Simplest way to get a rising edge (0 to 1) for a digital input from a selectable value of an analog signal?
What you're describing is a textbook case for a comparator. A comparator is superficially like an opamp, but it's specifically designed to output logic level values: high when the positive terminal voltage is higher than the negative terminal, and low otherwise. Most also have some hysteresis, to prevent rapid switching when the voltage is near the threshold.

When choosing one, note that most comparators have open collector outputs - they pull low, but not high. This makes interfacing between the logic level of your digital device, and the possibly different voltage levels of the signal you are testing, easier - just include a pullup on the output to logic high.







simulate this circuit &ndash; Schematic created using CircuitLab

In the example above, the voltage divider R1 R2 sets a reference voltage to compare to. Input on the + terminal is compared to the reference; if it's greater than the reference, the comparator output switches off, and the pullup R3 pulls the output voltage high. If it's less than the threshold, the comparator switches on, pulling the output voltage low.