[jojomodding]

Well, you can use tap code: https://en.wikipedia.org/wiki/Tap_code

Or you can use "Morse tap code" where two close knocks represent a dot, and if the gap between them is slightly longer it's a dash. That takes two knocks for one dot/dash but it works and is (was?) used in practice.

[king_zee]

Yes JofArnolds response is similar where you just switch a flag on/off to draw dots and dashes and dits