How to Build a Smart Ring (that Tracks Your Activity, Motions)

For years, smartwatches have been the go-to wearable for tracking health and fitness data. But for someone like me—interested in comfort, miniaturization, and seamless integration with daily life—smartwatches always felt a bit bulky. That led me to ask: can we create something smaller, more ergonomic, and just as powerful? Could a ring replace a smartwatch?

This question became the foundation of my project: building a smart ring from scratch—capable of tracking real-time motion and heart rate, all while fitting into a space smaller than a coin.

Here’s a concise Bill of Materials (BOM)

1. Nordic nRF52832 BLE SoC (ARM Cortex-M4) — 1
2. ICM-42670P 6-axis IMU — 1
3. MAX30102 PPG heart-rate sensor — 1
4. TXS0108E 8-channel voltage level shifter — 1
5. Arc-shaped Li-Po battery — 1
6. Custom 2-layer flexible PCB — 1
7. 32 MHz crystal oscillator (for BLE radio timing) — 1
8. 32.768 kHz low-frequency crystal (for RTC/power management) — 1
9. Ceramic decoupling capacitors (0.1 µF and 1 µF) — assorted
10. Pull-up resistors for I²C (≈4.7 kΩ) — 2
11. Test pads or board-level I²C connectors — as needed
12. Battery protection/polymer fuse (integrated with pack or discrete) — 1

You can of course adjust passive counts (bypass caps, pull-ups) based on your schematic.

Part of my inspiration came from watching Becky Stern’s teardown video on commercial smart rings. Her breakdown of the internals of mass-manufactured smart rings helped me understand how commercial manufacturers manage space, sensor alignment, battery integration, and mechanical design in such compact enclosures. This deep dive helped me visualize how to stack components and reinforced the idea that building a smart ring wasn’t just possible—it was achievable.

My smart ring prototype includes the following features:

1. 6-axis motion tracking (accelerometer + gyroscope)
2. Heart-rate monitoring using optical PPG
3. Bluetooth Low Energy (BLE) data streaming to a smartphone
4. Powered by a 100 mAh curved Li-Po battery
5. Built on a 2-layer flexible PCB, housed inside a custom 3D-printed ring enclosure

Notably, I intentionally avoided adding an LED indicator or wireless charging features in this version. These were excluded to conserve space, minimize power consumption, and keep the hardware and firmware simple for the first few iteration.

The heart of the Smart Ring lies in its flexible PCB, which allows the circuit to conform to the curvature of a finger. Using Autodesk Eagle, the schematic and PCB layout were designed to minimize space while maximizing functionality.

Key Design Considerations:

1. Component Placement: Centralized to maintain balance and comfort.
2. Trace Routing: Optimized for signal integrity and minimal interference.
3. Connector Pads: Positioned for easy access during assembly and charging.

Once the design is finalized, send the Gerber files to a PCB manufacturer that supports flexible substrates,

CAD & PCB Design

1. Used Fusion 360 to design the ring’s mechanical enclosure and cavity layout.
2. Designed the 2-layer flex PCB in EasyEDA

Component Assembly

1. I choose to get assembled from pcb manufacturers
2. Connect Battery: Solder the Li-Po battery, ensuring correct polarity.
3. Insert into Enclosure: Gently bend the flexible PCB and insert it into the 3D printed ring enclosure.
4. Seal the Enclosure: Use a small amount of flexible adhesive to seal the enclosure, protecting the electronics from moisture and wear.

Testing and Debugging

1. All I2C devices were tested independently.
2. Power rails were verified using a multimeter before the first power-on.
3. BLE communication was tested using the nRF Connect app on Android.

All firmware was written in Segger Embedded Studio using Nordic’s SDK. Key components included:

1. Initialization: Set up peripherals and initialize sensors.
2. Data Acquisition: Continuously read data from sensors.
3. Data Processing: Apply filters and preprocess data for transmission.
4. Communication: Transmit data to the mobile device via BLE using NRF Connect.
5. Power Management: Implement sleep modes to conserve battery life.

BLE data was streamed every 20ms and could be visualized using mobile dashboards.

Using data from the IMU, the smart ring can:

1. Count steps based on acceleration patterns
2. Detect repetitive motions such as arm curls, punches, or typing gestures
3. Recognize posture changes like sitting-to-standing transitions

With more advanced filtering or machine learning models or mathematical model, this data can be expanded to support full activity recognition systems, including automatic workout detection or fatigue monitoring.

To visualize and analyze the data transmitted by the Smart Ring:

1. Install NRF Connect: Available on both Android and iOS platforms.
2. Pair the Device: Scan for BLE devices and connect to the Smart Ring.
3. Monitor Data: Use the app's interface to view real-time sensor data.
4. Data Logging: Optionally, log data for further analysis or machine learning applications.

The Smart Ring's capabilities can be significantly expanded by integrating offboard machine learning algorithms:

1. Activity Recognition: Classify different physical activities (e.g., walking, running, typing).
2. Health Monitoring: Detect anomalies in heart rate or movement patterns.
3. Gesture Control: Recognize specific gestures to control other devices.
4. VR hand movement: Using motion tracking data
5. By transmitting sensor data to a smartphone or cloud service, complex algorithms can process the information without burdening the ring's microcontroller, allowing for sophisticated features without compromising battery life.

Hardware Upgrades for Future Versions

1. Integration of wireless charging via an ultra-thin coil
2. Optional capacitive touch sensor for tap/gesture input
3. Addition of a vibration haptic motor for alerts (later abandoned this considering vibration may interfere with electronics, size and power factors)

The Smart Ring exemplifies how wearable technology can be both functional and unobtrusive. I will try to work on this and keep it updated in instructables