This circuit uses a power component that is not common today, being recommended by Motorola in the 90's. Note the need for symmetrical power supply.
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....