The device I took apart is what I believe to be the Fitbit Flex. Released as one of Fitbit’s earlier activity trackers, the Flex stood out for its minimalist design: no screen, just a slim core module that tracked steps, activity, and sleep while sitting inside a flexible rubber wristband. Its simplicity and lightweight build made it popular, and the removable band design allowed users to swap colors and styles with ease.
Tools I used for this teardown:
- My hands
- Tweezers
- Precision screwdriver handle
- Extra smaller precision screwdriver handle for the tiniest screws
To begin the teardown, I first removed the tracker from the band. Since the Flex was designed to be interchangeable, this step was fairly easy—I just used my hands to press and slide the small tracker module out of the wristband. Once I had the core in hand, I noticed there were three tiny screws securing it to the band, although one was already missing. Using my precision screwdriver, I carefully removed the remaining screws. Because of their size, I switched to an even smaller screwdriver handle to make sure I didn’t strip or lose them.
With the screws removed, the tracker was free from the strap, leaving me with the main module ready for further disassembly.
After removing the module from the band, we can see the silicone strap with its inner plastic casing that holds the electronics in place. Inside the cavity is a rubber insert, which helps reinforce the housing and keep the capsule secure. Next to it is the main capsule, which contains the circuit board, chips, and the LED indicator section. There’s also some adhesive residue around the housing, showing how the unit was sealed into the strap.
Upon further examination of the unit, we can see that the capsule houses all the core electronics. The main PCB (printed circuit board) is exposed, with various chips, sensors, and contact pads visible. At one end sits the LED indicator array, responsible for displaying the familiar row of dots on the tracker. The construction shows how compactly everything is integrated into a single module, designed to slide neatly into the strap while remaining water-resistant and durable.
I continued to pry open the Fitbit Flex using tweezers and observed the battery section. There’s a circular coin-cell battery, which looks like a standard lithium battery for small devices. Next to it, there’s a square replaceable battery with the markings +LSSP031420AB – 531869887, connected to the PCB via a small connector.
The NFC antenna is also visible. It’s a thin coil integrated into a flexible PCB, and during the teardown, it was broken. On the Flex, this antenna isn’t used for payments; it primarily handles wireless signal functions, like aiding Bluetooth syncing or device detection.
Opening the case exposes the main circuit board. At the center is a white flex connector, which links the board to the battery and sensors. Around it are gold test pads used in manufacturing to check the circuits.
On the left is the cavity for the vibration motor, responsible for buzz alerts. The right side shows the battery compartment, where the lithium-polymer cell normally sits, with contacts connecting to the board. The yellow gasket material around the edges helps seal and protect the electronics from sweat and movement.
The main printed circuit board (PCB) sits in the module, holding the microcontroller, memory, accelerometer, and power management circuitry that make the Flex track steps, monitor sleep, and sync with your phone.
On the PCB, there are small white label stickers with QR codes. These are identification labels that contain information like serial numbers, batch numbers, and part identifiers. They are used during manufacturing for tracking, testing, and quality control, and don’t affect the device’s normal operation.
The Fitbit Flex is built starting with the rigid multilayer PCB, where copper traces and vias are etched and laminated. Surface-mount components like the microcontroller, memory, accelerometer, PMIC, LEDs, and tiny resistors and capacitors are placed on the board using pick-and-place machines and soldered in reflow ovens. The NFC antenna is etched onto a flexible PCB, while the battery and connectors are attached with solder and adhesive. The silicone band is injection-molded, and the tracker module is snapped into place. After assembly, each unit is tested for step tracking, Bluetooth syncing, vibration, and LEDs using test fixtures and optical inspection.
Two design elements that interested me are the removable rechargeable battery and the NFC antenna. I found it really interesting that the Fitbit Flex has a replaceable battery, and even an extra one tucked in the module. The designers likely added this to extend the device’s lifespan, letting users swap batteries instead of replacing the whole tracker. It also adds convenience for servicing and ensures the device remains reliable over time. The thin copper NFC antenna also caught my attention. Even though the Flex doesn’t use it for payments, it’s included to enhance wireless communication, like improving Bluetooth syncing or device detection. Its compact and flexible placement shows careful consideration for space, signal strength, and interference with other components.
In conclusion, the Fitbit Flex’s tracker module showcases a compact, efficient design. From the replaceable battery to the PCB with the microcontroller and accelerometer, and the LEDs and NFC antenna, every component serves a specific function while minimizing size. The device’s modularity, careful component placement, and lightweight design highlight the thoughtfulness of the engineering and make the Flex both functional and durable.