"HOLOGRAPHIC" VIDEO HALO
2006 W. Beaty

ANYWAY, while flying to Florida for 2005 thanksgiving, I was sitting there with a blank pad of paper and wondering what cool stuff I could build someday, when I had a detailed vision of an attention-getting device to sell at SF conventions: glowing advertisements which float around your head with no apparent cause. Very appropriate for cons: it's a kind of propellor-beanie with lots of whirling LEDs. The perfect geek fashion accessory, one which screams "huge flaming Nerd." (Actually I think I had a similar LED-propellor-beanie idea many years ago at a con, but that was long before single-chip CPUs or ultra-bright LEDs existed ...and I didn't write it down.)

A motor spins a cylindrical rotor having vertical stripes of ~20 LEDs driven by a PIC/AVR/etc. The rotor must be fairly smooth and low-speed so it doesn't drain the batteries by acting as a fan. Rather than dangerously spinning just one LED array at 60Hz/3600RPM, multiple synchronized LED stripes allow low RPM with little flicker: 24Hz flicker isn't too bad, and with six arrays it gives four rotor revs per second. Most important: the mechanism and rotor would be almost invisible. It wouldn't be very bright, so it would only work indoors. Walk up to victims and "talk" by triggering stored sentences responding to typical questions. "IT'S A MECHANICALLY-SCANNED LED ARRAY." "YOU CAN'T BUY THESE, IT'S ONE OF A KIND." "NO, THERE ARE NO GLOWING LETTERS HERE, YOU MUST BE HALLUCINATING." "HI THERE, DO YOU COME HERE OFTEN?" Display a personal ad, or rent out the space to advertisers?

A DC motor is mounted on a transparent, heat-formed "plexiglas yarlmuke" adhered to my bald scalp. Even though the transparent rotor spins at only a few revs per second, it's almost invisible because it's built from multiple transparent "blades." As with bicycle wheels, slow rotation of numerous spokes causes invisibility via total blurring.

My first prototype had six blades made of adhesive tape supporting a transparent 12" dia. cylinder about 3" tall. It spun at three or four revs per second, and the six white stripes I stuck to the cylinder would totally vanish at this low speed. I demonstrated it at 2005 december Dorkbot in Seattle.

I didn't get very far beyond the crude proto of the rotor section wo/LEDs. I hope to use (six?) synched, phase-shifted POV strips to allow low, non-dangerous RPM. The rotor structure is made from bent 1/64" transparent acrylic sheet taken from a Walgreens "poster frame." A 4" plexi disk on the small DC motor's shaft has six horizontal blades supporting the cylinder. The cylinder is very visible in this case, but if large diamond-shaped "teeth" are cut in the edges of the cylinder, the edges (and therefore the entire cylinder) will become nearly invisible. With low mass at this low RPM, the edge moves at 10'/sec or 7MPH and can be safely stopped with a finger. Even at this low speed it drew a few hundred mA at a few volts (it takes lots of work to stir the air.) Letters composed of red LEDs, 6x20 = 120 LEDs at 20mA would draw two or three watts. More brightness and more wattage: each strip could be three LEDs, RGB, with tiny phase-shift to lay the color LED flashes on top of each other. Next task: adapt a Radio Shack 4-conductor slip-ring "phone cord untangler" to supply signal and power to the rotor section.

The real challenge would be to align all the LEDs so the six images appear very static, and don't jump around vertically per each revolution. Stretched tape is the wrong idea: the supporting "blades" need to be stiff in order to prevent distortion of the rotor or vertical wobble of the cylinder. Another challenge: to walk around without moving my head (since any head-tilting causes a huge gyroscopic effect!)