Teardown Tuesday: Fitbit Charge Fitness Tracker

April 19, 2016 by Alex Udanis

In this week’s Teardown Tuesday we're going to take a look at the inside of a Fitbit Charge.

Want to see what's inside a Fitbit?

Fitness Trackers have become more and more popular and are typically packed with sensors and proprietary software. Often, these wearables will have Bluetooth, a barometer, an accelerometer, and sometimes even a heart rate monitor. In this week’s Teardown Tuesday we are going to take a look at the inside of a Fitbit Charge.


The Fitbit Charge Fitness Tracker - Source: Fitbit 



The Fitbit Charge uses a small single cell Lithium-Ion Battery. This battery measures approximately 2.8x13x20mm and has a capacity of ~46mAh. It appears that this cell does not have the protection circuitry that is found on a lot of lithium cells. This cell carries the part number of “281320” that denotes the physical dimensions of the battery.


The Lithium Battery in the Fitbit Charge 


The battery is attached to the circuit board through two solder connections. The battery is charged by a TI BQ24232. This is a small IC in a 16-VQFN package and has a wide input voltage range of 4.35 V to 26 V. The charger is capable of charging with an input current of either a 100mA input current, making it ideal for USB applications, or a 500mA input current for faster charging. Due to the small battery size and application, the Fitbit most likely charges at a slow charge rate.


The Battery Charging IC marked with NXK 



The Front of the OLED Display 


The display on the Fitbit Charge is a small monochrome PMOLED display made by WiseChip. This display is connected to the circuit board through a flexible circuit board. The display has a small driver that is located on the back of it. Displays like these are often referred to Chip on Glass (COG) displays.


The Rear of the OLED Display 




To track activity, the Fitbit Charge uses two types of sensors, the first is an accelerometer. The accelerometer for the Fitbit Charge can be seen in the image with the 2-D barcode on the top of it. Based on images of the accelerometer found in other Fitbits, it appears that the accelerometer could be an ST LIS2DH.



Also in the Fitbit Charge, is a barometric pressure sensor that is used to measure the change in altitude for activities such as climbing stairs. The pressure sensor used in the Fitbit Charge is a TE Connectivity/Measurement Specialties MS5805


Vibration Motor

The Vibration Motor and Water Gasket 


The Fitbit Charge can relay information to the user through a small vibration motor that can be seen in the image above. Since the ST microcontroller can not drive this motor directly, there is a MOSFET that drives the vibration motor.


The MOSFET to Drive the Vibration Motor 



At the heart of this Fitbit is an ST Microelectronics STM32L151C6 microcontroller. This is a 32-bit microcontroller with an ARM cortex M3 core inside of it.


The ARM microcontroller from ST Microelectronics 



In order to communicate with the Fitbit Smartphone app, the Fitbit Charge uses a small Bluetooth radio. The fitness tracker uses a Nordic NRF 8001 Bluetooth IC. This small IC was designed for applications just like the Fitbit according to the datasheet. This Bluetooth module supports Bluetooth low energy, making it ideal for battery powered and wearable applications.


The Nordic Bluetooth Low Energy Module 


Fitbit Force and Other Fitbit's

Through researching this article, it appears that many of the devices that Fitbit manufactures are very similar. It appears that the Fitbit Charge, Fitbit Force, and Fitbit Flex all use the ST microcontrollers from the same line and the same Bluetooth IC’s. It's also worth a note that the discontinued/recalled Fitbit Force is nearly identically electrically to the Fitbit Charge. Below are images from Adafruits website that show the Fitbit Force compared to the Fitbit charge that was torn down for this article. 


Fitbit Force PCB (Source: Adafruit) vs the Fitbit Charge PCB 



Fitness trackers are very popular devices. From a hardware standpoint, most fitness trackers have relatively the same hardware, a microcontroller, a display, and an accelerometer. A lot of the technology that differentiates fitness trackers is in the hardware quality, software, and the mobile app.

Thanks for looking at this week's Teardown Tuesday.

Stop by next Tuesday for another teardown! We're always looking for new things to teardown, if you have any suggestions or would like to contribute an item for a future Teardown Tuesday, click here for my email address.



As an added bonus to this week's Teardown Tuesday, here are the insides of the Fitbit USB Dongle.


The Fitbit USB Dongle 


More of the Fitbit USB Dongle 

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