This variable frequency circuit was found in a Popular Electronics from the 90s. The circuit outputs to a small speaker and can be powered by voltages as low as 3 V.
This circuit is from a documentation from the 90s. The transistor can be any general-purpose FET...
This is the 1968 MR480 commercial cassette recorder circuit. The circuit was fully transistorized and...
Taking advantage of the two stable states of a tunnel diode it is possible to build a switching circuit...