Comparator Hysteresis Calculator
This calculator will compute the resistor ratio R1/R2 and reference voltage for given high and low threshold values for a hysteresis curve, or the threshold values given the reference voltage and resistor ratio.
The most commonly know comparator is the LM339 and it's various flavors. Rail to rail op-amps can also be used as comparators. The LMV324 is one such op-amp that is affordable, and works well as a comparator.
Comparator Circuit Schematic