thread monitor for monitoring double lock stitch shuttles or hooks and chain stitch machine shuttles or loopers . during normal operation of the sewing machine , the shuttle thread is used as a reflection surface . the scattered light beams reflected by the thread enter photodetector 10 of the thread monitor 8 . if the support surface 7 , 28 is covered by thread , light beams will reach said photodetector 10 , as a result of which a control circuit 12 connected to it issues a warning signal .