An open physical and electrical interconnect standard for wearable band modules

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The idea behind InterBand is to create a standard interface, both physical and electrical, that allows specialised modules to be created by different makers and easily used with each other to form a "smart band". Modules could include things like a Bluetooth host, battery, end strap connectors, sensor modules (accelerometers, temperature, humidity, sound, etc.), and outputs such as screens and speakers. The interface specification only defines the very ends of the module, and so the majority of a custom module can take any shape needed for its function. The electrical connection consists of 3.3V power rail and a data bus, with a band consisting of a host module and any number of slaves.

The drive behind creating InterBand comes from two places. The first is that while there are heaps of awesome wearable projects by makers popping up at the moment, many of them are having to cover the same ground before implementing the special features unique to their project. Things like Bluetooth, display drivers, and battery management circuitry are included in many of them without much reuse of designs. Wouldn't it be great if makers could instead use existing open source modules for these, mix-and-matching what was required for their project, and designing their own module to implement specialised features?

The second is to try and encourage people to use a bit more of the available real-estate on a wrist, arm, or ankle for their wearables. The established form-factor seems to be a small blocky thing on top of the wrist, with a flexible band holding it on. Bands created using InterBand would instead use the whole strap length to fit electronic modules, allowing more room for things like batteries, and making it easier to implement features without having to design everything to be extremely compact.

(side note: I'm not particularly attached to that name yet. It's simply the only thing I could come up with that doesn't already refer to another system or product. Suggestions welcome)

Project Goals:

To be a successful, useful standard, InterBand has several project goals.

  • Open. I really want people to able to use this, which means getting as much input as possible. I'm yet to see how practical it is, but even the physical interface could be included in the GitHub repo if it's in a textual form like SCAD
  • Easily constructed. In short, ideally the whole thing can be 3D printed, with the option of milling it. This means that parts of the physical interface cannot be too small or fine, particularly around load points like the hinge. Ideally it would also be able to be printed without support.
  • Cheap, and easily sourced. Any components that can't be easily fabricated need to be easily accessible.
  • Low entry barrier. Example module STLs, and good documentation on the bus protocol.

System Outline:

(pinched from the Electrical interconnection project log)


This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Australia License.

  • 2 × Module housing Currently being 3D printed in ABS.
  • 2 × M1.6x4mm Socket Head Screw Got these from Element 14, though the length isn't critical. The original silver ones I think came from an old photocopier...
  • 2 × Molex SlimStack connecter, male+female Element 14:
  • 1 × Jumper Board Not quite finished yet, possibly FPC?

  • Printed prototypes

    novirium08/17/2015 at 20:01 0 comments

    This is actually a prototype a did months ago, when first thinking about the project. Back then I had started with the idea of only using two lugs per side on the hinge, where the screws were set in permanently, and the modules slid together sideways. A cylinder held in place by the jumper (here shown as a piece of black cardboard) would then hold the two inner lugs apart. It worked reasonably well, but essentially relied on the jumper staying on place (being a piece of cardboard, it did not).

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  • Jumper design

    novirium08/17/2015 at 15:46 0 comments

    To progress with the design of the electrical connection between modules, I figured I really just needed to start mocking things up and trying them. To that end, I drew up a rough version of an FPC jumper using the Molex SlimStack connectors I've mentioned before.

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  • Physical Hinge

    novirium08/17/2015 at 14:09 0 comments

    The point of the physical part of the InterBand standard is to only define the shape of the end of modules, allowing them to happily interconnect with a range of modules without unnecessarily restricting the shape or form of the rest of the module. For instance, battery modules would probably lend themselves to a flatter, thinner form intended to sit underneath the wrist, possibly with a lug on the side to house a USB micro port for charging. Smaller sensor modules would happily sit on the side of the wrist, being shorter and allowing a greater curvature.

    There are several forms this hinge could take, and like the electrical connection, I'm yet to firmly settle on one. In general, it seems like a good idea to use two hinge sections, one on each side, with a gap in the middle for the electrical connection. While I'd like the body of the modules to be 3D printable, the hinge axis itself is something that should probably be a separate component. The goal is to have the hinge be similar in size to conventional watch bracelet links, 22mm wide and no more than 4mm high. At these sizes, a pin in a 3D printed hole would not be strong enough.

    So far I've found two options for the hinge axis: either use standard watch band pins, protruding all the way through the hinge, or small screws (likely M1.6). It's reasonably easy to get hold of either of these. Watchband pins have the advantage that they're fairly low profile, but I suspect that they rely on fitting tightly into a hard wearing hole to retain their position effectively (not how I'd describe the ABS and PLA used in 3D printing). Another disadvantage of these pins is that they obstruct the middle of the hinge, meaning any electrical connection would have to go around the center axis of the hinge, rather than being able to pass through it as it moves.

    Small screws would have to be used in pairs, one on each end of the hinge, and can self tap relatively easy into 3D printed plastic. A potential problem with them is that the thread might slowly wear on the sliding half of the hinge (possibly even loosening the screw)

    Of course, having a triplet of interleaving lugs on each side of the hinge isn't the only option. I've briefly thought having a short bar snap into a sort of cup (think a closed lug with a slot cut into the side of it), but am pretty sure that trying to print this at diameters under 4mm would fail.

  • ​Electrical interconnection​

    novirium08/17/2015 at 13:34 0 comments

    The electrical connection and bus between the modules is a big part of what would make the InterBand standard successfull. The current plan involves 5 connections: GND, 3.3V, SDA, SCL, and INT. GND and 3.3V are pretty self explanitory, and SDA and SCL essentially implement I2C or TWI. Even though these protocols support multiple hosts, the intention behind InterBand is to have a single host module with many slaves.

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