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Testing the encoding scheme hypothesis

A project log for Sniff the Wireless Data of a Sports Wrist Watch

Reverse engineer the wireless protocol between a chest belt heart rate monitor and it's wrist watch that displays the beats per minute

RoGeorgeRoGeorge 02/16/2017 at 01:220 Comments

A computer controlled radio Tx


First, thank you all for the effort put in reverse engineering the encoding scheme, great job!

Now, it's time to test some more the hypothesis.

To do this, it would be helpful to be able to transmit any code, valid or invalid, and see how the receiver will react. The Crivit chest belt can't do that, so we need to build our own radio transmitter. With a carrier frequency of only 110 KHz, it should be easy to digitally synthesize the entire modulated carrier.

A few lines of code later, it proves out that a simple wire connected to a digital output is good enough as a Tx antenna, and an Arduino UNO is fast enough to generate the carrier, modulate it, and in the same time talk to a computer over the serial port:

This will allow us to put on air any combination of 0's and 1's that we might want to test.

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New findings


Manually typing each code to be tested proves to be useful, but also very time consuming and prone to errors. Since our radio Tx is now able to transmit any codes coming from the serial port, it will allow us to do automated testing. This will be the next step.




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