Freddie Temperton (via Jon Masters ):
One thing we noticed when playing with AirPods is that your phone knows the charging status of the AirPods individually as well as the case itself. How did they do this? Our interest was piqued! We also noticed that if you do not have any AirPod in the charger's cradle, the phone will not be able to read the charging status of the charger. It was very unlikely that the case has another Bluetooth radio in it as it seems totally overkill and power hungry, so we discussed other options. Since there are only two contacts between the charger and the AirPods, we theorized that they must use some form of power communication and hijack the radio in one of the AirPods to transfer information from the charger holder to the phone.[…]
With the first glimpse of the interiors, we knew this was much more than a simple mute charger. There's an ARM Cortex-M0 + in there![…]
The small button on the back is actually a whole custom mount. Two metal contacts are molded into a piece of plastic to contact the springs on board, then soldered an SMD switch. The button that is pressed down even has a slightly milled milled metal insert to press the button! Very impressive ME work!
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