The info probably does come for free. The laptops don’t use the magnets along the top edge of the screen for detecting if the screen is closed, those magnets are just there to provide the latching effect when the screen is closed, so it doesn’t open accidentally.

The sensor used for detecting if the lid is closed is an “angle” sensor, although really it’s an Hall effect sensor and a magnet in the hinge. If you have a Hall effect sensor, getting angle data from it is pretty much free, because the Hall effect produces a continuously varying signal, you need thresholding logic to turn it into a binary output.

Given Hall effect ICs are so cheap and plentiful there no reason to use anything else. Also given they mass-produced ICs it’s probably cheaper to buy a fully featured Hall Effect IC, because the manufacturing cost between a basic IC and an advanced IC is almost certainly zero these days.

In short, modern IC manufacturing has just made magnetic angle sensors as cheap, if not cheaper, than dump non-angle sensing Hall sensors. After all you can always use an angle sensing Hall sensor as binary switch if you want, but the reverse isn’t true, so if the ICs basically cost the same, you can expect the less capable ICs to be completely outcompeted by the more capable ICs.

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macNchz 7 days ago | parent | next [-]

Once upon a time Mac laptops used reed switches to detect closed lids, and they were a common point of failure, presumably since they contained moving parts.

	▲	cosmic_cheese 7 days ago \| parent [-]
		They can be erroneously triggered or prevented from working as expected by nearby magnetic objects too, which can be annoying. No such issue with a hinge angle sensor.

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londons_explore 6 days ago | parent | prev | next [-]

Angle sensing IC's tend to need to be on the end of the shaft they sense, which can make some packaging and assembly headaches.

I personally am surprised they don't put an accelerometer in both halves of the laptop and use math to calculate the angle based on gravity.

	▲	avianlyric 6 days ago \| parent [-]
		They only need to be co-linear to the shaft if you care about accurate measurements, such as in a motor controller. I doubt the error introduced by being off-axis would make much difference in this application. There are also packaging considerations when putting a hall sensor elsewhere. Packaging it in the hinge has the advantage you can use the same hinge and sensor setup in all laptop models. Packaging the sensor elsewhere means custom packaging setups for each laptop to work around all the other components in the body of the machine. Doing the extra work for packaging in the hinge once is probably quite a bit cheaper than having to constantly redo the packaging work in every new model.

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ChocolateGod 7 days ago | parent | prev | next [-]

So basically as free as the glowing Apple logo that used to be on the back of Macbooks.

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userbinator 6 days ago | parent | prev [-]

The cost of the software is higher for an angle sensor than a binary switch, but perhaps they consider it NRE (which is actually not true if you consider "maintenance" work.)