This circuit with the PLL4046 was found in the October 1994 Electronics Now magazine. The transistor is low power, and the volume obtained is small.
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...