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I have a cheap ECG/HRV/heart rate sensor module for Arduino, bought from Aliexpress. It has 3 electrodes: RA, LA, RL, which I guess mean right arm, left arm, right leg. The module outputs one single time series data, like one referenced electrode of an EEG.

I would like to use this cheap module to record blinks (one coarse channel is enough for this), with its electrodes placed around one eye. But I am stuck with this problem:

What I have found so far on the internet about EOG electrode placement says that the ground electrode should be placed somewhere on the forehead or near the ear/neck. But which of these 3 electrodes should be deemed as ground? What arrangement of the 3 electrodes would you advise? (What are the equivalents of these 3 electrodes in a commercial EOG sensor?)

I appreciate any helpful comment. Thanks in advance!

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  • $\begingroup$ Can you share any documentation for your sensor, manuals or instructions? Why doesn't that manual answer your questions? $\endgroup$ Commented Nov 17, 2022 at 20:36
  • $\begingroup$ I agree with @MaximilianPress -- It depends on how the device is wired, whether there is a fixed ground electrode etc. $\endgroup$
    – AliceD
    Commented Nov 18, 2022 at 9:41
  • $\begingroup$ As it is the case of almost all Aliexpress-marketed no-brand Arduino modules, there is zero documentation, except how to wire it up with Arduino. And no matter how much I googled, all applications only involve HRV/ECG sensor wiring of the module, even though is could be capable of acting as an EOG sensor I think. $\endgroup$ Commented Nov 19, 2022 at 11:01

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Despite initial difficulties, after inspecting the module really closely, I could successfully identify the (only) microchip on it, which is an Analog Devices IC, with all necessary functionality embedded in one piece. Name is AD8232: https://www.analog.com/media/en/technical-documentation/data-sheets/ad8232.pdf

It seems that pin 4 and 5 are connected with a resistor. Pin 5 goes to "RL" pin of the module board. Pin 3 goes to "RA", which at the same time IC docs refer to as IN-, and pin 2 goes to "LA" which the docs refer to as IN+. So I suppose the mystery is solved, but I can not try and check the setup in action in near future

(So far no matter in what configuration I wired up the module with my eyes, it could not work, but emitted a rhythmic noise. That is why I asked the question originally. But I found out that the source was the problematic power supply, not the electrode arrangements I tried. Now I need to get a proper power supply to try the module then.)

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