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 circuit produces a signal at a frequency of 455 kHz, based on a ceramic resonator CSA455E, by...
This simple unijunction circuit is from a Radio Electronics from July 1963. It can be assembled with a...
Hughes suggests this circuit in a 1968 documentation. The circuit uses old transistors that can be...