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.

It took me quite some time to get the pcb done right: the long, thin traces are somewhat hard to get right the first time when you homebrew your PCBs. After fixing the traces, I ended up with a nice seven-segment-display. I put some of the paint on it and dropped the two wires of my power supply momentarily on both ends of the wire which made up one of the segments. It worked! I immediately decided that this concept would be great to make a digital clock out of: the idea could be scaled to arbitrary sizes, the clock would be readable both in sunlight as well as in most low-light conditions and it wouldn't have any noisy mechanical parts.

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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