The TT555: a discrete 555 timer plug-in replacement

After dissecting a whole batch of 555 timer ICs I thought it would be an interesting project to make a working copy of the 555’s internal circuit out of discrete components, in the same physical space as the original IC. I’ve done this before with the 741 opamp, and the steps are similar. First, I drew the schematic in KiCAD:

I used the original circuit as designed by Hans Camenzind, as a tribute to his design but also because it uses the smallest number of transistors among all different designs I found while dissecting the various 555 ICs. Discrete transistors are larger than discrete resistors, so the original design saves space compared to newer versions that include several more transistors. Note that it’s the opposite situation when you’re designing an actual IC: integrated resistors are usually the largest parts in your layout.

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The 555 timer

If there’s a classic analog chip even more iconic than the 741 op amp, it has to be the 555 timer. Released just three years after the 741, it similarly took the world by storm, selling billions of units over five decades. Quite unlike the 741, which established op amps as a common IC type, the 555 has remained largely in a class of its own. There are many ICs that can generate square or triangle waves, but I can’t think of any chip that can function as a one-shot, a flip-flop, Schmitt trigger, or one of a million different oscillator types like the 555 can.

Designed by Hans Camenzind in 1972, its story is described in detail in Camenzind’s own book Designing Analog Chips. I highly recommend reading it (available on paper or as a free download) if you’re interested in analog IC design. In Chapter 11, Camenzind shows the schematic of the original 555 timer:

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