Ancient Web: Ken Shirriff Opened a 555 Timer and Mapped Its Silicon

Part of the Ancient Web Collection.

The 555 timer spent decades hiding a surprisingly clever circuit inside an unremarkable little package. Ken Shirriff decided the appropriate response was to cut one open and explain every interesting part.

Read the original 555 timer teardown.

Published in February 2016, this article is newer than most Ancient Web subjects. It qualifies as a piece of technical preservation: Shirriff documents the physical design of a chip created by Hans Camenzind in 1970, using his own microscope photographs, annotated diagrams, and an interactive comparison of a die image with a schematic.

The circuit is inside the picture

Shirriff begins with the timer's familiar behavior. Three internal resistors divide the supply voltage into thresholds at one-third and two-thirds. Comparators, a flip-flop, and a discharge transistor let an external capacitor repeatedly charge and discharge. What looks like a mysterious eight-pin device becomes a repeating sequence of decisions.

The remarkable part comes when he moves from the usual block diagram to the actual silicon. An NPN transistor does not resemble the tidy symmetrical drawing in a textbook. PNP transistors in the photographed chip use a different, lateral layout. Shirriff points out the shapes of emitters, collectors, isolation rings, resistors, and metal connections.

He also explains why integrated-circuit designers used current mirrors and differential pairs, and how high-current output transistors occupied visibly more real estate than smaller components.

The page's interactive die-and-schematic explorer lets a reader associate circuit components with their places on the photographed chip. This is a particularly effective use of the web: the archival image becomes a teaching tool.

A failed successor and an enduring original

Shirriff discusses an improved 555 that Camenzind redesigned in 1997. It addressed several weaknesses of the old circuit, but, as the article explains, did not displace the established design. That contrast is part of the history of electronics: technically better is not automatically commercially successful.

The author even describes using a metal-can version of the chip because opening a conventional epoxy package is more difficult and hazardous. It is a practical account of where the photos came from, rather than an unexplained gallery of silicon.

A 2016 web essay may not itself be ancient, but it preserves knowledge of how an early mass-produced integrated circuit was physically made. That specificity gives this page a reason to survive.

Explore the photographed 555 chip and interactive schematic.

Part of the Ancient Web Collection.