I painted my wall with glow in the dark paint and built this double pendulum to which I could attach a small UV light.  In the dark, the UV light will leave trails on the wall that trace out the chaotic motion of the double pendulum.

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This involved figuring out how to cut acrylic cleanly (thank you Bosch T102BF blade), because too much heat and it melts.  Also got a bunch of bearings from ebay and steel shafts that I had to cut with a hacksaw.  I designed the mount with a tightening screw to hold it in place.  It works pretty well.