The disclosure relates to an apparatus for generating an elec­tric signal corresponding to thread movement in a thread brake in textile machines or different types, a transducer element (9, 21) of piezoelectric type being disposed in direct contact with an element (3, 18) included in the brake and being, by the intermediary of a further element included in the brake, or directly, in contact with that thread (13) which is to be braked and sensed, in order that those movements which occur in the thread (13) on braking thereof shall be transmitted to the transducer element (9, 21) and the elec­tric signal corresponding to thread movements is impressed on a monitoring circuit for indication of whether the signal ceases during a period of time when the signal should exist, and possible arrest of the operation of the machine on signal loss.