In the figure we have one of the many options found in this section for an intercom using ordinary phones not in use. This was obtained in a Popular Electronics from the 90s.
The purpose of this 1985 circuit is to make the sound test of type 106 SCRs (MCR106, TIC106,...
Using the CMOS 4001 we found this circuit in the October 1974 Radio Electronics magazine. The...
We found this circuit in a 1957 Raytheon transistor publication. The circuit can be assembled with...