Special thanks to UPNALab (https://www.instructables.com/SonicSu...) for providing all the tools necessary to build this Halloween project. Much of the heavy lifting was done by their work.

Introduction:

I've wanted to make a halloween project for a while now. The neighborhood kids always get a kick out of the decorations we put out. When I came across the SonicSurface project I knew I could retask it to create something scary for the local kids.

My current intention is to conceal two 256 transducer arrays above a dusty surface and remotely trigger (or via simple motion sensor) "Spirit" writing on the surface. I want to make the surface obvious that nothing is concealed within, so the kids can actually touch the dust too... Resetting the dust (baking soda?) would have a bin of baking soda and a servo with compressed air to "redistribute" between groups of kids.
https://3dprintzothar.blogspot.com/2021/10/ghost-writing-halloween-project-using.ht