Hot Resistors Used For Color-changing Clock Face

[Sprite_TM] built a full clock display using thermochromic paint. This picks up where he left off with his paint-based 7-segment display prototype. He never really saw that design through to a finished project, but he recently came across the leftover paint and decided to do something with it. Instead of making thin traces on a PCB he’s heating up resistors mounted on protoboard. Each resistor has been coated with the black/light grey paint after getting a rough sanding on the tops of the packages. Run around 500mW through a segment and they heat up enough to change the paint to light grey. Once shut off, the segments gradually fade over the next 60 seconds.

37 thoughts on “Hot Resistors Used For Color-changing Clock Face

  1. leave off the paint, and make a clock that can only be seen with a thermal imaging camera. If you get the resistors hot enough for shortwave IR, a web-cam w/o the IR filter might see it too.

  2. They used to make incandescent 7-segment displays that looked a lot like Nixie tubes. They were called Numatrons. I think the basic idea here would have worked better with Nichrome wire; you’d get the paint changing temperature with less current and it would cool down faster.

  3. @osgeld – I’m thinking you would have a hard time making it fugly.

    @sprite_tm – maybe a analog type clock with 12 radians (each with one short and one long arm). That way you could let the old one fade slowly as you heat up the new one – since they’re 5 minutes apart it might look neat.

    But come on guys – clocks are the “hello world” of electronics. Everyone’s done at least 6 or 7 in their tadpole years. It’s time to move on. Especially since practically every device made has a clock built in AND everyone carries a mobile so whats the point.

    1. New tadpoles are born every year, and they need somewhere to start. You may have outgrown clocks, but clock projects will be at the level of a lot of new folks.

      The same is true for a lot of other “not a hack” posts. If all that ever gets featured is the truly elaborate hacks that can only be done with a big budget, it just guarantees that the community here will stagnate, grown old, and then wither.

  4. @vonskippy
    I like making clocks, because time is a universal data source which is constantly changing and has some meaning.
    Say you built this neat display – what would you show on it?

  5. Nichrome wire (as has been suggested), but behind a layer of something that is opaque and conducts heat (aluminium foil?), so that it looks like a smooth black panel when powered off.

  6. @vonskippy:
    A clock is the perfect way to show off a novel means of displaying information.
    The clock isn’t the hack here. You’re focusing on something that’s totally irrelevant.

  7. You don’t “run 500mW” through something. You would apply a voltage, which causes a current to “run through it”. This would be in mA (mili amperes).

    Power (measured in Watts) is the product of this. It is the work done, the heat generated. You don’t run power through things.

  8. Also for “hello world” electronics, one of my first memories was building a speaker at school. Way before we got our hands on PICs or even LEDs.

    A cone made out of cardboard, a tube made out of cardboard and attached to the cone. Wrap a wire around the tube and wait for the single magnet to make its way around to your table. Then apply a signal to the wire using a sig gen. Beeeeep.

    That was pretty cool back in the day!

  9. Interesting, I never heard of such paint.
    Does it react to low power lasers? This could be used to build a high persistence screen for servo driven lasers.
    Does it also exist for higher temperatures? If yes it may be painted on critical components (power transistors, etc) to get immediate visual feedback if they heat too much.

  10. i also cast in my vote for banning people like vonskippy from here. we need more creative ppl and fewer angry kiddies with nothing to do bu whine.

    sadly, a moderated system that requires membership to post seems to be the way here now.

  11. @vonskippy Obvious Stupid American is Obvious Stupid American

    Anyways this is a cool project and I wonder if I can get some of that paint…

    And Yes I am American so dont go off on me :)

  12. If it takes a whole minute to cool down, it seems like you could turn off a segment early if you knew it was going to change (not the ones that are unchanging because they would fade all over the place… but the minute hand for sure…)

    So if you know a segment is going to change, then like at 00 secs turn it on, and at 30 secs turn it off — so that 30-60 is “cooling down” and gradually fading… — or even only turn it on for 5 or 10 seconds (however long it takes to heat it) — though then it might cool off quicker too — so maybe you could mix in some sort of PWM… to have a quickly fading “off” state.

  13. Doh – just put some negative current through the resistors, they will cool down.
    Now seriously: you could run some resistive wire in loops behind a plate painted with this thermo paint and you would have a nicer display. Maybe you could display temperature outside with it? Or… make a speedometer for your car…

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.