HARNode: A Time-Synchronised, Open-Source, Multi-Device, Wearable System for Ad Hoc Field Studies

Human activity recognition (HAR) research often lacks accessi-
ble, comprehensive field data. Commercial systems are rarely open
source, hard to expand, and limited by issues like node synchro-
nisation, data throughput, unclear sensor placement, complexity,
and high cost. As a result, researchers typically use only a few
intuitively placed sensors and conduct limited field trials. HARN-
ode overcomes these challenges with a fully open-source hardware
and software platform. Each node includes an ESP32-S3 module
(AtomS3), a 9-axis IMU (Bosch BMX160), pressure and tempera-
ture sensors (Bosch BMP388), a display, and an I2C port. Data is
streamed via Wi-Fi, with NTP-based time synchronisation achiev-
ing 1 ms accuracy. The system runs for up to 8 hours and is built
using off-the-shelf parts, a simple online PCB service, and a com-
pact 3D-printed housing with Velcro straps, enabling flexible and
scalable body placement while requiring little hardware knowledge.
In a study with ten subjects wearing eleven HARNodes each, setup
took under five minutes per person. A random forest classifier dis-
tinguished walking from stair-climbing transitions, showing the
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benefits of sensor-overprovisioning: Seven nodes achieved≈98%
accuracy, matching the performance of all eleven. These findings
confirm HARNode’s value as a fast-deploying, scalable tool for
field-based HAR research and optimised sensor placement.