Notes

This ultrasonic device may be used to deter small mammal pests from encroaching upon your part of garden.

The circuitry consists of a phase locked loop - CMOS 4047 - wired as a 23KHz oscillator. Oppositely phased voltage outputs are current amplified by complementary pairs of unbiased output transistors to drive 24V peak square-wave into a Motorola 3.25 inch Piezo Horn Tweeter.

The current draw is around 120mA, so an external power supply is recommended, but the small output transistors do not need heatsinks because most of the energy is dissipated by the transducer.  The potentionmeter is used to preset the output frequency.
 

The piezo was a standard 85mm square Motorola Horn, Maplin part number WF09K or WF55K. These are rated +/-3dB to 28kHz.

Do not put your ear near the tweeter when tuning – you might not hear the output, but you will feel it!