Decimal Oscilloclock harks back to 1927 movie

Metropolis is a classic, silent film produced in 1927 and was one of the very first full length feature films of the science fiction genre, and very influential. (C-3PO was inspired by Maria, the “Machine human” in Metropolis.) Within the first couple of minutes in the film, we get to see two clocks — one with a 24-hour dial and another larger one with a 10-hour dial. The human overlords of Metropolis lived a utopian 24 hour day, while the worker scum who were forced to live and work underground, were subjected to work in two ten-hour shifts during the same period.

[Aaron]’s client was setting up a Metropolis themed man-cave and commissioned him to build a Metropolis Oscilloclock which would not only show the 24 hour and 10 hour clocks from the film, but also accurately reproduce the clock movements and its fonts. [Aaron]’s Oscilloclock is his latest project in the series of bespoke CRT clocks which he has been building since he was a teen.

The clock is built around a Toshiba ST-1248D vintage oscilloscope that has been beautifully restored. There are some modern additions – such as LED glow indicators for the various valves and an external X-Y input to allow rendering Lissajous figures on the CRT. He’s also added some animations derived from the original poster of the film. Doing a project of this magnitude is not trivial and its taken him almost eight months to bring it from concept to reality. We recommend looking through some of his other blog posts too, where he describes how oscilloclocks work, how he builds the HV power supplies needed to drive the CRT’s, and how he ensures vibration and noise damping for the cooling fans used for the HV power supplies. It’s this attention to detail which results in such well-built clocks. Check out some of [Aaron]’s other awesome Oscilloclock builds that we have featured over the years.

The film itself has undergone several restoration attempts, with most of it being recovered from prints which were discovered in old archives. If you wish to go down that rabbit hole, check out Wikipedia for more details and then head over to YouTube where several versions appear to be hosted.


25 thoughts on “Decimal Oscilloclock harks back to 1927 movie

  1. This is a fantastic project.

    Makes me wonder how difficult it would be to make my old HP 120B scope work as a clock without causing it any harm. The 120B isn’t terribly useful these days with only a single channel and 1 MHz bandwidth, but it’s a great example of a scope from the time period (1960-ish) and it has an awesome retro-futuristic look to it.
    Something to think about, anyway.

    1. There’s a bit of trickery involved to ensure that both the 20 hour and 24 hour timekeeping lines up, while still keeping the length of a “second” the same in both cases (a limitation of the current hardware design).

      I plan a post on the Oscilloclock site relating to this, so if you like, Subscribe there for updates.

  2. Beautiful. Similar looks in a much thinner form factor and less power consumption / heat production would be a nice tabletop deco. Though both of those point at dumping the CRT and going for a more modern solution.

    1. I was thinking the same thing – how close could we get with a TFT, 1/4″ glass and a ~5% diffuser. And of course a plate + brass ring around the outside to hide the rectangularity of the TFT.

      1. I have a feeling, never. You’ll just never get a passable CRT appearance from LCD. Different colour rendering, no pixels (with a smooth continuous covering of phosphor), no backlight. Just too many things, that even with a very high-res, very high colour rendering LCD, you still couldn’t match. Maybe OLEDs in the future, but probably still not.

        Besides that, the entire point of this is the CRT. There’s hundreds of ways of doing LCD clocks, this one requires a CRT.

        Still, how hard is it to find old round ‘scope-style CRTs? The demand must be pretty low. Worst case, you could always make one, get hold of some phosphor and a chemistry flask. Deflection plates can go inside it, or coils outside the glass if necessary. Things like linearity can be handled in the software that drives it, so it doesn’t need to be as complex as a TV screen.

        Talking of TV screens, early CRT TVs had 5 and 6 inch screens, one of those might also be suitable. I think they can be remanufactured, though it’s not easy.

        1. Dammit, one last thing… remember the “portable” TVs from the 1970s and 80s? With a tape recorder and radio built in, and a small 4 inch or so CRT TV. Built into something like a small suitcase and weighed a ton. Would run off mains, 12V, or a load of “C” batteries. Must be plenty of those hanging around, they still sold them in the 1980s.

          I used one of those at my grandad’s house to play The Hobbit on the ZX Spectrum. I used to rack my brains trying to think of a way to get it to record video onto the cassette tape.

  3. O-scope clocks, spacewar, asteroids, etc are fun but unless it is a disposable scope the burn-in concerns me.
    Not too difficult as the FOSS code is out there for both ready to play and cut/pasting into projects for both Arduino and Linux both for audio output and resistor DAC setups.

    1. I wonder if you could borrow a trick from modern televisions and do some sort of “pixel shifting” to try to prevent burn in. My Plasma TV has a feature that moves pixels around and uses a sort of 2D interlacing to attempt to unnoticeably dim or disable pixels and keep them from getting too burnt. Unfortunately, this didn’t actually work in practice, as my TV has a permanent CBS logo in the bottom right corner, thanks to the station’s insistence on 24/7 advertising and my family’s love for their prime time lineup (at least, their lineup as it was up to a couple of years ago).

      Maybe you could cause the display to randomly flicker every second or so to make it look as if there is some electrical problem that it is fighting against to valiantly bring you the time. :D

        1. That’s not a bad idea, but I bet even a bit of drifting the image around would help a lot. In an X/Y direction, but also rotating the image a little, slowly so nobody would notice. Particularly the hour “tick” markings would benefit from that. Do it purely in software, rather than the nightmare of having the tube rotate, and have all the wires flexing.

        2. That’s an idea I haven’t thought of! But you don’t even have to recalculate the image in software. All that’s needed is a beam rotation coil and drive that in the opposite direction to the physical CRT rotation. Another project for another day!

          But regarding burn-in in general, this isn’t really an issue. I’ll try to put a post on the Oscilloclock site soon about this.

  4. I have an old oscilloscope that uses tubes. I got it as a visual piece (I don’t obviously actually use it)… It’s tempting to try to come up with a use like this. It too has the round display, like the one in this article. I actually know very little about the long term durability of these CRTs though. As genuinely useful as it would be, I’d hate to hurt the original tube. I could imagine having a motion detector and a timer with a time out to blank the display, but that’s still running the filament of the tube.

    Anyone have experience with CRT clocks in the long term? Can you underdrive the filament to save it from wear, but keep the tube “nearly” warmed up” for a more rapid response to detected motion? Is that stressful to the CRT? Is it more or less stressful than simply turning it off altogether and having it fully powercycle? Is it even something that significantly maters, in terns of tube life, say, in comparison to just blanking the image to save the phosphors?

    I’ve got an old Tektronix tube stored away somewhere too, but it’s a square tube, and I can’t remember if the reticule marks are built into it or not. It’s also a raw tube… No support electronics, so I’d have to build from scratch. I just DO like the look of the old vintage scope, and I’d love a reason to pull it from storage. I’m also reluctant to “hurt it” though.

    1. It is exactly as you say, one thing is phosphor burn-in, another is the lifespan of oxide-coated cathode. It is quite easy to avoid the burn-in – just avoid perfectly still images at full intensity and you are fine. The cathode burnout though is rather unavoidable. Running the cathode too hot accelerates the emission loss. Running it too cold might lead to very rapid degradation through what is called “cathode poisoning” when the cathode is cold enough to absorb contaminants as getter normally should. The lifespan is a very individual thing, you could get 1k, 5k or 20k hours out of your CRT. Yet, knowing your beautiful creation has a defined finite lifespan is something that brought me out from using tubes entirely.

        1. Would running the filaments at low power help them last longer? I’ve a feeling it would. It’s the same as incandescent lightbulbs (VERY much the same!), mostly they fail when you switch them on, from the cold inrush current.

          I believe some valves in old TVs had some sort of NTC resistor to limit inrush, or some similar method.

          The other advantage of keeping the filaments warm would be quicker on-time, but how slow was that anyway? Surely didn’t take more than a few seconds for a valve TV to start working?

          1. According to many folks, running the filaments at lower voltages than specified can result in “cathode poisoning”, and isn’t recommended.

            Instead, the best way to preserve the filament is a slow-start circuit to limit the inrush current (as the filament has a low resistance when cold), and to simply not run the clock 24 hours a day but flick it on only when needed.

            I also never use CRTs where the specification actually mentions a lifetime of hundreds or even thousands of hours. (Many Russian CRTs easily obtainable actually have this limitation.) The brand makes a huge difference.

            But above all, for every Oscilloclock made, I ensure to keep lots of spare CRTs. Some folks request spare CRTs to be shipped together with their clock so they have peace of mind! In 6 years of making these devices, I have still not had a unit come back for CRT replacement. It will happen eventually, though.

      1. Still you’ve only got so many K hours in your own lifespan. No real need to save CRTs. If they don’t ever get used, and enjoyed, then eventually they’ll just die from old age. It’s a shame they don’t make them any more, but society doesn’t need them, you’re not depriving the world of anything essential. And we always could make more if we actually needed them. It’s just a matter of price.

        I think a hoarding mentality is certainly a part of this whole geek / hacker thing. But eventually you’re gonna reach your own biological MTBF, and all your treasure becomes some old loony’s junk. Or maybe “eccentric” if you’re lucky!

        An object only has value if somebody wants it, and since you love and appreciate the gear, it’s worth much more to you than to anyone else. That means if you use it, and enjoy it, then the object is supplying the most value it ever can.

  5. Actually… what’d be super-nifty, would be if this thing could show video, as a hidden easter egg mode.

    You’d need something like a Raspberry Pi, with composite video output. Then a composite to CRT driver, from an old CRT TV. Surely by the later days of CRT TVs, these got simpler and more integrated? Sinclair had a pocket CRT TV in around 1981. Sony had one in the mid-1980s. So you must be able to get it small enough. Actually 1950s TVs must’ve used simple circuits, since you could only put so many valves in there.

    So… do that, have a Raspi driving the CRT as a video display from composite. With a switch, perhaps just a relay, between the video circuit, and the direct drive that the clock display uses.

    Then either play videos from SD card, or use a TV tuner dongle to actually tune in TV signals! Run that through the Pi, Bob’s yer uncle! A very configurable, versatile TV set with a tiny round green display. That’s also an art-clock, for timing art.

    Other method might be to have the Pi drive the CRT directly as before, and get it to generate a full analogue TV signal in software. Scanning the beam and controlling intensity itself.

    Either way. Just think it’d be such a mad thing to be able to watch TV on an old ‘scope screen.

    Actually didn’t old TV engineers used to have some box that let them watch video on their scope?

    I’ve got a travel-size 6″ CRT TV or so in the back of a cupboard. Half-driven to sticking a Pi into it and giving it some clever function, now. Would make a nice clock+. Plus what, I’m not entirely sure.

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