An assembly for detecting and signalling when an object enters a work zone including a plurality of transmitter-sensor pairs serially connected along a section of the work zone perimeter for detecting when an object passes therethrough. Once an object is detected, an optical warning signal is activated....http://www.google.de/patents/US5552767?utm_source=gb-gplus-sharePatent US5552767 - Assembly for, and method of, detecting and signalling when an object enters a work zone
