This circuit was found in old documentation, but the setting applies to any operational. Resistors and capacitors determine the frequency. For the indicated values the frequency is 10 kHz.
Using SCRs, this circuit from a 1967 General Electric manual allows full wave load control with SCRs....
This interesting circuit from a publication from the 1970s, measures the intensity of the sound...
This simple NPN and PNP transistor tester configuration can use the low current range of a common...