Many users see the smart bracelet as a magical gadget that just wears on the wrist and starts showing numbers. However, inside the compact body of the Mi Band or Xiaomi Smart Band, there is a complex set of microelectronics. Understanding the principles of these devices helps not only to correctly interpret health data, but also to extend the battery life.
The basis of any tracker is a bundle of sensors and a microcontroller that processes incoming signals in real time. Unlike a classic watch, time is only one of many functions available to the user, the main task of the device is the continuous monitoring of physiological parameters and physical activity of a person.
In this article, we will examine the physics underlying measurements and look at software algorithms that turn raw data into comprehensible statistics in the Mi Fitness or Zepp Life app.
Optical plethysmography and pulse measurement
At the heart of the health monitoring system is an optical heart rate sensor that works like photoelectric plethysmography, and on the bottom of the bracelet are green-light LEDs and a photodetector, and the green light is not chosen by chance, because the hemoglobin in the blood actively absorbs this spectrum of radiation.
When the heart contracts, the volume of blood in the wrist capillaries increases, and the absorption of light increases. When the heart relaxes, the blood flow decreases, and the light passes better. The sensor records these vibrations at a high frequency, converting them into a pulse wave graph.
Algorithmic signal processing allows you to sift out noise caused by the movement of the hand or a bad fit of the bracelet. Modern Xiaomi models use an array of several LEDs to improve the accuracy of measurements even during intensive training.
⚠️ Warning: Tattoos, thick hair on the wrist or too weak fit of the strap can block the green light, which will lead to incorrect pulse readings or complete absence of data.
To measure blood oxygen saturation (SpO2), the device switches to the red spectrum of radiation, oxyhemoglobin and deoxyhemoglobin absorb red and infrared light differently, allowing the microcontroller to calculate the percentage ratio.
Why can measurements be inaccurate?
Accelerometer and gyroscope: how the tracker counts the steps
The tracking of movement is performed by an inertial measuring module consisting of a three-axis accelerometer and, in more advanced models, a gyroscope. The accelerometer responds to the acceleration of the device in space, recording every movement of your hand, and the gyroscope determines the orientation of the bracelet in three-dimensional space.
When you walk, your hand makes pendulum-like movements. The accelerometer detects the characteristic bursts of acceleration. The software filter analyzes the amplitude and frequency of these bursts to distinguish walking from waving your hands when talking or clapping your hands.
- 🚶♂️ False-step filtering: The algorithm ignores short series of motions that don’t look like rhythmic walking.
- 🏃♂️ Determining the type of activity: by the pattern of movement and the frequency of steps, the system knows whether you are running or walking fast.
- 🛌 Sleep analysis: At night, sensor sensitivity increases to detect even minimal movement to detect sleep phases.
It's important to understand that step counting is a mathematical model, not an absolute truth, and it's calibrated based on your height and weight, which you type in when you first set up your profile.
Monitoring of sleep and its phases
Rest analysis is one of the most integrated features of Xiaomi wearables, and at night, the bracelet goes into a constant but energy-efficient motion and pulse scan, and a sleep hypnogram is built on that data.
There are several stages that the algorithm distinguishes between: wakefulness, light sleep, deep sleep, and REM sleep. Light sleep is characterized by periodic body movements and an unstable but slow heart rate, and it is in this phase that we spend most of the night.
Deep sleep is a period of complete recovery, during which the muscles are relaxed, there is little movement, and the heart rate becomes minimal and very smooth, and the disruption of this phase leads to a feeling of weakness in the morning.
Phase REM (Rapid eye movement is associated with dreams, during which the brain is active, the pulse may increase, but the body remains immobilized. REM-The phase is only available in new models with improved algorithms.
⚠️ Warning: If you sleep in a position where your wrist is above heart level or clamped by a pillow, the pulse sensor may lose the signal and the deep sleep phase will be recorded incorrectly.
Comparison of technologies in Xiaomi models
Xiaomi fitness bracelets evolved along the path of increasing the number of sensors and improving the energy efficiency of processors. Whereas the first models relied mainly on an accelerometer, modern devices are full-fledged medical gadgets.
The table below compares key characteristics and technologies used across different device generations.
| Characteristics | Mi Band 4 | Mi Band 6/7 | Xiaomi Smart Band 8 |
|---|---|---|---|
| Pulse sensor | PPG (1 diode) | PPG (2 diodes) | PPG (improved) |
| SpO2 sensor | Absent. | There is. | Got it (automatic) |
| accelerometer | 3-axis | 6-axis (with gyroscope) | 6-axis (high accuracy) |
| Autobrightness | No. | There is. | Aye (adaptive) |
The presence of a gyroscope in the new models allowed the introduction of automatic screen rotation detection and more accurate tracking of swimming, where hand movements differ from walking.
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The newer the bracelet model, the more complex the noise filtering algorithms are, which is critical for the accuracy of measurements when driving actively.
Synchronization and data transmission
The bracelet itself is just a data-collection terminal, all analytics are done in the cloud or on a smartphone after synchronization, and Bluetooth Low Energy (BLE) protocol is used to transmit information, which provides minimal power consumption.
The process of data exchange is as follows: the bracelet accumulates information in the internal buffer memory, when you connect to a smartphone through an application (Mi Fitness or Zepp Life), the accumulated data is transferred in batches.
It's important to note that the bracelet doesn't store data forever. In the case of overflowing memory, old records can be rewritten with new ones if the synchronization has not been done for a long time. 7-14 squirrel-day.
- 📡 BLE 5.0 and higher: Provides more stable connection and longer range than older versions.
- 🔄 Background synchronization: the application periodically polls the bracelet to update the widgets on the smartphone screen.
- 🔒 Encryption: Health data is transmitted in encrypted form to protect user privacy.
If the connection is interrupted, the bracelet continues to write, marking the time stamps, and when the connection is restored, the data is "stitched" into a single lossless timeline.
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For quick synchronization, open the app immediately after workouts. Don't wait for the smartphone to find the device itself - this may take longer due to the energy-saving Android/iOS modes.
Energy consumption and autonomy
One of the main engineering challenges in creating fitness trackers is the balance between functionality and operating time. Xiaomi uses specialized microcontrollers with ultra-low energy consumption, which are in deep sleep most of the time.
Wake-ups are only due to an event: raising the wrist, receiving a notification or interval measurement of the pulse. AMOLED screens also contribute to savings, since black on them means the pixel is off and zero power consumption.
The main energy consumers are:
- Constant monitoring of SpO2 and pulse.
- The brightness of the screen and the frequency of its inclusion.
- Bluetooth module activity with a bad signal.
The use of the original charger and cable guarantees the correct charging currents, which affects the durability of the lithium polymer battery. BMS (Battery Management System).
⚠️ Attention: Regular battery discharge to zero (until completely turned off) is harmful to Li-Pol batteries. 10-15%.