Found in a 90's EDN this circuit can be used as a shield. The transistor is a general-purpose type and the frequency of the emitted sound is determined by R1 / C1.
Found in a 1967 American publication, this circuit with feedback for two transistors has a gain of...
This circuit was found in the Spanish magazine No longer in existence, Mundo Electrónico, in a...
This circuit uses transistors from the time it was published but can be mounted with current...