This power supply protection circuit using a relay was found in a 1967 documentation. The components are from the time, but current equivalents can be used depending on the power source current.
The frequency of this oscillator depends on T, which for the audio band can be a common output...
This circuit can work with almost any JFET. The circuit is from an Electronics Now from April...
This voltage-controlled oscillator uses components that are still common in our market. The...