I want the LED to be off when there is a certain input voltage lets say 5v (the exact value doesn't matter), but when that voltage drops I want the LED to turn on. I would also like the LED to remain on even when the voltage goes back up to 5v. I was thinking of using some sort of logic gate combination to keep the LED on even when the voltage goes back up to 5v, but I'm not sure what to do to keep the LED off for a high voltage value and on for a lower value. Would I need some sort of transistor? Any help would be great!