Analog Angle Blog

At 50 years, the 555 timer IC defies conventional wisdom

You’re well aware of the IC shortage and its impact. Commentators and analysts have provided many explanations, including unexpectedly high demand, capacity limitations, Covid, and too much reliance on latest-process ICs. They also talk about floods and other disasters that have disrupted some major facilities as well as increased call for “legacy” analog-centric ICs, which can only be fabbed at smaller, older sites. Probably, there is no single primary cause, but instead smaller supply-chain ripples that are aligned in phase and add up to a major issue.

So, I’ll add my reason to the list: the wide use of the 555 timer IC. “What’s that?” you’d ask. “How’s that possible?”

I’ll tell you how. This IC has been on the market and in wide use since it was introduced in 1972. In IC technology chronicles, that’s the days of dinosaurs. This largely unchanged IC has been in production for that long and is still available from about a dozen vendors. I can’t find specific numbers, but I suspect many millions are still used in both legacy and new designs every year. So maybe it’s time to retire the 555 and make room in those legacy fab queues for other, newer analog ICs?

Of course, I’m only kidding. The 555, with the deceptively simple description of “Timer” on its datasheets, can be configured for monostable (timing) or astable (oscillation) modes. It’s a wonderful 8-pin device, using just 28 internal transistors in analog mode (Figure 1). It doesn’t do much by itself, but it can be made to do so many things when enhanced with few passive external components. And with active ones, it can do even more. It’s been a remarkable device for hobbyists and professionals alike.

Figure 1 The block diagram of the 555 timer IC is simple, but its applications are varied and numerous. Source: Texas Instruments

For hobbyists, the 555 has been a valuable entry point and platform for experimenting with electronics and seeing the imaginative way building-block components can be configured to do so many things. Since it’s a low-frequency and bipolar device, it doesn’t have critical layout considerations, while its pin functions can be observed with a low-end oscilloscope or even a voltmeter (yes, they can). For professionals, it has been a planned-in-advance, low-cost solution to a headache-inducing design requirement, or the solution to an “oh-oh, we have a problem here” fix-it late in the design or in the field.

555 timer history

I won’t recount the history of the 555 timers as it has been covered extensively elsewhere. The best source is the oral history provided by Hans Camenzind himself, who, while working solo under a freelance contract for Signetics, created the 555. He conceived of, designed, laid out, cut the rubylith masking-film masters, and tested it (don’t know what rubylith is? look it up!). You can see the original datasheet here. Although he passed away in 2012, his legacy truly lives on via his oral-history interview with a transcript posted here. Camenzind also wrote a fascinating article, “Redesigning the old 555,” published in IEEE Spectrum in September 1997.

If you’re not familiar with the 555 or know a budding engineer who might want to do some good hands-on analog-circuit projects, there are many clever Design Ideas about it posted on EDN. Also, check out the “555 Cookbook” by Walt Jung, the classic compendium of 555 circuit and applications ideas. Even better, you can literally get into the 555 by getting the Discrete 555 Timer kit from Evil Mad Scientist, which uses discrete transistors and passive components (Figure 2).

Figure 2 If you want to probe within the 555, this emulation kit makes it possible. It uses only discrete active and passive components to let you observe all of its internal points. Source: Evil Mad Scientist

Still alive and well

Conventional wisdom is that ICs don’t have a long new-product design-in life. As with so much of such presumed “wisdom,“ it is both true and not-so-true. Certainly, many advanced digital ICs have a viable life of just three, five, or perhaps a few more years, and then they are no longer preferred for new designs. On the other hand, a good analog IC, whether a basic op-amp or perhaps a DC/DC converter, maybe a new-design candidate for 10 or more years.

There are several reasons for this. One is that vendors like it, as the engineering and up-front costs are paid for and the margins can be very high. At the same time, the subtleties of the analog process are all resolved, so yields are high, which is good for both the vendor and the user.

Moreover, many system designers like to manage their risks in a new product, and while they may need to use the latest digital IC, they don’t necessarily want to change everything in a new design. Therefore, by continuing to use an established, fully-understood analog IC, they minimize their overall risks by taking some uncertainty out of the project. That makes a lot of sense.

The fact that this IC has turned 50, has been used on NASA space missions, is still widely designed-in, and is the result of the efforts of one person working mostly alone tells a worthwhile story. I wonder if any of the media outlets that cover celebrities and milestones will pick up on this story. Don’t worry, I am not holding my breath for that to happen, as the story lacks the “splash” and celebrity elements that the media craves. That’s too bad in many ways, as it’s a story worth telling.

Have you recently relied on the 555 in a new design? Or have you used it to solve a problem that came up late in the design cycle? Was the use of the 555 accepted at the design review or ridiculed?

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