so ive got a ATmega 2560 which ill be putting into my car, etc etc, the story doesn't really matter... what im wondering, is ill be measuring data from the car (as in measuring devices powered directly from car battery), but the board will be powered by a laptop in the car, so for the power to flow into the board for it to be measured it has to be grounded back to the car battery?, now would it ground though the board, though the usb, though the laptop, though the charger to the battery, or do i have to hook up some other ground to the car (if so where off the board? like can the ground pin be used that way?)
background story: will use the AtMega to capture all the data from the dashboard (voltage, temp, fuel, oil pressure, speedo, tacho) and transferring the data over the usb to the pc to be displayed a "virtual dashboard"
"The story" is often also called "the application" and does matter very much.
now would it ground though the board, though the usb, though the laptop, though the charger to the battery, or do i have to hook up some other ground to the car
Ground is a common reference point. All electrical devices must share a common reference point.
background story: will use the AtMega to capture all the data from the dashboard (voltage, temp, fuel, oil pressure, speedo, tacho) and transferring the data over the usb to the pc to be displayed a "virtual dashboard"
You can likely connect the arduino ground to some clean metal on the car frame. If you look in the engine compartment you will see the - of the battery doing the same (in the US anyway - we used to have '66 MGB that had Positive to ground instead).
If you have a cigar lighter or some other power receptacle, you can pick it up there also.
So... ? yes/no i need a ground wire from arduino board to car battery? if so how
As CR said. However if you still want a real ground, just drive a ground rod into your driveway and attach a real long cable to your car frame, that should keep your grounded for awhile.
