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.
This circuit appears elsewhere on the site. The circuit shown is from an American 1980s...
This circuit can produce signals up to 100 kHz depending on C1 and R1 adjustment. The output has a...
This circuit is from an old publication from 1977 and can be implemented with more modern...