This circuit was found in a 1985 documentation. It provides an audiovisual indication of the presence of moisture or water in the sensor. The circuit can be powered by voltages from 6 to 9 V.
The XR2206 is not a common component today, but if you have one you can build this function...
This PUT circuit was found in a 1967 General Electric SCR manual. The basic component is unusual, and...
This ultra-low power metronome was found in a 1991 documentation. The output transistor is general...