This audio oscillator is from a 1957 Raytheon publication. The tone generated depends on C2 and C2 and the handset must be high-impedance magnetic. Equivalent transistors can be tested.
This very simple oscillator, with only one transistor, was found in a Japanese magazine in 1967....
This simple bridge circuit can be used for impedance or resistance measurement using a high...
This circuit has the intensity of the signals controlled by the light that falls on the sensor....