For the guys asking what I am trying to do, I have a switch in my car that is fed with a 12v source (feeding it with 5v is not an option) and when pressed it sends 12v out that i want to send into the arduino to trigger various actions
Now I understand what you want do do.
1st. NO need for a 7805.
2nd. Use opto-coupler. So when a 12 V ( let call it HIGH ) and 0 V ( Let call it LOW ) Got me so far ?
Look into the 4N35 datasheet. http://search.datasheetcatalog.net/key/4N35
Use a 910 ohm limiting resistor for the LED inside the opto-coupler.
12 V - --- 910 ohm ------- Opto LED ----- GND of the car. Got me so far ?
3rd. A resistor of 10 K -- +5 --- 10 K --- opto-transistor --- GND of the Arduino
And use a digitalRead ( optocouplerpin ); to read the info.
That is the simple idea I can think off.