Schmidt shaping circuit is used to convert sine wave into rectangular wave. According to relevant public information, the step-down sine wave voltage of transformer is directly applied to the base of VT 1. In the positive half cycle of alternating current, diode VD is turned on, and the base potential of VT 1 is positive, so VT 1 is turned off and VT2 is turned on. In the negative half cycle of alternating current, the base potential of VT 1 is negative, VT 1 is turned on, and VT2 is turned off, and a rectangular wave output of 5OHz can be obtained from the collector of VT2.