We found this circuit in a documentation from the 70s. The circuit uses amplifiers from the time, but it can be implemented with modern equivalents.
This circuit was found in a 1975 documentation. Current general-purpose transistors can be used,...
We found this circuit in a publication for radio amateurs in 1974. The bipolar transistors can be...
This conventional humidity or water alarm circuit was found in an English documentation from 1974....