There's no per-switch sensor at all!
Instead, each key-press just results in one wide metal bar being hit. And to figure out which key was pressed, there's a sensor on the right and the left side which are used to calculate what key was pressed based on how long it took for the signal (i.e. vibration in the bar) to reach the left and right sensor.
It's so clever and so ridiculous at the same time!
[1] Video presentation (Talk about acoustic switches starts at 1:46:00): https://youtu.be/AvszDsr1js8?t=6361
[2] Slides: https://docs.google.com/presentation/d/1jfWpf8cABnH54yVjPIU3...
Just the single tweet, a small "Show this thread" link and then 5 random other tweets from other people not connected or related to this tweet at all.
https://oldcomputers.net/compucolor-8001.html
had a keyboard that was optical, it ran beams of light through masks attached to the keys, and looked at the beam interruptions as keys were pressed.
The keyboarding experience was absolutely miserable; you had to COMPLETELY release a key before pressing another. Ghosting and mangled keystrokes were almost impossible to avoid. I'd rather type on a card punch than one of those . . . well, no. But it's darned close.
These are linear sensors that work by measuring the output of a transformer with a movable core. They have a reputation for being extremely reliable, even under harsh conditions. Therefore they are commonly used in aircraft, turbines, nuclear reactors, etc...
[1] https://en.wikipedia.org/wiki/Linear_variable_differential_t...
I think Topre is the only one that manufactures keyboards with contactless key switches today. Topre key-switches measure the change in capacitance instead, and their keyboards (of the HHKB fame) sell for greater than $200.
They use infrared and they are also analog. You can bind different event based on the actuation force you put on the switch.
Wooting also has a Hall effect switch [3]. You can't buy any keyboards with it yet but you can pre-order one at least [4].
[1] https://deskthority.net/wiki/Keystroke_sensing#Hall_effect
[2] https://geekhack.org/index.php?topic=84525.0