Glow In The Dark Computer Memory Illuminates The Fundamentals

Two circuit boards connected with wires

Computer memory has taken on many forms over the years, from mercury-based delay-line tubes to handwoven magnetic core. These days, volatile storage using semiconductors has become ubiquitous with computing, but what if there was a better way? [Michael Kohn] has been working on a new standard for computer memory that uses glow in the dark stickers.

Clearly we jest, however we’re still mighty impressed by the demonstration. Eight delightful star-shaped phosphorescent stickers represent eight bits of memory, totaling one byte. The glow in the dark material is stuck to the inside of short cylinders, each of which contains a white LED and a phototransistor. The memory array is wired up to an iceFUN FPGA board, which is then connected via level shifters to a Western Design Center MENSCH single board computer.

To write a ‘1’ to memory is as simple as writing to the corresponding memory address using 6502/65C816 assembly language. Using the STA command will illuminate the white LED at that memory address, in turn irradiating the glow in the dark sticker and ‘saving’ the state. Conversely, LDA at the same address will read from the phototransistor, which picks up the glow (or lack thereof) emitted from the sticker.

A refresh cycle is needed to sustain the 0s and 1s across the memory array as the phosphorescence fades, not dissimilar to modern DRAM requiring frequent recharging to maintain memory contents. The entire setup is a tangible demonstration of the fundamentals of volatile computer memory, and would make for a fun beginner project. [Michael] has more details on his website and GitHub page.

While the FPGA board has its own little set of blinkenlights, an 8-bit RGB LED array would make this project even brighter.

22 thoughts on “Glow In The Dark Computer Memory Illuminates The Fundamentals

  1. That was fun. I didn’t hear mention of clearing bits in the video though. While writing a one seems pretty straightforward, won’t erasing it be a time intensive operation that can’t exceed the natural decay rate of the material? Use of a minimum glow threshold to represent a zero could speed things up but that would increase the refresh rate needed to maintain the enabled bits and in turn reduce the IO rate. Still pretty fun looking.

    1. I think one could use two glow in the dark stars, two leds, two photodiodes and. Differential amp. Writing a 1 would energize the one star, writing a zero would energize the other. The decay still slows it down but this method would be faster.

      1. This. The diff amp means that the retention time is limited mainly by the noise level in the system. But there is still a hazard, where if you write a number of times in either state, a single write to the other state may not be enough to flip the read value. This is because the writes get integrated by the phosphor compound.

    2. Can a lower wavelength of light be used to encourage the pigment to emit and discharge without having enough energy to reenergize it? I swear I dimly remember something like that being possible.

        1. I remember it much less dimly. There was an article on HaD that was similar to this one, where a glow-in-the-dark drum or tape loop was used as a storage device, and someone in the comments section mentioned that phosphorescent compounds can be “discharged” by specific wavelengths of light. Unfortunately I have not been able to find that article again.

        1. Sure, done it on the kitchen counter years ago. They made IR test cards that you’d have to charge up before using. Sold them at RadioShack to verify that your remote was working.

  2. If you build this on a whole wall – and so that, the glow can be seen would be impressing to view while the computer runs.

    The Williams and Selectron tubes use similar approaches – would like to see them in action.

    1. You know, bistable storage CRTs used a flood gun to refresh the charge on what is essentially an array of capacitors on their faces, but these were of course limited in size. BUT, if you can read it and write it, for example by having a row of photodiodes as well as the row of UV LEDs, you could re-energize any bits that are above a certain threshold, which would create a persistent memory array. Not unlike dynamic RAM, but with the added benefit of being directly viewable.

  3. I liked it the most when you shared that we should choose a material that picks up the glow emitted from the sticker. My friend wants photoluminescent exit signs for their building. I should advise him to go for it to ensure it’s visibility.

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