In a general context, however... and note that there are likely better solutions in this specific case based on what I've read above... but in a general context you can turn an analog input into a digital output by using a comparator
https://en.wikipedia.org/wiki/Comparator
There are a number of cheap op-amp/comparator chips out there... you feed your analog sensor into one input on the comparator, and use a potentiometer as a voltage divider on the other input to provide a reference voltage. When your input voltage exceeds the reference voltage, your signal goes high (or low, if you want to hook it up that way).
Google a bit and you'll find sites like this
http://home.cogeco.ca/~rpaisley4/Comparators.html but maybe check out some of the options above before using this solution in this specific case.
Jason