This circuit was found in a 1966 General Electric documentation. It makes use of direct contact water sensors and therefore requires an isolation or low voltage relay drive transformer.
This circuit from an old Argentine publication shows how it is possible to use a tunnel diode in a...
Found in documentation from the 90's, this circuit has a narrow pass band around 1 kHz. The...
With this circuit it is possible to obtain an input resistance of 100Gohms for a multimeter, with...