Smallest USB Device… So Far

For better or worse it seems to be human nature to compete with one another, as individuals or teams, rather than experience contentedness while moving to the woods and admiring nature Thoreau-style. On the plus side, competition often results in benefits for all of us, driving down costs for everything from agriculture to medical care to technology. Although perhaps a niche area of competition, the realm of “smallest USB device” seems to have a new champion: this PCB built by [Emma] that’s barely larger than the USB connector pads themselves.

With one side hosting the pads to make contact with a standard USB type-A connector, the other side’s real estate is taken up by a tiny STM32 microcontroller, four phototransistors that can arm or disarm the microcontroller, and a tiny voltage regulator that drops the 5V provided by the USB port to the 3.3V the STM32 needs to operate. This is an impressive amount of computing power for less than three millimeters of vertical space, and can operate as a HID device with a wide variety of possible use cases.

Perhaps the most obvious thing to do with a device like this would be to build a more stealthy version of this handy tool to manage micromanagers, but there are certainly other tasks that a tiny HID can be put to use towards. And, as far as the smallest USB device competition goes, we’d also note that USB-A is not the smallest connector available and, therefore, the competition still has some potential if someone can figure out how to do something similar with an even smaller USB connector.

Thanks to [JohnU] for the tip!

37 thoughts on “Smallest USB Device… So Far

        1. Maybe a plastic tab that you can tuck back inside.
          I think one could prototype that idea with some kapton tape, but it might not have strong enough adhesive for multiple uses.

    1. A few inches of dental floss wrapped around it before insertion?
      Leave it long enough to get a good grip on and pull.
      Maybe do a double loop and have it circle the board twice, so that you can even out the pull to keep it strait as you pull it out.
      Much like the way battery compartments used to have a strap in them to leave underneath the cells and you pulled it to pop out the cells.

  1. Impressively small and with more components crammed on there than I would have guessed. The tiny 300 mA voltage regulator impresses me the most. I’m wondering what else could be squeezed in if the MCU was a WL-CSP package instead.

    1. Hey there, I didn’t start this project with the explicit goal of making the smallest USB device. My goal was to make a rubber ducky that couldn’t be seen from the outside and I only noticed after it was complete that it also seemed to be the smallest USB device currently around 😉

      1. [knotta] is correct in some ways. Just because you can’t fit the electronics inside the connector doesn’t mean it can’t be smaller than this project. Let’s say one were to make use of a smaller uC, and a double sided board with a USB-C edge connector, it could shave a few more mm of size for sure.

      2. If I were to attempt it. I would first try using two flex-pcbs with components sandwiched in the middle. Pads exposed on the outside to make contact with the USB-C.
        Getting everything the right thickness would be hard. If the radius of the bend for the flex-pcb is tight enough, then you could use a single pcb that is folded onto itself, providing the spring tension needed to maintain good contact without having to have such a tight fit, making it easier to pull out.

        Myself, I can’t do it. I’d at least have to buy some microscopes to do the work because there’s no way I can look at tiny stuff for hours anymore.

    1. The photodiodes have both a mechanical function (hold the PCB in place inside the port and maintain contact) as well as an electrical function (remote activation). I came up with the electrical function only after deciding I needed some sort of taller SMD component to use as a spacer. Ofc the range isn’t great but a surprising amount of light does get through

    2. Apparently a 3W IR source is needed at short distance (0.5 to 1m), so it barely works, but it works. A keylogger would need software on the device to send the info to the USB device.

  2. I’m probably just missing the point (and I suppose making something small is fun in and of itself), but why have a connector on something you can’t remove again at all? If you’re building a one-time insert USB device, why not just put it inside a usb c cable’s connector like you already get with things like the usb pd cables that have a little OLED screen that tells you the voltage and current being drawn? Or like how lightning cables used to do some conversions or some weirder converters..

    If you want it to be stealthy and pass-through you can just add a usb-c port at the end of it. So it is just a usb-c cable. That way there isn’t even a suspicious little board blocking the port.

    1. what jumps to my mind is enhancements for laptops. if you don’t have or can’t use an internal like M2 sort of connector then the USB is handy…but you don’t want a big dongle or cable hanging off of your laptop if it ever actually leaves your desk. so this fits the bill nicely. and the market has already produced usb bluetooth dongles that only stick out about 4mm…bigger than this device (and easier to remove), but just barely.

      of course i don’t know too many devices that are useful and happy inside a usb connector. even the bluetooth dongles seem to get lousey performance as a compromise for being mostly encased in the shielded connector. (?)

  3. Perfect for pranking stores. Stick one containing the file of Rick Astley’s Never Gonna GIve Up video and had it sit and do nothing for a few hours (tri-state D+ and D- pins) and wait x minutes or hours since power on, then connect the pin, behave like an USB storage, and offer video file. Most TV often auto plays video file if there’s only one.

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