Melssj5:
Voltage regulators
DVDDough, I still dont get why I shouldnt use Voltage regulators.... are they supposed to reduce any voltage to 5volts or they are not reliable???
They are for power supplies. Signals often have near-zero currents. That's a big difference.
Use this for a car signal:
Complete electric isolation with the optocoupler.
Mind the two different earth connections! They are NOT connected. One is the car chassis and car battery GND, the other is the Arduino GND pin.
C1 takes care of most spikes; R1 limits the current to the optocoupler (can handle up to 100V - 50 mA spikes are OK for this optocoupler), D1 protects the optocoupler from reverse voltages.
Set the pinMode to INPUT_PULLUP to avoid the pin from floating. You may instead add an external pull-up, 4k7 or 10k, which would make the circuit more robust to electric noise. Only needed if your car is particularly noisy.