Karhu Active Pulse teardown

Karhu Active Pulse (W311) was a fitness tracker product that was sold cheaply few years ago. It was quite comfortable on the wrist the UFit application running on Android was just fine (not very good or bad).


Most functions go on by pressing that white circle (pulse, sleep monitoring) for about three seconds. You can scroll through the functions by pressing the white circle. The bracelet has that picture of the sun and the moon. When you press the button on the wristband for about 3 seconds, the night mode switches on and sleep monitoring starts. In the morning change the mode back to the day ..

The product was very light and quite comfortable on the hand. It used a custom charging cable for charging. The mechanical construction could have been better, it broke down quite easily: did not properly charge and the clear plastic over the display fell off causing some other damage to inside electronics.

Let’s look inside the device.



The main components inside device are nRF51822 Bluetooth Low Energy and 2.4 GHz SoC, display and LiPo battery.

The nRF51822 is a general purpose, ultra-low power SoC ideally suited for Bluetooth® Low Energy and 2.4 GHz proprietary wireless applications. It is built around the 32-bit ARM® Cortex™-M0 CPU with 256/128 KB flash and 32/16 KB RAM. The flexible 2.4 GHz radio supports Bluetooth Low Energy and 2.4 GHz proprietary protocols, such as Gazell.


Karhu Pulse W311



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