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Guy Marsden
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David Bruckenstein
"Comet" Clock

May 2020

This is my 5th collaborative project with Dave Bruckenstein aka "The Crazy Clock Guy".  We call this one "Comet" and it uses thermochromic paper to display time.  Thermochromic paper converts heat into colors like a thermal imaging camera - except much more slowly.  Small light bulbs are used as heat sources behind the clock face.

The clock is completely black until you wave your hand in front of the display.  For the next sixty seconds tiny light bulbs heat the thermochromic paper from behind - the larger dot is the hour "hand" and the smaller dot is the 5 minute "hand".  For sixty seconds the dots expand from the center outwards. The hottest area in the center of the dot is black.  Concentric rings of color-blue, green, yellow and red-are seen as you move outwards from the center to the perimeter of the dot.  When the bulbs turn off the dots shrink and eventually disappear after about a minute.

We spent many weeks researching and developing ways to heat the paper from behind.  An early approach was to use the same LEDs we had used in 2 previous clocks since they produced a surprising amount of heat.  A first test with a range of brightness looked promising - shown below in time-lapse as it warmed up the paper.  Later we learned that even when the LED was "off" it remained so warm that it showed a "dot" and we did not want that.
After more testing we decided to use tiny "grain of wheat" lamps because they produced more heat.  This necessitated making a layered structure to place the lamps in circular wells behind each clock position and Dave sketched out the whole plan once we had it figured out based on his extensive testing and prototyping.  (click on any of  the images below to embiggen)
I fabricated the "mechanism" from layers of 1/4" acrylic sheet, and drilled holes and milled slots for the wires.  There is reflective foil behind the lamps to constrain the heat inside each well.
I hand wired the electronics and went through many design revisions as I worked out how best to control the lamps.  There is a battery backed real time clock so it never loses time when unplugged.  Buttons allow setting hour and minute, and a blue knob can set the size of the smaller minute "blob".
We had some discussion about how to trigger the clock to show the time because watching the blobs appear slowly is a big part of the clock.  We decided on an optical sensor that you can wave your hand near to trigger it.  This is similar to the hands free faucet controls in public washrooms.  It is visible on the right above, and looks out through 2 small round holes in the face of the clock.

The design of the case evolved continuously throughout the project.  The original idea was to use the same 1:4:9 proportions as the monolith in the movie "2001 a Space Odyssey".   Then we tried variations on that theme and Dave sketched them out on CAD.  The final design is tapered so it is wider at the bottom as you see it from the front.  Dave is considering making it into a grandfather clock with a tall tapered wood base.
Once we settled on the tapered design, I got to work fabricating it from sheet acrylic.  A final coat of satin black paint completed the project.
 The back removes to allow setting time, replacing the lamps, and the thermochromic paper - because it will likely wear out after a year or so.  I engineered the module to be easily removable for this reason.  The thermochromic paper is adhered to the front of the mechanism with repositionable adhesive - like post-it note adhesive.
Here are front and back views.  The back can be removed via 4 screws to access the electronics.
And here's a video showing it working.

Visit Dave's web site:
DAB's Clocks

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