What the MacBook lid-angle sensor is

Nearly every laptop has some way of knowing the lid is shut, so it can go to sleep. On a MacBook that is typically a magnetic sensor, and it answers one question: closed, or not closed. It is a switch.

The continuous lid-angle sensor is a different component answering a different question. It reports the hinge angle as a value — around 100° when you are working, falling toward 0° as you close it — and it keeps reporting while the lid is in motion. That is a measurement, not a switch, and only the measurement can drive something that follows your hand.

This is why a Mac can genuinely have “a lid sensor” and still not be able to run Mac Duo.

If you only know that a lid closed, the best you can do is start an animation and hope it finishes around the same time the lid does. Move slowly and the animation runs ahead of you. Stop halfway and it carries on without you.

With an angle, there is nothing to guess. Every position of the lid has one corresponding state of the fold, so the fold can be wherever the lid is, including halfway and including on the way back up. How Mac Duo turns that reading into glass walks through the rest of the pipeline.

macOS ships with hidutil, which can list the sensor interfaces your Mac exposes. This asks for the lid-angle one specifically and prints one plain answer. It only reads: it installs nothing, writes nothing and changes nothing.

hidutil list --matching '{"PrimaryUsagePage":32,"PrimaryUsage":138}' 2>/dev/null | awk '$4==32 && $5==138 {found=1} END {print "Mac Duo compatibility check"; print ""; if (found) {print "✓ Required lid-angle sensor detected."; print "  Your Mac appears compatible with Mac Duo."} else {print "✕ Required lid-angle sensor not detected."; print "  Your Mac does not appear to expose the continuous lid-angle sensor"; print "  Mac Duo requires. Please check the supported MacBook list before buying."}}'

“Detected” means your Mac appears compatible. “Not detected” means Mac Duo is not expected to work on it. Either way it is a compatibility check rather than an absolute guarantee, because Mac Duo looks more widely than one command can and checks that whatever it finds really does report a continuous angle. The supported list and the compatibility guarantee are on the compatibility page.

An accelerometer, a gyroscope and an ambient light sensor are all separate components, and none of them can stand in. A Mac can report plenty of sensor data and still have nothing that describes the hinge.

Apple does not publish a compatibility list for this, which is why Mac Duo checks the sensor on your own machine on first launch and says plainly when it cannot find one. How Mac Duo reads your MacBook's lid angle covers how it searches and what it checks before trusting a reading. If you are not sure about your own Mac, run the sensor check on the compatibility page before purchasing. That page also carries the compatibility guarantee.