The operational amplifier used in this circuit is uncommon nowadays. We obtained this circuit from documentation from the 1980s, but it can be adapted to use more modern equivalents.
This circuit is from a Popular Electronics from the 90s. It uses an opto-coupler with SCR directly...
This high selectivity circuit for single tone decoding was found in 1970s documentation. The coil...
This sensitive voltage measuring circuit has a very high input resistance, of the order of Mohms,...