This circuit was obtained in the 1990 Motorola Linear Databook. The circuit uses a 139 series comparator, but other series can be used. The power source need not be symmetric.
This circuit was found in a 1977 publication, but it can be easily assembled, although unijunction...
This alarm circuit was found in a 1982 publication. It uses components that can still be found...
Found in 90s 303 Circuits, this circuit is intended for interfacing analog black / white computer...