The input sine wave voltage provides working power for IC 1 through a half-wave voltage doubling circuit composed of C1,C2, D 1 and D2. IC 1A constitutes an amplifier to amplify the input signal, which is inverted and shaped into a square wave signal by IC 1B and IC 1C, and then amplified by IC 1D, IC 1E and IC 1F for output. R2 is used to adjust the amplitude of the output signal. The circuit shown in the figure can output a square wave with good performance from 20 Hz to 20 Hz. In use, it is suggested that the effective value of input sine wave should be greater than 1.5V to ensure the quality of output signal. The minimum value of input sine wave voltage should be greater than 750mV, and the peak value of output signal should be about 2 volts. The amplitude of the output square wave is proportional to the amplitude of the input waveform. When making D 1 and D2, diodes made of germanium materials, such as 2AF series, can also be selected, and the minimum effective value of the input signal can be lower.