to detect when the ignition is in the on, which is obviously going to be a 12V+ signal. What is the easiest way to reduce this to be able to use as a input to an AtMega? I guess a voltage divider may work, but do I need to concern myself about load dump etc? Analog electronics isn't my strong point.