Like many a nerd, I’ve a sure fascination with locks. This originates in studying the memoir Surely You’re Joking, Mr. Feynman as a young person, by which Richard Feynman recounts selecting locks and cracking combos whereas at his day job engaged on the Manhattan Project at Los Alamos. I presently have a few locksport decide units and coaching locks that I bought at hacker conferences through the years (though I’m nonetheless horrible at it), and I delight within the YouTube exploits of oldsters just like the LockPickingLawyer.
All of which left me questioning if I might attempt constructing an attention-grabbing lock. One dodge that lockmakers have lengthy used to problem would-be plunderers is to conceal the existence of the lock itself. A contemporary variant of that is the magnetic lock, the place a magnet is held or slid in opposition to an innocuous portion of a door or field, releasing a catch. The issue with that is that, when you guess the placement of the lock, any robust magnet will open it.
So I began fascinated by embedding an RFID reader under the surface and resigning myself to conserving observe of yet one more entry card. However then I stumbled throughout one thing far more attention-grabbing: SparkFun’s US $100 magnetic imaging tile. I noticed I might use this to construct a magnetic lock that opens with a programmable secret gesture.
I used just a few discrete parts to drive the servo motor and put the lock into program mode [top], managed by a Teensy 4.1 microcontroller [middle]. An array of Corridor-effect sensors detect magnetic fields [bottom].James Provost
The magnetic imaging tile is a small-run SparkFun board based on the work of Peter Jansen. It options an 8-by-8 grid of Hall-effect sensors, and makes use of an SPI interface to speak to the microcontroller of your alternative. These sensors can detect each the magnitude and polarity of a magnetic discipline, and the board can report its measurements a whole lot to 1000’s of instances per second, relying on how a lot processing energy your microcontroller has obtainable. That is simply quick sufficient to seize the movement of a magnet being moved by hand throughout the board’s sensor space—my secret gesture.
An attention-grabbing factor about utilizing magnetic fields is that if I used to be in excessive paranoia mode, I might defend the complete lock behind copper sheeting, one thing I couldn’t do with an RFID reader. The copper sheet would act as a Faraday cage, blocking any RFID sign, together with every other electromagnetic fields an attacker with an EMF meter would possibly use to find the lock.
Selecting the Brains of the Field
I selected a Teensy 4.1 as my microcontroller. I just like the Teensy as a result of it provides a great deal of enter/output, a 600-megahertz processor, a megabyte of RAM, and a built-in SD card, all in a slim package deal with an equally slim $32 price ticket.
Wiring the Teensy to the magnetic-imaging tile was trivial, solely sophisticated by the truth that one aspect of the board has a silk-screen error that misidentifies the SPI connections (SparkFun says this will probably be mounted in future runs of the board).
In excessive paranoia mode, I might defend the complete lock behind copper sheeting.
To create the gesture-recognition software program, I as soon as once more turned to vibe coding. In the event you learn my Fingers On piece within the April 2026 difficulty of IEEE Spectrum on creating a teletext suite for ham radio, you’ll know I’ve been grappling with how vibe coding must be utilized in DIY initiatives, if in any respect. So this time, as an alternative of letting Claude write all of the software program, I confined it to writing simply two help libraries, which you’ll find in our on-line repository at https://github.com/IEEESpectrumHandsOn.
One library is in command of recording a lock gesture and saving it to the Teensy’s nonvolatile storage. The opposite library displays the tile and experiences whether or not or not a newly detected gesture passes a selectable threshold to be thought-about a match.
I wrote the top-level code myself to maintain it as DIY as potential. This code handles issues like a button that places the system in gesture-recording mode after a quick delay, the indicator LED for recording mode, and the code that operates the bodily locking mechanism.
Corridor-effect sensors depend on a present passing via a p-type slab of silicon. A magnetic discipline passing via the silicon deflects cost carriers towards the sides of the slab, making a voltage proportional to the sphere [left]. By wanting on the intensities of the voltages throughout an 8-by-8 grid of sensors, the microcontroller can reconstruct a gesture [right]. James Provost
In fact, I wanted one thing to lock to display my proof of idea. I purchased a wooden craft box and eliminated the prevailing lid latch. I glued a notched beam contained in the lid and constructed a mechanism that makes use of a servo motor to rotate one other beam out and in of this notch. I would be the first to confess this mechanism goes past “easy” and into “simplistic,” but it surely’s sufficiently sturdy for my demo. The field has ample room for the magnetic imaging tile to be mounted in opposition to one wall, together with the Teensy and different supporting electronics. These embody a battery pack, the recording button, and a small further board with two NPN transistors. These convert the Teensy’s 3.3-volt management alerts to a 5-V sign wanted to drive the servo.
As soon as it was all put collectively, I quickly found that the small dimension of the magnetic tile’s sensing space—simply 3 centimeters sq.—made it simple to suit the tile contained in the field however onerous to do the unlock gesture on the surface in order that it’s learn appropriately. Marking the define of the sensor space on the surface defeated the purpose of getting a hidden lock, so as an alternative I once again turned to mining my Irish heritage. I created a Celtic knotwork pattern that runs in a band throughout the entrance of the lockbox (I used an online knotwork generator after which projected and traced the sample onto the field). Utilizing the sample as a camouflaged reference grid I could make a repeatable gesture throughout the confines of the sensor space. (To make the gestures I’m utilizing a magnetic stylus borrowed from a ferrite core-memory kit we featured in 2022.)
There’s apparent room for enchancment in any extra critical set up. The largest downside presently is that if the field is locked when the battery runs out, you may’t open it once more with out breaking one thing. This may very well be solved by including a hidden inductive charging coil. Nonetheless, the demo lockbox works properly—now I simply must assume what I ought to retailer in it!
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