About a year ago, I had to order some physics-related stuff on ebay. While browsing through the other items that particular shop sold, I came across some thermochromic paint. Thermochromic paint is paint that changes color when the temperature raises above a certain treshhold; the particular type I bought was normally dark-grey/black-colored but would change to white when its temperature raises above 30 degrees celcius. The paint didn't cost too much and I figured I probably could do some neat things with it, so I bought two syringes filled with it.
When I got the paint, I first played with it for a bit. It would become white easily when heated to body temperature; when cooling down the paint would slowly go through a grey phase until it was again completely black. (I also found out that it makes some nasty stains in dark pants: forget that the stain is there and the fabric looks completely OK, until you decide to wear it and you have a spot that goes all white... Luckily the paint was water-based and came out quite easily.)
When I was done with playing with the paint, I figured it would be a nice idea to actually try and do something interesting with it: the only thing the paint needed to change colour was warmth, and warmth I could generate with electronic means. I started working on a design of a pcb. It consisted of long, thin traces folded into the shape of a thick line, seven of them in the shape of a seven-segment-display. The idea was that these traces have a certain resistance, so they will generate heat when a current is put through them. Glob some paint on it and you have an instant high-contrast, visible in daylight seven-segment-display.
Unfortunately, from there on the project became stagnant, and this time, it wasn't just due to me running off to other, more interesting projects. The problem was that I wasn't going to be able to make the needed PCB at home; the prototype print already was full of repaired traces and while testing I already had segments failing for no particular reason. I would need to design a PCB and send it off to a factory to have it produced. While that wouldn't be that much of a problem, I still had questions about the feasibility of that pcb: what would the resistance of the traces be? Would the epoxy cool down quickly enough to allow the digits to change once a minute? Would the traces be uniform enough not to require a fine-tuned current supply per segment? All in all, I didn't feel comfortable spending quite a sum of money to have PCBs produced which had a fairly high chance of failing out-of-the-box; I dropped the project and went on to work on other stuff.
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