This circuit generates frequencies determined by R3 and C1. We found it in the Radio Electronics magazine of May 1988. The power supply must be symmetrical.
This circuit generates sinusoidal signals between 300 and 3000 Hz. We find a 303 Circuits from the 90's....
This amplifier for audio transmission lines was found in a 1961 American publication. The critical...
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...