This circuit is a 555 timer IC connected as a square-wave generator. Its base frequency is approximately 45 kHz, as determined by the values of R1, R2, and C1. The 45-kHz carrier is frequency modulated by a modified trapizoidal voltage waveform applied to pin 5 of the 555 timer. That modulating voltage is developed by a network consisting of C2, R

3, and R4 connected across one leg of the bridge rectifier. The sweep is approximately 20 kHz on each side of the base frequency. The speaker is a 2-inch piezoelectric tweeter.

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