This circuit is from a Radio Electronics magazine from July 1979. It uses the well-known IC 7107, a liquid crystal exciter that became popular in circuits of this type at the time.
This circuit using an operational amplifier was found in old American documentation. It can be...
If the circuit oscillates, emitting sound, it is because the transistor is good. This simple...
Like the previous circuit, this is used in the analog part of the telephone lines. The same...