The RCA integrated circuit used in this control is uncommon today. The circuit is from an Electronics Now from July 1995. The sensor determines the value of the adjustment potentiometer.
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...