In the 1980s a company called Quotron developed a contactless keyboard. The main circuit board contained a bunch of LEDs and some sort of photo diodes or photo cells, all arranged in a grid. The keys were simply plungers that when pressed down interrupted the light beams shining into some of the receivers. The firmware then decided which key had been pressed and generated the appropriate code. The keyboard itself was huge. Us software people used to joke that, in a pinch, it could be used as a surfboard.

One of the advantages was that you could pour a cup of coffee into the keyboard and it would just keep working.