A system is provided for sensing the presence of a standard component of a locking mechanism of a door of a trailer when the door is moved to the closed position and is secured. A sensor is provided on the trailer proximate to the locking mechanism. The sensor automatically reacts when a ferrous target...http://www.google.sh/patents/US6400266?utm_source=gb-gplus-sharePatent US6400266 - Door sensor for a trailer
