Fitness Tracker Band

Overview

A fitness tracker band is a wrist computer tuned for measuring the body in motion: heart rate, steps, sleep, blood oxygen, and outdoor routes, all logged and synced to a phone. The Display Module shows stats and notifications on a small touchscreen, while the Mainboard, a low-power SoC with a sensor co-processor, flash, and Bluetooth, runs the firmware and stores activity. Sensing is split across three modules: the Optical Heart-Rate Sensor reads the pulse optically through the skin, the Motion Sensor tracks movement and posture, and the GNSS Module fixes outdoor position. A small Battery Pack gives multi-day life, the Vibration Motor delivers silent alerts, and the Strap keeps the sensor pressed to the wrist. The whole stack sits in a sealed Case & Caseback with a sensor window on the back.

How it works

Heart rate is measured by photoplethysmography. The Optical Heart-Rate Sensor shines its PPG LED light into the wrist and the Photodiode reads how much bounces back; blood absorbs more light at each pulse, so the reflected signal pulses too, and the optical front-end converts that ripple into beats per minute. Switching to red and infrared LEDs lets the same module estimate SpO2 from how the two wavelengths are absorbed differently.

Steps, sleep stages, and workout type come from the Motion Sensor IMU (6-axis): the firmware watches the acceleration pattern of each arm swing and classifies the activity. The barometric altimeter adds floors climbed. For an outdoor run the GNSS Module receiver locks onto satellites and records the track and pace. All of it is timestamped and buffered to flash on the Mainboard, then handed to the phone over Bluetooth LE when in range. Because every part is low-power and the radio sleeps between syncs, the Battery Pack lasts the better part of a week.