Reading +/- 12V with 3.3V micro
Eg. 

Consider the input at -12V we can write 3.3/R1 = 12/R3
Consider the input at +12V we can write 3.3/R2 = (12-3.3)/R3
Pick R3 to be something in a good range, say 10K

Then the results fall out as below. 

If you want to allow for a bit more such as +/-14V, just substitute the values, and you can scale the three resistors as desired up to the maximum your ADC likes to see at the input, or the minimum your loading permits. 







simulate this circuit &ndash; Schematic created using CircuitLab