This circuit, from a 303 Circuits from the 90s, uses a telephone pickup coil as a sensor to capture power grid noise, thus making it possible to locate electrical wiring.
This circuit was found in an old American documentation. The integrated circuit used is uncommon...
SST OXY Flex Oxygen Sensor The purpose of this SST sensor is to measure oxygen level in enclosed...
Circuits similar to this can be found in this section. This is from 1967 General Electric...