A yarn processing system (T) consists of a yarn feeding device (F) and an accessory device (Z, Z1, Z2, Z3) associated to the yarn feeding device (F), said accessory device comprising a device part (K, M, 4) which is driven using a signal representing the rotational speed of a component of the yarn feeding device. Said signal is generated by an electronic sensor (S) galvanically separated from the control system (C) and the drive means (M) of the yarn feeding device (F). Said sensor (S) also is structurally independent from said yarn feeding device (F). The sensor is located at or close to the yarn feeding device (F) at a mounting location (X, X1) where a part of a rotating magnetic field is detected by said sensor at the mounting location. Said part of the magnetic field originating from inside of the yarn feeding device is not used for the operation of the yarn feeding device at the mounting location of the sensor (S). The sensor comprises a pick-up head responding to said part of the rotating magnetic field which part is present at the location of the sensor but is not used for the function of the yarn feeding device at said location.