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Conclusion

So, does it work? With the current setup, after a few days of testing, I can tell you it works mostly. Usually when I walk past the clock, I can just see what time it is without even thinking. The clock is mounted in a somewhat dark corner of my house, but even that doesn't impede readability too much.

Ah, it's eighteen past noon.

At other times, I walk past the clock when the minutes have just changed. Dependent on the temperature of the room, the segments have a tendency to stay warm for a bit too long and the two numbers overlayed don't make sense. For example, a bit less than two minutes later than the previous picture, you would see this:

Ah, it's garblesmarble minutes past noon?

The 'ghosting'-effect caused by the slow cooldown of the resistors could be mitigated in a few ways. The resistors could be cooled by e.g. thermically connecting them to a big cooling block or perhaps a Peltier-element, but that would take up more energy than I'm willing to spend for a single clock.

A better alternative would be getting a different kind of thermochromic paint: the one I used discolors at 30 degrees celcius and my room temperature is 20 degrees, which means there's only a 10 degree temperature gradient. The speed of heat dissipation is, amongst others, a function of the temperature gradient: a larger gradient means more dissipation. Ofcourse, I could set the temperature in my room a lot cooler, but a more comfortable way would be to find a brand of thermochromic paint that changes color at 40 or 50 degrees. I have searched for it, and while these do exist, demand for it is so low that you can only get it in bulk. (If anyone does know a place where I can get just a few milliliter of this higher-temperature paint, please do contact me; I'd love to try it out on my clock!)

A third alternative would be to re-think the design of the display. A 7-segment-display gets quite unreadable when multiple digits overlap. An analog clock-lookalike based on the same resistor/thermochromic paint-principle would not have this disadvantage: the 'ghosting' digits would form a nice trail behind the clocks virtual hands, not impeding the time read-out at all. Perhaps I should make me one of those... Perhaps in another year.

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10 comments

C.Rama Murthy wrote at 24 Nov 2014, 8.27:

Dear Jeroen Domburg, Your interesting display is demonstration of a new type of seven segment display namely "Segmented seven segment Thermochrome Display".Nice work.Keep it up.

glowingMice wrote at 3 Jan 2011, 17.21:

Awesome project! I just had an idea though : Instead of discrete resistors, what if you used heating paint (like windshield defrosting paint, available at all automotive repair store)? You could get sharp well-defined segments. Etch two contacts at each end of the segment on the PCB, paint the segment with heating paint, let dry, then paint a 2nd layer with thermochromic paint. The heating paint should also have less thermal mass than the ceramic of discrete resistors, so less problems with number blending. Happy New Year!

Lucky Luke wrote at 9 Dec 2010, 10.03:

Nice project! Hmm... I do have some temperature sensitive paper somewhere... About the flat nichrome wire: As eric said: try an old toaster. I don't know if it is easy to solder, but if it is you could use it like a jumper. (Draadbrug, niet zo'n plastic dingetje) (The round stuff too of course) About the plastic film idea: maybe something metal cools quicker? It also spreads the heat so you don't see separate dots where the resistors are. Only it should not spread heat too much because then the display gets all white and unreadable... Maybe tinfoil could be used, with plastic film for electrical insulation and strength.

aterentiev wrote at 8 Dec 2010, 10.44:

How about following idea: Not to paint the resistors directly, instead, make a thin (maybe plastic) film, colored with the paint, and lay it on the resistors.

aterentiev wrote at 8 Dec 2010, 10.10:

I think it should be something like this one: http://cgi.ebay.de/ws/eBayISAPI.dll?ViewItem&item=390127314941

Eric wrote at 7 Dec 2010, 16.19:

Search for nichrome ribbon. It is also used in every toaster I have ever seen if you have an old one.

Sprite_tm wrote at 7 Dec 2010, 12.25:

Flat nichrome wire? I've never heard of it and an ebay search turned up nothing. I do have some of the thin, round stuff, but I can't see that being easy to make digits out of...

mr nobody wrote at 7 Dec 2010, 8.18:

what about the thin tape-like nichrome(the flat type) commonly found at ebay

Sprite_tm wrote at 5 Dec 2010, 21.01:

Christian: The firmware actually does that. It heats up the resistors twice per minute, but forgoes the second heating cycle if the segment will be off the next minute, so these segments are only heated at the start of the minute.

Christian wrote at 5 Dec 2010, 20.21:

What about shutting the resistors off 30 seconds before the next minute kicks in? The PCB version looked great too.

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