DABs Clocks is a collection of hand made, one of a kind devices that each display time in unconventional and interesting ways. These time pieces are both functionally and aesthetically pleasing. I design and build some of my clocks without any assistance. For others I turn to Guy Marsden for the engineering and fabrication phase. In these situations I bring a concept to Guy and through an iterative and collaborative process I'll eventually create a 3D representation of the clock that Guy will build.
Comet
Thermochromic Display
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 you can watch the dots shrink and cool- black and blue colors disappear first with just the faintest hint of red as the dot prepares to completely disappear. In the video below the clock show's 11:50.
Thermochromic Liquid Crystals: You've Seen it Before
The video below shows how thermochromic paper responds to the heat produced by your hand. Liquid crystals are designed to change color over different temperature ranges. This video shows paper that changes over the range of 25-30C. I discovered this range didn't work well for the clock and instead I ended up using paper that changed over the range of 30-35C. For a brief explanation of how liquid crystals work click this
The 3D rendering on which Comet's case is based
Design Optimization
The thermochromic paper transition temperature was selected to be 30-35C as it allowed the dots to form fast enough at low room temperature (67F) yet kept the paper dark black even at higher room temperature (77F). I chose this temperature range because it reflects my home in the winter and summer when room temperatures reach their greatest extremes. The first prototype used paper with a lower transition temperature and while the dots grew faster and larger the paper turned grey at 75F. The video below shows how I measured the rate at which dot size formed as a function of time and room temperature. Using this approach we were able to optimize the manner in which the bulbs were placed behind the paper and the best way to attach the paper to the underlying support without interfering with the heating of the paper. It was trickier than it sounds but in the end it worked!. The chart shows a summary of performance at 70F vs 67F when the bulb is oriented in the perpendicular (P) and horizontal (H) positions. Y-axis = time bulb is on (seconds); X-axis = "blob" diameter (inches).
The grain of wheat bulbs (5V) used for heating the thermochromic paper were tested in two different configurations- perpendicular and parallel to the face of the paper. Using videos (as shown above) it was determined that the optimal installation was as shown in the photo to the right. It was also determined that more heat was transferred to the paper when the backside of the bulb was covered with aluminum foil.
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Two sheets of acrylic were used for the clock components. In the rear sheet a bulb was installed in 1/2" holes, at each of 12 positions, and covered on the backside with aluminum tape. Troughs were cut in this sheet so that wires from the bulbs could be neatly gathered and connected to the electronic components controlling the bulbs. Matching 1/2" holes were cut in the front sheet. Thermochromic paper was adhered to 5ml thick plastic sheets which were in turn attached to the front sheet of acrylic using double stick tape. The two pieces of acrylic are held together with screws to form a tight connection. This approach was taken because thermochromic paper has a useful life of perhaps 6 months to 1 year. Therefore it was important to design the clock to allow for easy disassembly and installation of new paper.
The electronics were meticulously hand wired. 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".
The back removes to allow setting time and for replacing the thermochromic paper which will likely "wear out" after a year or so.
The default appearance of the clock face is plain black. The time is only revealed when the clock is "interrogated". Waving your hand past an optical sensor built into the front of the clock triggers the bulbs to turn on for one minute. During this minute the "blobs" for hour and 5-minute continuously grow and change color. After the bulbs turn off another "show" begins as the blobs shrink, change color and disappear as the paper cools.
The case is made from 1/4" thick acrylic that has been sanded and painted with satin black paint.
Time Lapse (300X)
Schematic
Source Code