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.
This circuit was published in an article in Radio Electronics magazine in October 1988. It uses...
This two input simple mixer uses two transistor and can b e upgraded to more inputs. Input...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...