The idea behind the PixMob wristband is simple — at a concert, organizers hand these out to the concertgoers, and during the show, infrared projectors are used to transmit commands so they all light up in sync. Sometimes, attendees would be allowed to take these bracelets home after the event, and a few hackers have taken a shot at reusing them.

The protocol is proprietary, however, and we haven’t yet seen anyone reuse these wristbands without tearing them apart or reflashing the microcontroller. [Dani Weidman] tells us, how with [Zach Resmer], they have laid the groundwork for reverse-engineering the protocol of these wristbands.

Our pair of hackers started by obtaining a number of recordings from a helpful stranger online, and went onto replaying these IR recordings to their wristbands. Most of them caused no reaction – presumably, being configuration packets, but three of them caused the wristbands to flash in different colors. They translated these recordings into binary packets, and Dani went through different possible combinations, tweaking bits here and there, transmitting the packets and seeing which ones got accepted as valid. In the end, they had about 100 valid packets, and even figured out some protocol peculiarities like color animation bytes and motion sensitivity mode enable packets.

The GitHub repository provides some decent documentation and even a video, example code you can run on an Arduino with an IR transmitter, and even some packets you can send out with a  Flipper Zero. If you’re interested in learning more about the internals of this device, check out the teardown we featured back in 2019.

This aquatic animal thing looks interesting… Not cheap but obviously quite capable.

Override the signals at the concert and have them flash any message you like in Morse code.

I don’t see why people find that sort of thing appealing. Might as well just take an airhorn if you want to ruin a show for everyone.

It is hard to bring one in I guess

Something fun you can do with these (destructively): Their default mode of operation is to pulse using the accelerometer through different colors. If you are at a concert and want to rebel, you can simply pop open the battery port and break off the IR sensor, which is accessible, then pop the battery back in. When it powers off it (IME anyway) resets to this default operating mode. It will perma-break the IR part for you but the device will still work, and will pulse through a rotation of colors, reacting to clapping or similar jarring movements.

Depending on venue you can do this with a pen, a small enough key, or a stiff enough paperclip.

hehe, that sounds handy to know, thank you!

Hey! Sadly, the newer bracelets (past couple years) mostly don’t have the motion sensor in them, so it doesn’t work on those.

Having worked closely but never directly with pixmob in the real world (on some of the mentioned events) my understanding is that there is a “start” command at the top of the show to arm all the bracelets into receiving commands via IR. After the show there is essentially a “nuke” command sent to make the devices ignore IR and just use the accelerometer.

My other take is that these devices have per show programming on the micro and some of the FX are hardcoded. So a band from Taylor may not work with commands from Coldplay.

Hey! I’ve been able to control bracelets from old shows with even after they were (potentially) “nuke” command before. That includes taking ones from an event where they were put in motion sensitive mode after the show and making them do things (like in the video on the GitHub repo).

Regarding per-show programming, I have collected bracelets from 6 different concerts/events (over a multi-year period) and they react *mostly* the same to the commands I have found. That said, there have been some differences. My working theory is that shows send a lot of programming data before the show starts/during breaks that queue up those per-show effects, rather than having them hardcoded. Part of my evidence for that is that the IR transmitters at events are very active for much of the time, even when the bracelets are doing nothing.

Would be interesting to know what else you have heard about how these work though!

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