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.
We found this project in a 1982 publication. What we have here is an audio signal relay trigger, which...
We found this circuit in an 80's Radio Shack publication. The circuit uses a company buzzer, but...
This circuit was found in a 90s Popular Electronics. It uses a varicap that admits equivalents. The...