are smd type leds i e extremely small surface mounted devices which are wired in advance to the flexible printed circuit 13 in the illustrated embodiment the emitting sensors e are 4 and precisely a sensor e 1 meant to detect the thread coils incoming onto drum t and then to monitor any thread breakage a sensor e 2 meant to detect the condition of complete filling of the thread stock on drum t and finally a pair of sensors e s and e z which allow to count the coils going out from the drum in case of thread stock accumulated in the rotation directions s or z of rotor 3 respectively sensors e s and e z are arranged at a same short distance on both sides of the hole f housing thread stopping pin 23 and hence in a perfectly symmetrical way both with respect to said hole f and to sensor e 2 this close and symmetrical arrangement of sensors e u2014 made possible by the particular structure of sector 4 s and by the one which will be described later of control block 10 u2014 allows to have a highly stable and accurate signal when the thread passes on the sensors said signal being perfectly symmetrical.