OpenHaystack Zephyr firmware

This project implements firmware with an OpenHaystack application based on the real-time operating system Zephyr.

OpenHaystack is a framework for tracking personal Bluetooth devices via Apple's massive Find My network. Thanks to this firmware based on Zephyr, you can create your own tracking tags with one of the many Bluetooth Low Energy devices that Zephyr supports.

After flashing the firmware to your device, it sends out Bluetooth Low Energy advertisements that will be visible in Apple's Find My network using the OpenHaystack application in macOS.

Disclaimer

The firmware is just a proof-of-concept and currently only implements advertising a single static key. This means that devices running this firmware are trackable by other devices in proximity.

There is also no power management yet. So if you're running this firmware on a battery-powered device, it won't be as energy-efficient as possible. If you want to improve this, all patches are welcome.

Requirements

• A Bluetooth Low Energy device, supported by Zephyr
• A Zephyr development environment
• OpenHaystack's macOS application to view the location of your device

Initialization

The first step is to initialize a workspace folder (for instance zephyr-workspace) where the application and all Zephyr modules will be cloned. You can do that by running:

# Initialize Zephyr workspace folder for the application (main branch)
west init -m https://github.com/koenvervloesem/openhaystack-zephyr --mr main zephyr-workspace
# Update Zephyr modules
cd zephyr-workspace
west update
cd openhaystack-zephyr

Build

To build the firmware, run:

west build -p auto -b $BOARD -s app

Replace $BOARD by your target board.

Once you have built the application, the firmware image is available in build/zephyr.

Use your own key

You need to specify a public key in the firmware image. There are two ways to do this:

• Make the change directly in the source (the char array public_key in main.c) and then build the firmware. You have to initialize the public key like this: static char public_key[28] = {0x61, 0xc4, 0xc2, 0x55, ...} with all bytes of the key instead of the ellipsis. Note that this is the raw key, not the Base64 encoded key.
• Patch the default public key OFFLINEFINDINGPUBLICKEYHERE! in the bin file (build/zephyr/zephyr.bin) to your own key and save the resulting firmware image (see the script openhaypatch.sh for a way to do this).

Flash

How to flash the image to a device depends on the device and its bootloader. For many devices you can run:

west flash

Refer to your board's documentation for alternative flash instructions if your board doesn't support the flash target.

For the nRF52840 Dongle with the built-in bootloader, run:

nrfutil pkg generate --hw-version 52 --sd-req=0x00 \
        --application build/zephyr/zephyr.hex \
        --application-version 1 openhaystack.zip

This packages the application in the file openhaystack.zip. Now press the reset button and flash the package onto the board with:

nrfutil dfu usb-serial -pkg openhaystack.zip -p /dev/ttyACM0

Have a look at ls /dev/tty* for the right device on Linux and macOS. On Windows it should be something like COMx.

For devices with the Adafruit nRF52 bootloader such as the April USB Dongle 52840 or makerdiary nRF52840 MDK USB Dongle, first generate a UF2 file from the hex file with uf2conv.py:

python3 ../zephyr/scripts/uf2conv.py -f 0xADA52840 -c build/zephyr/zephyr.hex

And then drag and drop the file flash.uf2 to the storage device mounted by your operating system.

Supported devices

This procedure has been tested with:

• Nordic Semiconductor's nRF52840 Dongle (board name nrf52840dongle_nrf52840), as well as its derivatives April USB Dongle 52840 and makerdiary nRF52840 MDK USB Dongle, which are both using the Adafruit nRF52 bootloader
• the nRF52833-based BBC micro:bit v2 (board name bbc_microbit_v2)
• Ruuvi's nRF52832-based RuuviTag (board name ruuvi_ruuvitag) using the RuuviTag Development Kit

Other Bluetooth Low Energy devices supported by Zephyr should work as well. Please let me know if you manage to run this firmware on another board, so I can add it to the list of devices it has been tested with.

Using OpenHaystack as a module

The base code is written as a Zephyr module, in the directory modules/openhaystack. You can reuse this in your own Zephyr applications. For examples of how you do this, take a look at:

• the application of this repository in the directory app
• the Send My Sensor project, which uses the OpenHaystack module to upload sensor data via Apple's Find My network.

Debugging

A sample debug configuration to read logs from the USB UART is also provided. You can apply it by running:

west build -p auto -b $BOARD -s app -- -DOVERLAY_CONFIG=debug-usb-uart.conf

This only works with boards that support this, such as Nordic Semiconductor's nRF52840 Dongle.

For the UART logs: run ls /dev/tty* (Linux) or ls /dev/cu.* (macOS) in a terminal window, connect your board and run the command again to check which port appears. On Linux, this will probably be /dev/ttyACM0. Then run screen /dev/ttyACM0 115200 to connect to port /dev/ttyACM0 with a speed of 115200 bits per second.

Learn more about Bluetooth Low Energy development

If you want to learn more about Bluetooth Low Energy development, read my book Develop your own Bluetooth Low Energy Applications for Raspberry Pi, ESP32 and nRF52 with Python, Arduino and Zephyr and the accompanying GitHub repository koenvervloesem/bluetooth-low-energy-applications.

Acknowledgments

This project is inspired by and has used code from:

• the original OpenHaystack firmware for ESP32
• the original OpenHaystack firmware for nRF51822
• Antonio Calatrava's alternative OpenHaystack firmware using Nordic Semiconductor's Softdevice
• the Zephyr Example Application for the project structure and GitHub Actions workflow

License

This project is provided by Koen Vervloesem as open source software with the MIT license. See the LICENSE file for more information.