Based on a CA3059 zero-cross switch, this circuit was found in the July 1995 Electronics Now magazine. The circuit is not isolated from the mains.
Found in a 1967 Electronic Design, this transistorized circuit compresses and then expands a pulse....
This squelch is activated by its own noise, acting automatically. It is used in communications...
This is the battery-powered version of the previous circuit. The circuit is from the same Radio...