indicated by arrow 30 photoelectric cell 16 functions as a sensor and is mounted on bracket 14 so that it registers with the aperture 26 of the disc 21 when the aperture rotates in front of the cell the photoelectric cell 16 can be the type that includes its own light source and the disc 21 can be of a color that contrasts with the color of bracket 14 so that a change in light reflection is detected by the cell when the aperture 26 registers with the cell on the other hand a light source can be placed behind the rotatable element 15 so that light passes through the aperture 16 to the cell when the aperture registers with the cell as illustrated in fig3 the control circuitry for the thread monitor comprises a conventional on off switch 35 that is connected between ground and transistor t1 a normally open start switch 36 extends from a source of positive current through conductor 41 resistor 37 to the base 42 of transistor t1 the solenoid resistor 38 of the clutch 39 of the sewing machine motor 40 and sewing machine 10 is connected between a source of positive current and the other terminal of transistor t1 when the on off switch 35 is closed and when the start switch 36 is closed a circuit is made through conductor 41 resistor 37 to the base 42 of transistor t1 whereupon a circuit is made from the positive source of current through the clutch resistor 38 transistor t1 on off switch 35 to ground a circuit is also made.