A certain knock at the door may tell you if the person behind it is the one expected or just somebody else. Each knock on the door itself cannot tell you too much, it can be lighter or stronger and that’s all about it, there’s no information on pitch here. But a certain sequence of knocks can be an actual secret code. The secret knock is encoded in the actual timings of the knocks, the time between each knock creating a distinguished pattern.
Secret knock detector with 555 – [Link]