dc motor, hall sensor 10-20VDC to arduino
Open collector Hall sensors generally don't have any pull-up built in or on board, so although the supply to the sensors is a higher voltage, the output will be only as high as the pull-up resistors you add - which should be going to the 3.3 or 5V rail on the Arduino. Depending on Arduino model, the internal pull-up resistors alone may be sufficient, so long as the speed/response time requirements aren't that high.