Comparator Latch circuit
You could do something like this- the comparator output goes low when the input passes +2.50VDC (the voltage set by the R3/R3 resistor divider, which turns on the LED D1 and also Q1. Q1 pulls the inverting input up to +5 regardless of the voltage at the input so the LED will not turn off. SW1 shorts out the transistor drive so the LED will turn off and, if the input voltage is less than 2.5V, will stay off when the switch is released. Otherwise it will come on again immediately when the switch is released. 







simulate this circuit &ndash; Schematic created using CircuitLab