We find this circuit in an old documentation, but it can be made using modern transistors. The circuit trips, producing a continuous sound when the sensor is wet. The 1 mOhm resistor is actually 1 Mohm.
This power stage with transistors is stabilized by two lamps. The circuit was published in Radio...
This dual balanced modulator configuration was obtained in a 1980s telecommunications...
This circuit was obtained from the Linear Technology 2005 Application Manual. The circuit sensing...