[dcb]

You have a point about the key, I had kill switch in my head so it didn't occur to me that folks might be turning off the ignition.

I need to list out my assumptions I think.

1. The arduino is powered by battery power. Because:
A. there are a limited number of eeprom writes, the trips should be saved when the key is turned off, but the arduino can't do that if you are turning off it's power also.

B. there are some sleep modes for the arduino that it could utilize when the ignition is off, but I don't think the drain will be terribly noticable.

2. The arduino should sense the ignition power, Because:
A. it needs to write out the current state of the trips to the eeprom on the ignition off event.

B. If the arduino has control of the LED Backlight (i.e. fancy schmancy menu brightness option) it can turn it off on the ignition off event and save ~200 milliamps. This adds one component (transistor) to the design. Switching it with the ignition directly will add one component in the form of a resistor to limit the current, and not give user control.

So I was thinking have the arduino turn off the LED, since the arduino was already connected to ignition power (in my head) and it was a simple task to have a menu driven brightness. The LCD only takes about ~4ma so I'm not worried about it yet.

But, maybe, it just has to be a (hokey) delay thing. Like if the car hasnt shown any rpm or travelled any distance in, say, 10 minutes, then it saves the trips in their 10 minutes ago state, and turns off the backlight? Saves us a ignition power pin.