In the figure we have a simple way to produce tone in a piezoelectric transducer. The frequency depends on the capacitors and the supply can be done with voltages from 1.5 to 6 V.
This circuit is from a 1967 US publication. The configuration can serve as a basis for designs...
This ultrasonic insect repellent was found in American documentation from the 1980s. The...
Oscillators up to some megaherts can be made with comparators like the LM139, 239 or 339. Theses...