Originally Posted by

**2k1Toaster**
Hardware:

If you are getting a macimum of 9.99KOhms when idle, that is the value you need to use to determine the rest.

Assuming your car puts out 14.4v volts when on (some are 13.8, some are 14.4, but again always design for the worst) and you want a maximum of 5v minus some happy factor to make it about 4.5 (if it spikes for somereason, and better safe that sorry) then you get:

4.5 = 14.4 * (10k/(10k + Rmystery))

Through some voodoo magic of algebra you get Rmystery to be 22KOhms.

So that means you should get voltage out like this (at most, your car will be different depending on the voltage of it running):

Software:

I did redo the logic section recently, I may have goofed something. I will take a look at it.