Send My Sensor

This project uses OpenHaystack to upload sensor data from a device without internet connection by (ab)using Apple's Find My network. The data is broadcasted via Bluetooth Low Energy and forwarded by nearby Apple devices.

The application is based on the Send My project and runs on the real-time operating system Zephyr. Because this firmware is based on Zephyr, you can create your own sensor boards 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 with sensor data that will be visible in Send My's DataFetcher application in macOS.

Disclaimer

The firmware is just a proof-of-concept and the messages that it sends can be read by anyone who knows the modem ID; there's no extra encryption or authentication.

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.

The proof of concept reads the temperature from a Bosch BME280 sensor, but it can be easily adapted to use any other sensor that Zephyr supports.

Requirements

• A Bluetooth Low Energy device, supported by Zephyr
• A BME280 sensor, built-in (as in the earlier versions of the RuuviTag) or on a breakout board connected to your board's I²C bus
• A Zephyr development environment
• Send My's DataFetcher application on macOS to read the sensor data

Initialization

To build this firmware, you first need the OpenHaystack Zephyr module. Install this by initializing a workspace folder (for instance zephyr-workspace) where the application and all Zephyr modules will be cloned.

# 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

Then clone the Send My Sensor repository and enter its directory:

git clone https://github.com/koenvervloesem/send-my-sensor
cd send-my-sensor

Build

To build the firmware, run:

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

Replace $BOARD by your target board.

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.

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

Change the modem ID

You need to specify a modem ID in the firmware image. Change the 32-bit unsigned integer value modem_id in main.c) and then build the firmware.

Flash

How to flash the image to a device depends on the device and its bootloader. For many devices, including the RuuviTag if you use the RuuviTag Development Kit, 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 send-my-sensor.zip

This packages the application in the file send-my-sensor.zip. Now press the reset button and flash the package onto the board with:

nrfutil dfu usb-serial -pkg send-my-sensor.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.

Supported devices

This procedure has been tested with:

• Nordic Semiconductor's nRF52840 Dongle (board name nrf52840dongle_nrf52840) with an I²C breakout board of the Bosch BME280 temperature/humidity/pressure sensor. Solder headers to the dongle and connect the BME280's SDA to pin 0.31, SCL to pin 0.29, GND to GND and VCC to VDD. If your BME280 sensor has another I²C address than 0x76, change this in the device overlay nrf52840dongle_nrf52840.overlay.
• Ruuvi's nRF52832-based RuuviTag (board name ruuvi_ruuvitag), which has a built-in BME280 in earlier versions. Use the RuuviTag Development Kit to flash the firmware.

Other Bluetooth Low Energy devices supported by Zephyr should work as well, as long as they have a built-in BME280 or you can connect one over I²C. You may need a custom device overlay. Please let me know if you manage to run this firmware on another board, or if you need assistance, so I can add it to the list of devices it has been tested with.

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 Send My firmware for ESP32
• the Zephyr Example Application for the project structure

License

This project is provided by Koen Vervloesem as open source software with the GNU Affero General Public License v3.0. See the LICENSE file for more information.