In a line there're voltage levels representing words of dots and dashes. In particular, the device is intended to raise an output each time a given pattern of five short/long signals (say 1s and 2s durations, respectively) is recognized (and restart if nothing comes in for a given time). A sort of Morse code recognizer (Arduino Uno preferred, but others are also OK) would be a good starting point. Any hint is welcome, I'm new in this community.
Use a GPIO of an Arduino and check (at the speed you need) the HIGH/LOW state.
Depending on the speed at which is sent, you can check the dots and dashes being sent, e.g. by keeping a timer since the last change checking if it is a short or long 'HIGH'.
You have to sample often enough to check if there is a drop (between dots/dashes).