Monitoring circuit for a knitting machine

A monitoring circuit for a knitting machine, comprising at least one yarn feeding device having two yarn monitoring means provided with two switches, said switches being connected via a first rectifier circuit and an indicating lamp to a supply line connected to a supply voltage source in such a way that when one of said two yarn monitoring means is operated, said indicating lamp is only activated by one half-wave associated with said yarn monitoring means and belonging to an a.c. voltage signal generated by said supply voltage source, the improvement wherein said a.c. voltage signal generated by said supply voltage source comprises first intervals during which said signal has exclusively positive a.c. voltage half-waves and second intervals during which said signal has exclusively negative a.c. voltage half-waves, and said supply voltage source includes an a.c. voltage source, first and second rectifier elements which are of opposite polarity and connected to said a.c. voltage source, and a periodically operated change-over switch which alternately connects said rectifier elements to said supply line.