The 555 and How It Got That Way

By Dan Maloney

There’s a certain minimum set of stuff the typical Hackaday reader is likely to have within arm’s reach any time he or she is in the shop. Soldering station? Probably. Oscilloscope? Maybe. Multimeter? Quite likely. But there’s one thing so basic, something without which countless numbers of projects would be much more difficult to complete, that a shop without one or a dozen copies is almost unthinkable. It’s the humble 555 timer chip, a tiny chunk of black plastic with eight leads that in concert with just a few extra components can do everything from flashing an LED a couple of times a second to creating music and sound effects.

We’ve taken a look under the hood of the 555 before and featured many, many projects that show off the venerable chip’s multiple personalities quite well. But we haven’t looked at how Everyone’s First Chip came into being, and what inspired its design. Here’s the story of the 555 and how it got that way.

Perfect Timing

Hans R. Camenzind. Source: IEEE Spectrum

For Swiss-born engineer Hans Camenzind, the 1960s were a mixed bag. He came to the United States at the beginning of the decade and earned his Master’s from Northeastern University. In those days, the ring of communities around Boston was becoming a mecca for technology, and Hans wanted in on the action. But with a wife at home and kids on the way, one does what one must, and he landed a job with the P.R. Mallory Corporation, a Massachusetts company primarily in the dry cell business.

Mallory wasn’t exactly a cutting-edge tech firm, but Hans stuck with it for six years, hoping that the staid company would break into something more exciting than batteries. It didn’t, and Hans started bombarding tech companies from coast to coast with resumes. In 1968, he signed on with Signetics, a young Silicon Valley company started by former Fairchild engineers who bristled at their company’s focus on discrete components and believed that integrated circuits were going to be the wave of the future. Finally, here was something that Hans could sink his teeth into.

Sadly, it was not to last. Signetics had struggled right from the start, trying to build its business around custom ICs built to customer specifications. The company eventually found success in the defense market, but by the time Hans joined up, competition from other, larger manufacturers, ironically including Fairchild, had put the company on the ropes financially. That coupled with the downturn in the US economy as the 1970s rolled around led to round after round of layoffs at Signetics. Within two years, Hans saw half of his Signetics colleagues disappear.

Knowing how the story would end, Hans took a leap of faith. He resigned from Signetics, but not before convincing his management to hire him right back as a consultant. The company desperately needed a win, so they gave him a one-year contract to come up with something new. Hans was working at a fraction of his former salary, but he was doing what he loved, and more importantly, he was working when others weren’t. In the summer of 1970, working out of rented space in Sunnyvale wedged between two Chinese restaurants, Hans got to work.

One Pin Too Many

When he struck out on his own, Hans already had the basic idea for a timer chip in mind. His inspiration came from his technical education in Switzerland, which included a stint as a radio repairman, or “radio mechanic” as the Swiss called it. So he knew all about radio design, and during his time at Mallory he had pondered if it would be possible to build a radio on a chip. He knew that traditional designs using large coils wouldn’t work in silicon, so he looked for alternatives.

Scouring the MIT library at night, Hans discovered papers describing phase-locked loops, or PLLs, and he realized they would be perfect for the tuned circuits needed to build a radio receiver IC. When he jumped ship for Signetics, he convinced his new management that a PLL chip could be a winner. They agreed, and Hans went on to design the 565 PLL chip, a successful product in its own right, which came to market just before Hans left.

One of the main parts of the PLL that Hans designed was an oscillator whose frequency could be controlled by external components. It struck Hans that this could be easily modified into a timer circuit, one that could be used in either a free-running or one-shot mode, depending on how the external components were wired. He thought such a timer would be a handy component all by itself, and pitched it to Signetics as his contract project. They balked at first, fearing a timer would cannibalize sales of other Signetics chips, but they eventually approved the idea.

Hans spent the first six months of his contract breadboarding his circuit and optimizing the design. It worked, but it had one major problem: the chip would require nine pins, which would force it to be packaged in a 14-pin DIP rather than the much cheaper and more compact 8-pin part. Unhappy with the compromise but pressed for time, he submitted his design for review and got to work on laying out the lithography for the chip.

The problem wouldn’t leave him, though, and two weeks into the tedious sessions spent over a light table manually cutting out circuit elements from lithography film with a knife, he realized that he made a mistake. He could eliminate the need for the ninth pin by removing the voltage-to-current converter circuit and charging or discharging the external capacitor directly. The change meant losing two weeks of layout work, but the advantages of the improved circuit were too good to pass up.

Original 555 schematic. Note that Q18 and Q23 as configured as diodes. Source: Designing Analog Chips, by H. Camenzind

Layout of the chip, an entirely manual process in 1971, took almost the rest of his one-year contract. Hans beat the clock, though, despite some eleventh-hour chicanery from a former Signetics employee who went to another company that came out with their own timer chip that was eerily similar to the circuit Hans had presented in the original design review. That product was withdrawn from the market when Signetics released their chip, dubbed the “555” just because marketing-honcho Art Fury liked the way it sounded.

An Instant Hit

The new chip had a grand total of 23 transistors, 16 resistors, and two diodes on board, and was packaged in either an 8-pin DIP or a TO-5 metal can with eight leads. When it came out in 1972, it sold for a mere $0.75 and was an instant hit. Engineers fell in love with the chip because of its simplicity and flexibility. It could do just about anything, and Hans was constantly surprised by the applications designers thought up for it. He had seen it as timer that could be used as an oscillator, but couldn’t foresee all the uses for it.

Portrait of a classic: the 555 die. Source: Ken Shirriff

The 555 has found its way into hundreds of products from kitchen appliances to toys to game consoles and PCs, and it has even made it into space on satellites. Over a billion of the chips were produced between 1972 and 2003, and the design has remained the same in all that time. The 555 was joined by the 556 with two independent timers in one package, and later the 558 quad timer. A CMOS version was also released, and the chip is still in production to this day.

The 555 has endured not because it’s a solid design; by Camenzind’s own admission, it’s far from perfect. What made it a classic is that was purposely made as flexible as possible. Creativity favors open-ended designs that can be a sandbox for development, and the 555 has been delivering that for over half a century now with no sign of slowing down. That it came from a leap of faith intended to keep food on the family table only makes it more endearing.