The circle can be generated using a diffraction plate (like in the decorative winter snowflake projectors, etc). All you need is a laser pointing up, a diffraction plate for a circle shape, and a mirror to get the conical beam reversed to go down.

Or, without a mirror, just put the laser and the diffraction plate somewhat higher.

The circle can be generated using a diffraction plate (like in the decorative winter snowflake projectors, etc). All you need is a laser pointing up, a diffraction plate for a circle shape, and a mirror to get the conical beam reversed to go down.

Or, without a mirror, just put the laser and the diffraction plate somewhat higher.

Without a diffraction plate, the mirror could be mounted at an angle on a small rotating stage, with a hole in the center. Laser points up, shines through the center of the rotating stage, reflects off the rotating mirror, projecting a cone of light that looks like a circle when it hits a flat surface.