Errr... another thing.... so the other 2 circles on the back of the S2 isn’t failed tech?!
I’m looking into that now!
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Apple Watch uses four sensors to detect your pulse
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Regards
Martin
No worries, I didn't take anything as a poke - you mentioned something I'd never seen any indication of, so I was curious where that impression had come from.
To my understanding, on the S2 and S3, two of the four spots are LEDs emitting light, and the other two are photodetectors reading the light bouncing back. Thus all four used. With the S4, they merged those into one "spot" on the back of the watch.
Thus, I'm not aware of any unused tech on the S2 or S3 models.