Custom circuit drives a small round CRT display

[Svofski’s] latest hack seeks to do no more than look cool on his desk. We’d say mission accomplished. He doesn’t even need anyone around to be proud of the small round CRT display unit he put together. Just having it hum away next to him will be more than enough to keep him going when regular work gets a bit tedious.

One of the biggest challenges when working with a cathode ray tube is the supply. He compares the requirements with that of Nixie tubes, and this is quite a bit more challenging since he wants to generate the 750V from a 12V DC source. To pull it off he hand wound his own transformer. There are two secondary coils, one for the cathode heater and the other as the supply. You can see a brief clip of the unit in action after the break.

Take note of the PCB section of his writeup. He took a meandering route through several different software packages before printing the board. It started with Eagle, moved to, which produced a Specctra file that he converted to gEDA using a Python script.

31 thoughts on “Custom circuit drives a small round CRT display

  1. While I admit it looks cool, it looks extremely dangerous with all the high voltage wires exposed. Definitely does not belong to a work desk.

    1. I understand the concern. All high voltage traces pass beneath the tube or hidden by other parts. It takes some creativity to touch anything dangerous. It lives on my work desk.

    1. It would be ignored (indiscernible) if buried in a bag of adaptors. I have gotten my large hand-wired led panels through without pause, even a Geiger counter I forgot was in my carry on. They don’t care.

  2. It may LOOK dangerous but it’s not. As almost everyone knows, high voltage at very low amperage is not going to do any damage.

    1. Which is why I defillibrate myself once a day just to keep my heart in tip-top condition!

      Seriously, high voltage low current will mess you up, maybe not in as dramatic of a fashion as the other combinations of voltage and ampacity, but it can mess you up none-the-less.

      1. though this particular project likely wouldn’t lay you flat, standard commonsense rules still apply… (keep one hand behind your back, for instance, when coming into contact with the high volatge portions of the circuit if they’re live)

      2. A defibrillator applies 200 joules at 1000 volts in 5 milliseconds. This works out to 40 amps. This seems like it would be a lot for a little power supply like this (unless it has some insane capacitors). However I could see places where less energy could be applied causing more damage so take it with a grain of salt.

      3. High voltage at very low Amperes equals the kind of shock I get every time I touch the doornob when I wear these synthetic slippers.

        I didn’t actually measure it but I recall that static electricity can easily add up to 10000 Volts (but usually only a few tiny microamperes)…

        Yes, it is very unpleasant, but I wouldn’t call it dangerous.


      4. How about those electric fly swatters? I stupidly measured one at 1100 volts and then burned out a cheap dvm trying to measure another.

        A few years back, I had a broken swatter. Just the handle with a couple wires sticking out. I did not realize one of the wires was disconnected. So when I shorted the leads to discharge it, it did not actually discharge. Moments later I was futzing with the wires, one in each hand.. you know where this is going.. I took the stored charge through the arms and across my upper torso. Not fun!

        It’d be interesting to know the particulars of that zap.

      5. The one I had was no more than 1000V. That was the rating of the capacitor when I opened it up after it broke. My favorite part about it when I was younger was that I could stun flies without actually killing them. Was always fun to baffle people with the fly on a string trick.

  3. This is absolutely gorgeous. I love it and want one. I am becoming an electrical engineer so that I can make and understand things like this. It is an inspiration.

  4. Very cool. I have been wanting to build something like this. There are quite a few smaller electrostatic CRTs at a local surplus store that would be good candidates.

    1. Please inform us about the location of your local store. If I had to guess, it is either in California, or is EPO in Houston…

      1. R5-D3 Surplus in Portland, OR. No web presence, no credit cards. Bob Lee, the owner, does most if his sales in tubes and incredibly knowledgable in tubes.

      2. Ahh… I had a feeling it was at least on the west coast… I have been planning to visit Astoria, but I think Portland is a very long drive around the cove to get there, if I am not mistaken.

        I would love to visit his shop, though, and it does seem that most of the “abundant” outlets for surplus electronics don’t publish their inventory on the internet, probably because there is too much stuff to even get started listing online….

  5. Make one of these using a flat CRT
    or one of those G0 night vision
    tubes with an LCD panel+IR backlight
    for a more compact unit?

    Old CCFL inverters can be used for
    the HV supply here.

  6. Anyone else notice that ticking sound as he moves in front and to the side of it?

    Think that could be RF noise interfering with the mic?

    1. Heh, this is not a Tesla coil, just a modest CRT tube, there are no sparks flying around and it does not smell of ozone :) If there was RF interference capable of making this to the mic, the tube would not be able to form any clear picture. In the writeup I mention that the toroidal transformer does a great job of containing most of the magnetic field very close to itself. People using E-cores usually report interference problems, especially with these tiny tubes.

      The sound is me squeezing the phone while filming with it in some uncomfortable position, trying not to shake it too much.

      1. Haha… Yea, the invention of large screen TV’s. Just make sure you are sitting in exactly the right spot to see the proper focal point…

        And the fact that we couldn’t see this movie in hi-def may be a blessing. I mean, how detailed do you really want to see rubberized skin being stretched???

        Awesome hack, anyhow. These tubes can be sourced by obtaining legacy oscilloscopes and stripping them of their tubes(as I “duck and run”)…

  7. Could someone build a (custom) 12″ round CRT in color? I would like to replace an old, black & white Zenith round “porthole” screen tube with a color one.

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