Electrical equipment overheating "brake sound" sound warning circuit

The picture shows the analog sound warning circuit of electrical equipment. It consists of a temperature overheating electronic switch, a controllable multivibrator, an analog sound circuit, an audio power amplifier circuit and an AC step-down rectifier circuit. 555 and R2, R3, Cl, etc. form a controllable multivibrator. Its multivibrator frequency is: the oscillation period To (=1/fo) of the parameters shown in the figure is 10s.