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 General Electric circuit is from the 80's. The PUTs used in this meter are not common...
Found in a 1964 Electronic Design, this circuit generates pulses based on an avalanche diode of...
This circuit, which consists of an audio oscillator, was found in a Radio Electronics magazine from...