I2C Paper Tape Reader Is Not What You Think

We’re not quite sure what drove the development of this project, but [shapoco] has put together an intriguing device that reads I2C signals (Japanese Twitter link) which have been printed as black and white rectangles on paper tape. He wrote a program that prints an I2C byte stream onto strips containing the SCL and SDA signal patterns. Once printed, you cut the strips from the paper and glue them together into one long piece, making a complete message — in this case, commands to a small LCD screen that will display the phrase “Hello, Tape I2C”.

We’re not sure exactly sure what’s inside that rectangular widget epoxied to the bottom of that perf board through which the tape passes. But clearly, it must contain a pair of LEDs to illuminate the tape and a pair of sensors to detect the reflection off the tape (looking at the wiring, it seems unlikely that anything is mounted underneath the tape). According to one machine-translated Twitter message, detection is done using a Schmitt trigger made from an LM393 comparator with hysteresis (see this TI app note for a review of this type of circuit). Here’s a scope capture of the resulting signals. [Shapoco] notes that the circuit can operate much faster — the tape is being pulled slowly in the video to make it easier to see.

This is not [shapoco]’s first experiment in optical I2C communications. Check out the second video down below where he’s reading I2C signals from a computer’s display. One person tweeted about how software source code was sometimes printed optically in old Byte magazines many years ago, a topic we talked about in Hackaday Podcast #049 last year when discussing Cauzin strips.

Besides just being cool, and possibly helpful as an educational device, does this technique have any real-world applications these days? Let us know your thoughts in the comment section below. Thanks to [Manawyrm] for sending us this tip.

25 thoughts on “I2C Paper Tape Reader Is Not What You Think

  1. Yes, I have a use for this and was planning to build similar. You know how the program for colossus was written on a paper tape loop? Well I want to build something similar, only black/white printed not punched. I was planning on glueng A4 together to (hopefully) give about 24 bits on each line. If anyone has built similar please post here!

    1. Glueing A4s together ? you´re masochist, aren’t you ?
      There is a class of devices called “ticket printer”, i´m sure you encountered some in your life.
      The rolls in these come in various width, and lengths of dozens of meter. Bonus: use the printer to actually print the tape. Thermal paper has a high contrast.

      1. Yeah, @rok, I’m a masochist. Maybe even a sadist, the idea is to build a computer that costs no more than £50 in parts that schools could build so lots and lots of people can do the same. Schools have A4 printers, sellotape and glue. I do appreciate your reply though, unless I find easy ways to do this I’ll never finish.

        1. Aaaah you want cheaper and more ubiquitous ? then just attache a Edding to a 5USD servo, + one motor for unrolling, and well… toilet paper.

          Look mum, no glue, no fancy A4 printer with costly ink/toner printer !

  2. What a fun idea!

    At a standard speed of 400kbit/s and 5mm wide bits (guessed from near to two perf-board hole distances) that would be a paper speed of 2km/s

    One thing I do not like much is cutting and gluing all those paper strips.
    A sheet of paper could be wrapped quite easily around a cylinder, and further automation with a few stepper motors…

  3. If he flips one side over and re-attach to other end he can double his data for same length and repeat forever or until tape breaks. Dont need no stinkin disk. Science fair retard with oozing volcano is likely to win though.

  4. I first thought, “what a great way to get your head around I2C.”

    But there’s something wrong here. The ACK bit at the end of a packet needs to be set by the receiving party, and this is only one-way communication.

    (pedant hat)
    This isn’t I2C!
    (/pedant hat)

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