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Over on IEEE Spectrum, [Stephen Cass] has a project that would probably have seemed like a magic a few centuries ago. Imagine a box that uses a magnetic device to holds a locking bolt, requiring a magnet to disengage it. The only problem is that any strong magnet can open the box. [Stephen], instead, used an array of 64 Hall-effect sensors developed by [Peter Jansen] to look for a specific magnetic gesture.
Inside is a Teensy that can read the 8×8 grid of sensors. A magnetic stylus allows you to draw an unlock pattern. The system lets you train the gesture you want. There are a few problems. The sensor covers a small area, so it is hard to reliably produce the gesture. Putting an obvious mark on the box is a dead giveaway, so [Stephen] put a Celtic knotwork pattern on the box and uses part of it as a reference.
The other problem is that if you run out of battery, the box is locked forever. Well, at least until you apply mechanical force and break the box open. Wireless charging would be an option, but we might favor a small jack that would allow you to plug in an external battery.
This is a clever use of the sensor. However, we might have preferred a mechanical solution that didn’t require batteries. Or, perhaps, you could use a few normally open and normally closed reed switches to make a kind of magnetic combination lock. It would still need a battery, but it wouldn’t draw current until you connected the switches. Both methods also have drawbacks, of course. Like most things, you just get to pick the drawbacks you can live with.
You can see a video of the magnetic imaging tile from [Peter] below. We really want a setup like this for our secret door. We also wonder whether switchable magnets could provide a battery-free solution.
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