This transmitter is designed to work with the next circuit as a remote control transmitterand receiver. It is only a single channel and you could once get multi-channel chips for the whole job. However ultrasonics have fallen out of favour commercially so I think most of these chips are now obsolete.
This is a fairly commonly published self-oscillating driver for ceramic (piezo) resonators. However with some resonators it can be a bit temperamental: the 1n0 capacitor across the top transistor of the totem-pole helps it to start up. Some resonators won’t need this.
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Also, the impedance curve of a piezo device is very complex and I found that some of them resonated at the wrong frequency (too low). A 1n0 cap in series with the resonator cured this – this cap is not shown here.
The two transistors to the left are a conventional multivibrator oscillating at 480hZ to modulate the transmitter. If I re-designed the circuit today I would use a 555 here as the old multivibs used to have problems starting sometimes. Use of the asymmetrical resistors and capacitors values helps this.
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