EEVblog #665 - Polar Wearlink Heart Rate Transmitter Teardown

Teardown Tuesday.
Inside a Polar Wearlink chest strap fitness heart rate monitor monitor transmitter.
Also a look at and some attempted decoding of the signal with the Tektronix MDO3000 oscilloscope.

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[flubba]

"And now I have to Probe myself..." That is the best quote Dave has ever produced.

[CH_Pechiar]

Very nice design and excellent PCB and component quality. As expected and required in a heavy duty device like this one.

[Coolkeys]

You can use an inductor as a pickup coil, when I tried it there was over 50v across the inductor. If you place a dynamic microphone near the transmitter you can literally listen to your pulse rate change as you move around. I've always been astonished how accurately the watch can calculate calories used just from heart rate when compared to high gym equipment.

[joshcryer]

Man, Polar must be making serious bank on these, they go for some $50 usually. Very minimalistic inside and the transmission scheme is super simple. Nice teardown, as usual. :)

[douro]

This Finnish company developed, with the University of Oulu, the world's first ECG-based heart rate monitor in 1977.  A tiny device which can be worn on someone's finger, it displayed an accurately measured heart rate in seconds upon contact with a finger on the opposite hand.

[redtails]

12:23 neighbours think you're truly crazy.

[ricksonstudent]

Great video. Really enjoyed it. My inner analog signal self came out of hiding and appreciated it.

[toast_recon]

I was terrified that Dave was going land on the printer on one of those jumps and seriously hurt himself. 

Does your patent go over how this technique deal with such dangers?

[mnovo]

The data can also be save as an excel file to view the number data as well.

[JimGriffOne]

Perhaps the distance between the initial pulse and the ending 2 pulses denotes the unit ID, and the two pulses on the end would prevent interference from other units. The 1 single start pulse of another unit being received by your unit would not register on your unit, since it is looking for 2 pulses at a particular latency to denote the unit ID and switch your unit into listen mode again.

[Mefaso]

Very interesting to me, as a layperson, to see how you approach this, nice video.

[TheBananaPlug]

The coil might have a bifilar winding - hence the three wires?
Gotta have a largish coil for that low frequency.

[drkastenbrot]

I feel like the second coil could be used for receiving data (writing the individual codes).

[necessaryevil]

I would love to see an oscilloscope with the possibility to connect a mouse + keyboard for the advanced functions. In addition to 'normal' knobs/buttons of course!

[AmericanLocomotive]

Does that scope really have image tearing in roll mode? Seriously?

[Gunbudder]

I wear one of these to make sure my heartrate stays below 190 when i run. pretty cool to see it in a tear down

[NoMad]

That "encoding" looks like Time Division Multiplexing (TDM) to me. The first pulse may be shared by all transmitters to sync and there are 31 other time slots where the second pulse can fit in. Which would be just controlled by that time offset to the second pulse. I'm not sure, especially how timing is managed since your heartbeat varies. But that might be easily predictable, or the unit just delays transmission to be in the right time slot (after all it seems it just transmits 2 actual heartbeats). You probably could only tell by probing both sensors transmitting to different watches at the same time.

[proluxelectronics]

The transmitter could generate a random then fixed second pulse that would be picked up by the nearest receiver and within 2 repeats software would compare time delays and  lock to the transmitted delay time. I would assume coding it not critical as the transmitted power will have a very short range and having two transmitter within the same area with the same random time code unlikely. Top vid Dave.

[billysgeo]

Great video!!! And +1 for the Penn t-shirt! (I'm also a backer! Should be a great movie!)

[JoseFrancisco-poby]

Hi! . Can I connect it to my ft1 watch?