Abstract: Apparatus for detecting whether one thread selected from a number of threads is in motion, while no other thread is in motion, or whether a set of threads selected from a number of sets, each having two or more threads, is in motion, while no other set is in motion. The apparatus includes a signal generator associated with each thread for generating an electric signal in response to the movement of the associated thread. The signal generators are coupled sequentially to signal indication apparatus. If the prescribed thread movement is detected, the signal generating apparatus inhibits generation of an alarm indication. If the prescribed thread movement is not detected, the signal generating apparatus activates a trigger device to initiate an alarm.