several circuit sections the configuration of the operational circuits shown in fig4 is for use with essentially constant speed operation machines each circuit described in the following paragraphs computes conditions for a different failure mode b blocks 20 through 26 comprise a circuit which indicates that a thread break condition exists the failure of the thread 1 to move for more than one or several duty cycle time periods represents a thread break comparator 20 provides a high voltage output only if the signal from block 19 is above noise level i e thread movement is ongoing block 21 provides a peak detector and voltage fall time delay the time delay exceeds the maximum anticipated duty cycle off time comparator 22 provides a high voltage output if the output of block 21 falls below normal led 26 indicates that the thread break circuit senses a thread break disable circuit 24 cancels the output of comparator 22 if the latch 25 is energized prior to receipt of a break indication from thread break blocks 20 22 the purpose of the disable function is as follows after the machine being monitored is stopped several failure modes would be indicated by the circuits of fig4 the identification of which failure mode initiated the stop will expedite correction by the machine operator disable circuit sections 24 44 51 allow only one of the led indicators 26 45 and 52 to light the illuminated led indicates which failure identification circuit caused the stop c blocks 28 to 32 comprise a knot snag circuit that determines if a knot inconsistency or snag condition exists if the sensor signal magnitude or increasing rate of change exceeds normal the logic of the knot snag circuit indicates.