One of the main obstacles during typing on flat keyboard is deficiency of tactile feedback. So my idea is that each key vibrates with different amplitude or frequency when it pressed and when not. Space between keys not vibrating at all. G and H keys on qwerty vibrate differently from other keys (or just have usual marks). It will allow to create slim keyboards without moving parts and with tactile feedback.
Also, this idea can be used in desktop keyboards with different rows of keys having
different "feel". and in waterproof silicone keyboards to actuate tactile feedback.
P.S. Probably it is possible to detect pressing event by change of frequency of free silicon crystal when finger is touching it therefore simplifying construction
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