I have had a theory that maybe by placing an object like a large satalite rotating disc a certain distance away from the planet earth.
We can rotate the satalite at a reasonable speed to perform morse code or similar. Using the planet earth as a light and a disc type mirror to reflect the light.
Im sure this would be possible yet how much distance would it cover across space.
In theory star are extremely far away. Yet it one started flashing im sure we would notice it and visa versa.