Electronic – Replacing a tactile switch with an ultrasonic ranger

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

What you're describing is a textbook case for a comparator. A comparator is superficially like an opamp, but it's specifically designed to output logic level values: high when the positive terminal voltage is higher than the negative terminal, and low otherwise. Most also have some hysteresis, to prevent rapid switching when the voltage is near the threshold.

When choosing one, note that most comparators have open collector outputs - they pull low, but not high. This makes interfacing between the logic level of your digital device, and the possibly different voltage levels of the signal you are testing, easier - just include a pullup on the output to logic high.

^{simulate this circuit – Schematic created using CircuitLab}

In the example above, the voltage divider R1 R2 sets a reference voltage to compare to. Input on the + terminal is compared to the reference; if it's greater than the reference, the comparator output switches off, and the pullup R3 pulls the output voltage high. If it's less than the threshold, the comparator switches on, pulling the output voltage low.