With this circuit it is possible to use a pair of old telephones in a wired communication system. The circuit is from a Popular Electronics from the 90s.
The circuit below shows how to use an LM139 comparator or series equivalent as an amplifier. The...
We found this receiver circuit for modulated light in a 90s Popular Electronics. Equivalent audio...
The circuit was found in the February 1992 Radio Electronics magazine. The scale can be changed by...