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There are only a few devices in which it is not installed. The 555 timer. Developed in 1970 by Hans Camenzind, several billion of this IC have been built, whole books on circuits with the 555 have been published, and the net is full of articles and videos on how it works.

But when Ken Shirriff takes on a topic, it takes on a whole new quality. If you know Ken, you know that he's got a blog dedicated to vintage computers and the reverse engineering of old ICs. And that with an expertise and a level of detail that some of his articles could pass as a master thesis.

If you've ever gotten your feet wet building your own electronics, you've certainly used the 555 before. Ken's article on this IC provides an understanding of how the little device actually works. 

Worth reading, under all circumstances.