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.
Found in the American 73 magazine for radio amateurs, this circuit can test common quartz...
This circuit for the automatic activation of a small lamp (or high brightness LED) was found in...
This circuit provides a 5 V negative output with a maximum current of 1 A. The circuit is from the...