This sensitive circuit was found in the June 1995 Electronics Now magazine. R1 can be from 10k to 1M depending on the desired sensitivity. Power is supplied by the relay.
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...