This circuit uses a Motorola component from the 90's and is suitable for the diode-transistor logic of the time. The circuit can be adapted to use more modern equivalent logic.
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....