Beware of Fake Heart Rate Sensors in Low-Cost Fitness Trackers

Manufacturers always attempt to lower the BoM costs of their products to increase sales and boost profits. Most do it in an honest way, but some are either incompetent or deceptive as we’ve recently seen in an IR thermometer was the infrared unit was not connected to the mainboard.

I’ve just been informed about another trick that’s been going on for a while. Some ultra-cheap fitness trackers come with fake heart rate sensors that are simply replaced by two LED’s to simulate the real thing.

The trick was discovered last year by Aaron Christophel who purchased several fitness trackers believe to be based on Nordic Semi nRF52832 in order to as he wanted to use Arduino firmware with those. He managed to do so in several with IWOWNFIT I6HRC tracker being the best suited for the tasks.

But he also found some bad apples in the lot with several trackers using a “heart rate sensor” board with two LED’s only, and no heart rate sensor at all. Many of those fitness trackers cost around $5 and rely on Telic TLSR8266 (unrelated to ESP8266) Bluetooth LE SoC. Aaron found a “Super SDK” for TLSR8266 that is supposed to be programmed via an SWD interface but could not find any programming adapter.

I was told fitness trackers with fake heart rate monitors were still being sold on Aliexpress for about $5  despite being advertised with a photoelectric heart rate sensor. I suppose they use the accelerator to estimate the heart rate. Those trackers can still be useful to estimate the step count, but just don’t rely on the heart rate data.

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