a stitch countdown counter b and a stitch counter c whose functions will be explained in connection with the explanation of the mode of action of the means according to the present invention reference is made for this first to a first operating state of the sewing machine in which bobbin thread from the main chamber 9 is being processed as is shown in fig4 and 5 in fig4 the detection beam 12 emitted by the infrared laser light transmitter 14 falls on the flange 5 of the bobbin 2 and is reflected by same to the receiver 15 since the flange 5 has a degree of reflection as is usual for metals i e a relatively high degree of reflection a signal pulse s 1 with a relatively high voltage is sent to the microcomputer 17 during the time during which the detection beam 12 falls on the flange 5 if the detection beam 12 falls on one of the openings 11 fig5 through the bobbin 2 which continues to rotate due to thread being pulled off this beam is reflected by the surface structure of the thread roll since the degree of reflection of the surface of the thread is relatively low and substantially lower than that of the flange 5 this leads to a diffuse reflection so.