How do you convert a changing input (square wave) into a single high signal?
You need a retriggerable one-shot, like the 74HC423  The first rising edge will make the output high, and each subsequent rising edge will keep it high.  The output will not go low until there is a time period between rising edges that exceed the output pulse width.

In the world of logic chips, one-shots aren't the greatest of ideas, and are considered to be cheats in many cases, but here it seems to make OK sense.