Thread by @TubeTimeUS

now here is a very unusual keyboard keyswitch. let's examine it closely...

keycap removed

it has 4 pins.

the wires pass through a ferrite toroid. this key switch is a transformer! it has a 1 turn primary and a 1 turn secondary.

so how does it work? let's take a look at the patent. this is one of those rare cases where the patent *exactly* matches the physical object.

the normal position (up) is in Fig 2. the two magnets saturate the magnetic field in the ferrite core, which *prevents* it from acting as a transformer! when you push the key down, it moves the magnets away, and the transformer can operate normally.

to use this keyswitch you just have to present it with a pulse train and check to see if it couples from the input to the output. notice that the output (upper trace) is the derivative of the input due to Faraday's law (change in flux=>voltage)

so how do you wire this keyswitch into a matrix? simple, you place the keys in a grid and connect the drive lines in a column in series and the sense lines in a row in series (or vice versa). then you pulse each drive line in turn and amplify the signals from the sense lines...

if that's starting to sound familiar, that's because it's very similar to core memory, even down to the ferrite toroids!

this type of keyswitch has some huge advantages.
✔️no contacts to wear out--quite reliable!
✔️simple to manufacture
✔️only 1 moving part
✔️safe for explosive atmospheres

some cons though:
❌sense circuitry slightly more complex
❌patented (since expired)

really the only reason i can see why it never took off is because it was patented. the sense circuitry wouldn't be terribly complex (basically just a flip flop with a low threshold voltage input)

but what does the actual keycap mean? these are ASCII codes, meaning Start of TeXt and End of TeXt.

some old terminals needed this to delimit keystrokes to be transmitted to the central processor.