This circuit was found in ancient American documentation. It makes use of the operational amplifiers of the time, but it can be implemented with modern equivalents.
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...