Hi all,
I want to make a car knob sensing system. It should tell me which of my four knobs in the car are up(open) or down(close). I think there are already these systems in place on modern day cars but my own is a little old 
Which sensors are the best to use ? I was thinking about a photoelectric sensor or a push button but I am not sure.
Thank You ! 
Are you planning on taking apart your car doors to put the sensors inside? Something simple like a magnetic sensor or a microswitch could possibly be used depending on what the guts of your doors look like and how the lock mechanism travels.
Connect one side of the switch to ground with the other side connected to a digital input on the Arduino. Pullup the digital inputs and look for them to be low to know if the doors is locked.
If it's running all the time (I'm guessing you want to be able to query your car from a distance to see if the doors are locked or not), then most likely you want it to be as low power as possible to reduce the drain on your battery. Passive sensors are probably your best bet.