The circuit shown in the figure can be designed with more modern operational amplifiers, as it is from 1994 documentation by National Semiconductor. The power supply must be symmetrical.
This circuit is from a July 1970 Radio Electronics magazine. The sensors are metal probes. The 18 V...
This commercial medium and short-wave radio was manufactured in Japan in the 60s. The circuit has...
General Electric suggests this circuit in an 80's documentation. The circuit makes use of PUTs...