Electronic – arduino – How to drive an ultrasonic transducer with an Arduino

If you have an available timer channel, and are running at a fairly fast clock rate either off a crystal or with the calibrated internal oscillator, I'd argue you should at least give using that a solid try before resorting to external circuitry - which you are unlikely to get 1% accuracy out of anyway given that it has a capacitor in it.

You can likely time the return of an internally generated signal as accurately as an externally generated one - at worst, if you are willing to loop the signal back into a different pin and thus directly reach a hardware timer with no interrupt latency in the way.

In terms of other methods, I believe it is possible to drive the transducer using a resonant circuit, which could arguably be the most "simple", however getting an accurate frequency and clean start transition may prove challenging. And if there's an unused timer channel and pin available to drive it anyway, then the argument could be made that an external generator, no matter how "simple" is more complicated to fabricate.

Depending on the properties of your transducer you may still need a transistor or other drive amplifier at lest to get maximum output.

Recently, I fixed an ultrasonic cleaning device (f = 40 kHz). The circuit consists of two high voltage bipolar transistor feeding a resonant converter. The piezo actuator is connected to the secondary of the converter's transformer. I have strong reason to believe that the circuit is of the Royer resonant converter type because it has two bipolar transistors, see this answer for more info.

There's another self-oscillating schematic with just one transistor in Sam Goldwasser's Sci.Electronics.Repair FAQ

Some clever techniques allow you to build the ciruit in a way that it catches onto the resonant frequency of the piezo acutuator, maximizing the output amplitude just right.

For your circuit, it would be a good starting point to check whether the triac has the job to turn the rest of the circuit on or off or if it is used to modify the power using a phase angle control technique. The rest of the circuit around the two bipolar transistors will very likely be some sort of resonant oscillator or converter stage.

Use caution. The entire thing is likely connected to the mains supply, featuring high and dangerous voltages with reference to earth ground (= your body).

Related, with a schematic: Replacement Transistor for Ultrasound Cleaner