This circuit can be used as an answering password system in a store, office, or other location. The circuit is from Intersil using components of the time. The circuit is from the 80's.
The simplest flip-flop can be built with two NAND gates as shown here. This is a Reset-Set [S-R]...
This circuit uses a GE optical coupler. Equivalents can be tried. The circuit is from the GE...
This circuit is designed to activate an automotive alarm after 8 minutes of activation. Of course...