Teardown: Fitbit Flex doesn’t easily give up its secrets

Article By : Brian Dipert

Sometimes to understand where we are and where we might be going, it can be useful to look back at a product category’s origins. Herein we present to you the Fitbit Flex, introduced in May 2013 and the first wristband tracker in the company’s product line...

In the midst of all this talk about the latest-and-greatest wearables, it can also be useful from an expectation-calibration standpoint to look back at the product category’s origins.

Herein I present to you the Fitbit Flex, introduced in May 2013 and the first wristband tracker in the company’s product line (the company was founded in 2007 but its first few productswere clip-on trackers intended to be attached to a pocket or belt, like a pedometer).

Fitbit flex bands

As you can see, it came in multiple band color (and size, for that matter) options, and even more variants were available from third-party vendors. The main tracker unit, which fits inside the band, was identical in all cases. I purchased my particular black-band unit off Ebay last October for just over $14; its stated condition was “New condition in open box. May have some minor wear from being handled;” whatever that means.

Here’s what I eventually received (the Ebay merchant forgot to include the band in the initial shipment, but promptly rectified the omission):

Fitbit flex package

The translucent shiny plastic “ring” to one side on the front of the band is intended to allow status LEDs from the main unit underneath to shine through (there’s no OLED, LCD, or other fuller-featured display). And next to it is a two-prong clip that holds the band’s two ends together when it’s on your wrist (estimated battery life between recharges is around five days).

Fitbit flex status ring

Fitbit flex clip

Turn the band around, and you can see where the main unit slides into (and out of) it. The rubber is quite flexible, so the insertion and removal operations are actually easier than they might sound:

Fitbit flex main unit

Note that, unlike some of its successors, the Fitbit Flex didn’t integrate aphotoplethysmography sensor for optical heart rate measurement purposes, far from a SpO2 blood oxygen sensor to detect, for example, sleep apnea.

Also included in the packaging is a “micro” Bluetooth Low Energy (BLE) USB adapter for use with computers that don’t already contain relevant built-in wireless communication resources:

Fitbit flex BLE adapter

Here’s a closeup of the markings on one side of it:

Fitbit flex BLE markings

This is what the main unit looks like when inserted in its companion USB-based charger:

Fitbit flex charger

Fitbit flex charger USB

And here’s a peek at the inside of the charger with the main unit removed, showing the multi-pin power interconnect between them:

Fitbit flex power interconnect

Now for the main unit:

Fitbit flex main unit

One end is rounded and mates up with the aforementioned charger:

Fitbit flex charger

The other is angled, marked with faint “Fitbit” branding and translucent (as you’ll see more clearly soon) so that the LEDs underneath can shine through:

Fitbit flex end

Let the dissection begin, first of the BLE adapter, and aided by a “box cutter.” Removing one half of the plastic cap exposes the Bluetooth antenna underneath:

Fitbit flex Bluetooth antenna

Take off the cap’s other half, and what’s revealed isn’t quite as exciting, but never fear, we’re not yet done.

A-ha! ICs and such!

Fitbit flex Bluetooth antenna removed

Fitbit flex ICs

Dominating the landscape on one side is Texas Instruments’ CC2540Bluetooth Low Energy Wireless MCU with USB. The “F128” on the end indicates that it contains 128 KBytes of integrated flash memory; a “F256” flavor is also available from the manufacturer.

Removing the remaining plastic frame bits reveals a wee bit more than we’ve seen before:

Fitbit flex dongle

Next comes the main act of the show, the activity tracking unit, with disassembly again assisted by the box cutter, first along the visible seam at one end:

Fitbit flex activity tracking unit

Fitbit flex activity tracking unit

See, I told you it was translucent!

Fitbit flex activity tracking unit cap

Visible at the top is the multi-LED array; below it is the Bluetooth antenna:

Fitbit flex LEDs antenna

To get the guts fully out of the water-resistant case required switching to a different tool. I think it’s called a cuticle scissors (but truth be told, this is the first time I ever used it) from my nail manicure kit:

teardown scissors

As I snipped away, I realized that I was inadvertently following along another seam in the enclosure, this one between the top and bottom halves and much harder to discern with the naked eye. The lithium polymer battery (which thankfully did NOT explode this time) came into view first:

Fitbit flex battery

The precariously dangling Bluetooth antenna fell off shortly thereafter:

Fitbit flex antenna

Plugging onward, the ICs on the other side of the PCB came into initial view, along with another mysterious antenna below them:

Fitbit flex open

That’s the haptic vibration motor next to the battery, by the way:

Fitbit flex haptic vibration motor

A better, albeit still incomplete (stay tuned) view of the previously mentioned antenna and ICs:

Fitbit flex PCB antenna

And finally the guts are out, at least from part of the enclosure. Here’s what’s left, i.e. the charging contacts still in the chassis:

Fitbit flex chassis

The antenna’s still stuck to the other half:

Fitbit flex NFC tag

It’s a near field communications (NFC) tag, which found use in auto-launching the Fitbit app on a similarly equipped Android smartphone held in close proximity (versus the user needing to manually fire up the app him/herself). Its value apparently didn’t end up justifying its incremental bill-of-materials cost, because it disappeared from subsequent models, although it’s made a recent resurrection of sorts in the “Special Edition” version of the Fitbit Charge 3, where it supports the Fitbit Pay service.

Fitbit flex NFC back

With the chassis set aside, the guts come into clear view. Note, for example, the charging terminals (which mate with those in the chassis, shown previously) along the right edge:

Fitbit flex charging terminals

There’s not much underneath the battery besides a bunch of test points. Ithink the now-severed Bluetooth antenna solder connection is in the bottom left corner:

Fitbit flex test points

The other side is more interesting at least from a silicon standpoint:

Fitbit flex PCB

The markings aren’t clear, so you’re going to have to take my word (and that of my buddies at iFixit) when I tell you that the large IC on the left is Nordic Semiconductor’s nRF8001 Bluetooth Low Energy connectivity and proximity IC, the large IC on the right is STMicroelectronics’ 32L151C6 ultra-low-power ARM Cortex-M3 MCU, and between them are (at top) Texas Instruments’ BQ24040 single-input, single cell Li-ion battery charger and (at bottom) an unidentifiable vendor/model 3-axis accelerometer (we think).

Last but not least, in packing away all of the disassembled bits, I again came across the USB charger and realized there might be something(s) interesting inside it, too. Note, for example, what appears to be a passive ventilation hole on one side:

Fitbit flex ventilation hole

Digging in with the box cutter reveals that the hole corresponds to what I’m guessing is some sort of a voltage regulator:

Fitbit flex voltage regulator

Fitbit flex charger PCB

And here’s what the other side of the mini-PCB looks like:

Fitbit flex charger PCB back

Reader insights into what this particular IC is, or about anything else I’ve covered in this teardown, for that matter, are as-always welcomed in the comments!

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