The 1kHz Audio square wave generator mainly uses comparator LM324 and R2, R3, R4 and C1.  The 3-pin of LM324 receives an input reference voltage GB / 2 through the voltage divider circuit R1, R2. R3, R2 constitutes positive feedback, and along with that a  negative feedback is framed by the integral circuits R4, C1. R2 and R3 taken before the value, making R2 / (R2 + R3) = 0.5, the square wave repetition frequency f = 1/2R4 · For example, if C1 = 0.033pF, R4 = 15kΩ, the frequncy will be equal to  f = 1kHz. Changes in C1 or R4, can change the oscillation frequency.