Abstract: A system for producing a control signal upon intrusion of a human part into a protected area, for example the area about the needle of a sewing machine, whereby radio frequency signals at a predetermined frequency are coupled to the human machine operator from a plate on which he sits or stands. As the operator's fingers or other body part approaches the protected area, an antenna receives the rf signal and applies it to an rf phase lock loop receiver which produces an output pulse when the received signal is at the predetermined frequency and is above a predetermined amplitude to operate solenoids which prevent or terminate machine operation. Preferably, withdrawal of the body part triggers a timer which produces a signal permitting the machine to operate for a given time which is greater than the normal time between intrusions, thus continuously checking that the system is working properly.