as turns on the surface of the bobbin 18 under formation in well known manner in its continuous travel from the exit of the twisting spindle the twisted yarn 5 is guided by the eyelet 6 which contains it while it undergoes circular movement in the direction of the arrow 14 between the positions indicated by 5 and 5a and vice versa see fig2 said circular movement of the twisted yarn 5 is present between the emitter 21 and receiver 36 of the opto electrical transducer 1 of the present invention the emitter 21 emits a monitoring beam of inspection light which is projected onto the receiver 36 the receiver is a photo diode the signal which it receives being a signal amplitude modulated by the presence of the twisted yarn 5 at the receiver 36 said signal is filtered by the filter 34 to separate the useful signal from the optical interference of the surrounding environment it then passes through the amplifier which sufficiently amplifies the signal to make it usable in the subsequent stages i e it raises the signal to a level sufficient for subsequent processing it then passes through the demodulator 31 which extracts that signal part modulated by the presence of the twisted yarn and hence excludes the carrier part of the signal in order to ascertain moment by moment the soundness and quality of the twisted yarn 5 under observation the signal then passes through the filter 29.