Building a 555 timer's SR Flip Flop using Op Amps
Have a look at the Vo-Vi curve of a hysteresis cycle:







simulate this circuit &ndash; Schematic created using CircuitLab

If you are able to force Vi above the positive threshold, the output will go high and will stay there when Vi returns to 0. If you you force Vi below the negative threshold the output will go low and stay there forever.

You can achieve this (well, actually the inverse) with







simulate this circuit