Readers who survived the 1970s will no doubt remember the “mood ring” fad, where a liquid crystal mounted to a ring would magically reveal your current emotional state to all and sundry by changing color. This nifty thermochromic display is based on the same principle, and while it might not start a new craze, it’s still pretty mesmerizing to watch.
This isn’t [Moritz v. Sivers]’ first attempt at a thermochromic display. His earlier version was far more complicated, using separate copper plates clad with thermochromic film for each segment, with Peltier devices to cool and heat them individually. Version two is much simpler, using a printed circuit board with heating elements in the shape of seven-segment displays etched into it. The thermochromic film sits directly on the heater PCB; a control PCB below has the MCU and sensors on it. The display alternates between temperature and humidity, with the segments fading in an uneven and ghostly way that really makes this fun to watch. [Moritz] has made the build files available, and there’s a detailed Instructable as well.
We’re always on the lookout for alternate display modalities, especially when they look this cool. We’ve seen other thermochromic displays before, of course, and persistence of phosphorescence looks great, too.
Two questions – do thermochromic displays “burn in” after use? And, where does one get a display that large?
I have not seen any burn in, but at high temperatures (like if you put it outside in the sun) the display gets blurry or unreadable. Large liquid crystal film sheets are available from many sources, including Arbor Scientific and TeacherSource.
I haven’t seen them burn in, outside of clear damage (I’ve used this stuff to trace beams in IR laser systems). Edmund Optics sells sheets in various sensitivities. I think they’re 12″ x 12″.
One can gets sheets of thermochromic film. Apparently you can even find suppliers who will sell 50″ wide rolls of as much length as you want to buy.
Maybe add a peltier cooler on the back for a faster refresh rate. These thermochromatic displays can change pretty rapidly. If a grid of thin wire was used they could be energised in groups of three to rapidly scan the surface, one wire is pulled half way to temperature and then a pair perpendicular to that line are turned on to increase the voltage through the point they cross the first line. Or perhaps small coils could be used for a dot matrix display. Or it could just have cross linking of diagonal heating wires on a grid that itself stays cool and just address then using two wires. Hmm. Maybe I’m over thinking it.
Using incandescent bulbs to heat it from behind seems sort of moot. As does using an lcd as an infrared mask. Still. Lots of possibilities.
Hmmph. Goes to show there’s some merit in just trying it. I have some of that film and thought of the same heated-trace thing a few months ago after seeing the previous heated-plate version, and calculated the resistance of the 6-mil traces and power required. When I saw I could not get a high enough resistance in a segment trace to make it “practical” or “efficient” (I thought) I gave up. I see these segments are quite a bit bigger than the ones I was contemplating though: there’s more than 10 meters of trace length on that board (!)
Regarding refresh rate, Looks like its the cooling that takes the most time here. Perhaps use of chilled water, generate with peltier, through a thermal block on each segment (switched via solenoid valve) Would only need one to switch cooling for all segments like shaking an etcha-sketch.
I’d be interested t see what it looks like without a full blanking. Wait for the previous image to half or three-quarters fade before bringing up the next frame, that ought to give some interesting displays.