Method and apparatus for detecting so-called moire effect during spinning

In the method, the number of defective portions of the yarn, occurring at periodic spacings within a predetermined time period, is detected to provide a detected value. This detected value is compared with a predetermined desired value of the number of defective portions within such predetermined time period. If the detected value exceeds the desired value, an output signal is provided or released to a stop device, control device, or the like of the spinning machine. The device comprises a thread monitor connected to a pulser which produces pulses responsive to variations in the yarn thickness. The output of the pulser is supplied to a thick portion monitor which detects long pulses produced responsive to relatively long defects in the yarn, and the thick portion monitor is connected to a time switch which, in turn, is connected to a signalling device, stop device, or control device for the spinning machine. A moire monitor is connected, in parallel with the thick portion monitor, between the pulser and the time switch, and detects short pulses produced by the pulser responsive to the periodically occurring defective portions of the yarn. In one embodiment of the invention, the moire monitor comprises a RC circuit receiving the pulses and connected with an additional resistor. In another embodiment of the invention, the moire monitor comprises a counter and a timing pulse generator.