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 is from a 1968 publication. It uses germanium transistors from the time, having...
This circuit can be used as shield or to drive powerful loads from low current switches such as...
This is a 6-transistor transistor receiver manufactured in the 60s. The circuit is a...