Fitbit Charge HR Teardown

Before I purchased a Fitbit Charge HR I scrutinized over the details of different athletic trackers and their capabilities.  I chose the Fitbit Charge HR because it: tells time, acts like a pedometer, has a heart rate monitor, can track stair climbs, track your sleep, have alarms set, syncs with Bluetooth,  and has various notifications come through like phone calls.  Honestly, it is pretty neat that a watch that do all of that and still be about 1 inch wide.

Earlier this year, my partner had a Charge HR’s wrist strap break and the company sent him a new one.  I am grateful he let me take the old one apart.

Here is the complete teardown of the Fitbit Charge HR:

Tools Needed:

  • Fitbit Charge HR
  • Torx Screwdriver: T2 (I have this set and it is awesome Floureon 38-piece Precision Screwdriver)
  • Spudger
  • Small bowl or container for screws
  • Self Healing mat or light workspace – I use this because my counter is dark and shiny so it is hard to see small parts.

 

Teardown

When you remove your Fitbit and flip it over you see the sensor.  Looking at the corners of the sensor plate you will see 4 torx shaped screws.  Torx screws are six sided star screws and are common in cars, bikes and other electronics.  When created, torx were initially tamper proof because the shape was strange, kind of like the 5 star pentalobe screw that Apple started using in the 2010s.  To remove the torx screws use the size T2 and they will come out right away.  I recommend putting them in a small container (I used a ramekin) so you can find them later.

torxamp

Now that the screws are out take the spudger and push the plastic backing out of the housing case. Using a spugder is easy and it helps ensure that you won’t damage your screwdrivers or accidentally stab your fingers or workspace.  Spudgers are also nylon so you won’t damage your Fitbit or its case if you want to put it back together.

Once I removed circuit board from the Fitbit I glanced back at the sensor, or the part that touches my wrist.  It is somewhat arguable that the fitbit already displays the the most interesting things, because I think the sensor is pretty neat.

fitbit_housing

The sensor is the rectangular part that looks clear.  It is an optical sensor, so basically the same thing that is inside a computer mouse.  The two dots above and below the sensor are infrared LEDs and they blink the green light. To work together, the LEDs send off light into you arm and sensor detects different light levels being reflected back by oxygenated and deoxygenated blood.  The detection in different light measurement is how the pulse is recored. You can purchase a pulse oximeter at a drugstore, and they are found in hospitals – they are the little sensors that are put on your finger to make you look like E.T.  Because pulse oximeters are used in medical devices to also determine oxygen saturation, it is possible that Fitbit is measuring O2 levels in blood and simply not reporting it back, or the calculation between oxygenated and deoxygenated blood is not made to tell.

 

Once you flip over the band, or where the circuit board was, you will see a yellow circuit (flex circuit) that connects the sensor back to the main circuit board.  To the right, there is a black chip.  It is kind of funny that the chip is not on the main circuit board but that is postulated that the chip is an amp to measure the pulse as there is so much other stuff going on inside the Fitbit.

amp

 

Now, looking at the main circuit board, you will see two yellow flex circuits.  One of the flex circuits connects to the sensor, as mentioned before.  The other flex circuit connects to the display, and part you interact with when wearing the device.

back_circut

Next is the battery, which is the biggest part of the electronic, unsurprisingly. Below the battery is the lithium ion battery charger, which regulates charge current. The white plastic circle is an altitude sensor (barometer), which assists in measuring how many flights of stairs you climb.

Flipping over the circuit board is the main side that has more “stuff” on it. This is the slide you see the display. The display is an OLED (organic LED), and uses very little battery, which is important as the device is busy collecting other data. Above it is a Bluetooth antenna—this is how you sync your device and beat the pants off your buddies in challenges! The display and antenna are soldered onto the circuit board on the upper right side, and also held by two additional torx screws. I accidentally tore the circuit board removing the display, so take caution when removing this part. It is possible that this is connected so snugly to provide more stability for the display.

initial_look

Near the vibration motor is a chip that is the OLED power supply. Then there are two shiny chips, one is 8 bit and one is a 32-bit. Because there is so much going on in the Fitbit it is postulated that is the function for having 2 processors: the 8-bit may handle the Bluetooth stuff (coming from the big chip that says NRF) and display, and the big shiny 32-bit chip handles all the data that is being measured.

processor1

Another cool thing that has 2 of the same pieces are the accelerometers. It is possible that there are 2 accelerometers to more accurately estimate all the movement that is happening in your arm.  (Fun fact: accelerometers are in computer mice, too!)

acccelerometer

Hope you enjoyed the teardown!  I know I learned a lot, and I was also interested to be able to see similar parts as I have seen in other devices.

 

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2 Comments

  1. very cool! it’s wild to see how many differently functioning parts there are. I’ve been curious about how fitbit tracks ‘sleep’ – presumably it has to do with pulse measurements? but often mine will count me asleep when maybe I’m just lying down. also. I had to google image search ‘spudger’ out of intense curiosity after reading this lol (whoever named that tool was a genius). anyway nice post!

    1. It is wild! I think I forgot how much fitbit does, and it easy to do so with how info is reported back.
      So I did some reading on your sleep question, and fitbit tracks sleep by using the accelerometers . The accelerometers measures movement in your body and help determine if you’re lifting a pint glass or actually taking a step, so it would make sense that if you were lying down it would think you are sleeping as your movements resemble what it is like when you sleep. Sleep can be detected using the accelerometers and a heart rate sensor, which would make a lot of sense and give a more accurate reading, but that it is currently unknown how fitbit incorporates the heart rate data into the sleep calculation.

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