First of all, I never said it wasn't possible. Two different approaches have been suggested , one using analog input the other using comparators, both are viable solutions. My point is that you still haven't provided any explanation of "your system" or why it is necessary for you to implement either of the two suggested solutions. :
Let's say we have a 0-48V digital signal which we want to measure with our Arduino.
We use voltage divider to minimize it to 0-5V.
Is there a way then, to a have a LOW input when signal is =0 (or almost 0) and a HIGH input when signal is from approximately 0.1V-5V?
Tells us virtually nothing. What is "the system" ? Where does the 48 V come from ? What are you sensing ? Why do you want to do this ? You haven't told us any of these things. If a person goes to the doctor and asks him if he has a medication that will do this or that, do you think the doctor will just prescribe it ? Of course not. His job is to treat the problem, not the symptom. He will try to find out why you are asking for thsose meds. Patients who can't get the drugs they want from their doctor buy them on the street. We still don't know why you want it . What is your problem ? Incidentally, Do you understand how the comparator circuit works ? (what the thresholds are, how to calculate a voltage divider and how a comparator works ?)