Speaking of tolerances. I am wanting to attach this circuit to a run off a car battery. How do i prevent the Amperage of the battery from frying my Mega? Would a level shifter do this?
Can you explain what a level shifter does?
Level shifters are for shifting logic levels between two ICs that operate at different voltages on the logic pins (not power pins), i.e. interfacing a 3.3V GPS to a 5V Arduino is an example. What you need for the battery is simply a voltage regulator. The 7805 is the cheap and standard choice for this, though there are many options out there.
You can also get a 5V regulator on eBay and many are more efficient than a 7805 or have more capacity. Search there for a pre-built one.
Output will be held very near +5V on this circuit for a +12V input, it may not be the most efficient option.