An ARM Powered Business Card, Part Two


While most microcontroller powered business cards opt for something small and cheap, [Brian] is going in an entirely different direction. His business card features an ARM processor, some Flash storage, a USB connection, and enough peripherals to do some really cool stuff.

This is the second iteration of [Brian]’s business card. We saw the first version, but this new version makes up for a few mistakes in the previous version. The biggest improvement is the replacement of the Molex USB plug with bare traces on the board. [Brian] couldn’t find a board house that could fab a board with the proper thickness for a USB plug, but a few strips of masking tape did enough to beef up the thickness and make his plug nice and snug. Also, the earlier version had a few pins sticking out of the board for programming purposes. This wasn’t an idea solution for a business card where it would be carried around in a pocket, so these pins were replaced with a connectorless programming adapter. Just a few exposed pads gives [Brian] all the programming abilities of the last version, without all those prickly pins to catch on clothing.

With his new business card, [Brian] has an excellent display of his engineering prowess and a very cool toy; he has a project that will turn this card into a keyboard emulator, randomly activating the Caps Lock button for a few seconds every few minutes. A great prank, and a great board to give to future employers.

39 thoughts on “An ARM Powered Business Card, Part Two

        1. +1
          I’d assume the people he is giving these cards to do not know him that well. If someone that I didn’t know gave me something and told me to plug it into my computer I’d be more than suspicious…

  1. Some pointers for a person looking for a job with this. These are the
    things I would pick at in a HW review/interview. (I know you can get away a lot IRL.)

    The USB _series termination_ resistor should be closer to the
    processor than the connector. The USB traces should be controlled
    impedance and treated and routed as differential pairs.

    Should put a filter cap to input power close to USB fingers and ESD
    protection should not optional on the exposed USB pins. One should also
    make sure that the blue LED brightness isn’t blinding like cheap Chinese

    Tape is icky if you do intend to hand this out to people. May be a thin
    plastic or FR4 super glued to the back side of board to make up for the
    right thickness.

    1. Yyyyeah… if your goal is a job where this is what you will be doing, you had better make sure the example of your work does not include fairly elementary design errors.

      To continue what tekkieneet started:
      – This appears to be a two-layer board. It is very difficult (read: impractical) to do proper differential impedance routing without a nearby ground plane. Additionally (as tekkieneet noted) the traces are not routed as pairs, they are not width controlled nor length matched, and there are stubs galore. Also it is recommended to make the series termination resistors as small as possible to minimize impedance mismatches and subsequent reflections. This board will work, but only by virtue of running at only USB Full Speed (11Mbit) and by USB’s inherent robustness against poor interconnect quality.
      – Less than in the original version, but still a few 90-degree angles. Argue if you want about whether or not it makes a difference in this application, but it looks wrong, and the name of the game with this project is impression – hand them an amateur looking example project, they’ll think of you as an amateur.
      – Where did the ground plane under the oscillator go? That was a good feature on V.1… although the plane is usually expected to follow the oscillator traces back to the MCU and terminate at a via underneath the package, typically sharing with an adjacent ground pin. That’s what Pin 5 of the device is for. But it appears to be missing altogether in this version.
      – There’s a decoupling capacitor over the noisy clock line. Not a big fan of that decision.
      – Agree on the use of tape on the back, not real professional. Yes, per the mechanical specifications for USB, the thickness should be 2.4mm (2.25mm for the inner tongue and 0.15mm for the metal shell), but Seeed’s 2mm boards should be pretty close to that. But I guess for $32 added cost ($16 thickness surcharge plus $16 ENIG plating) that’s a tradeoff. Maybe something else though, like soldering copper shim stock to the bottom? Tape is so garage-shop, and I wouldn’t want to put it in my computer and risk gumming up the works.
      – Reddit said it and I’ll repeat: The hand soldering is not clean, a reflow job would be so much nicer. A toaster oven is under $40, invest in one and some paste and be amazed at the increase in quality. (PS: you can’t say you were “Featured on Reddit” if you yourself submitted the project)
      – Have some pride in your work! Complete the title block in the schematic and clean up your PCB files. You’re releasing this to the world, and you’ve given your potential employer a direct invitation to look by including your URL. And (and this is not a criticism of this project per se but of a vast number of DIY projects) release a PDF copy of the schematic, so people don’t have to install Eagle to see your work! V1 had a screenshot of it, at least.
      – P.S. the Github version of your board still contains your phone number…

      1. Where do you guys learn all this routing knowledge?

        I come across some of these things here and there in different application notes, but I’d prefer some central source of good routing practices.

        I would appreciate being pointed in the right direction.

        1. ‘Read app notes’ is about the best thing I know to do. I too would love for there to be an ultimate encyclopedia of do’s and do-not’s for electronic design, but I don’t know of one.

          Actually, that’s a great idea. Now who wants to start it with me? :D

      2. There’s nothing wrong with making a design for full speed USB if the chip doesn’t support high speed anyway. It’s only 12 Mbps, so using a double layer board is fine. Of course, within the limits of 2 layers, the layout should still be made as nice as possible.

        1. I’m aware that the chip doesn’t support high speed, but just because it’s not hundreds of megabits doesn’t mean there isn’t a specification for it and that it shouldn’t be followed. I can run DisplayPort over a foot of single non-twisted-pair hookup wires too, but that doesn’t mean that that’s the way it’s intended to be done, only that there’s enough noise immunity in the receiver for it to struggle through it.

          Ultimately as I have previously noted, this is an example of ‘this is what I can do’, so it’s about proving that you know just what the heck you’re doing – otherwise you’re much better off just handing them a piece of paper and leaving these things at home. I also wouldn’t hand a business card with clipart on it and all the text in Comic Sans to a job interviewer for a professional graphic design position – same basic concept.

  2. Why not put some sm parts on the underside of the board in the correct
    location and thickness to make the USB fit tightly. They would not need to be functional, they just need to be the right thickness and fairly robust.

      1. well I used to deal in pager motors for the micro rc hacking scene about ten years ago and made 15 Grand in six months. my total costs including said motors; $250. There is a market for everything.

      1. Nope. Proper procedure is to not allow any outside devices to connect to any internal computers or networks. Only authorized devices acquired, secured and issued by IT are permitted access. Any device that has been connected to an external device or network is considered compromised and must be scanned or wiped before being allowed on the internal network.

  3. I’ve got a relative who works for a company that makes business cards out of plastic, with a snap-off USB memory stick on the end. Among other things. I think that’s rather neat! Of course it could be full of viruses, but it’s a big company. If they give people viruses you’re not gonna spend large amounts of money with them. Same way you trust Kelloggs not to put rat shit in your cornflakes.

  4. it’s interesting but i’m not a fan of his part selection. specifically, it uses parts with exposed pins. oh yeah, the ultrabright blue LED is just obnoxious. i go with a 5V (avoids making a power system) QNF chip or maybe even a BGA, make all the passives 0604 or smaller and put the parts fairly close to the plug. for the final version, use a black PCB, put all the text in exposed copper then add solder so that the text is shiny.

    malcontent? i dont know what you are talking about? ;P

    1. Not to mention that no network administrator in his right mind would have a usage policy that would permit a device such as this to be used on workplace devices. From a security standpoint, its a really stupid idea to get into the habit of running up to the nearest computer to plug in some device some random stranger handed you on the street. I don’t even allow thumb drives.

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