For analog lines, this circuit was found in the August 1994 issue of Electronics Now magazine. The transistors are common general-purpose types.
This SCS counter circuit was found in a 1964 General Electric documentation. The circuit has a...
This simple 1-transistor reflex receiver was published in Radio Electronics magazine in September 1959....
Operational amplifiers used in this 80's circuit is not very common today, but the setting can be...