This sensitive circuit for triggering relays was found in an old English publication. It works with alternating signals applied to its input. Transistors are general purpose types.
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...