Designing The 555, by Hans Camezind

Probably the most famous chip ever made was the 555. Hans Camenzind recalled how he designed it.

“In 1967 designing repeatable integrated tuned circuits was impossible. I was interviewed by Signetics that year and proposed that they let me try to designe one using a phase-locked loop.

“I found the PLL in a 1935 Proceedings of the IEE and had seen that it wouldn’t need on-chip repeatability since it can set itself in step with an external frequency.

“So with Alan Grebene (later with Exar and Micro Linear) and Graham Righby we set out to design the circuit blocks for a phase-locked loop.

“Righby came up with a clever voltage-controlled oscillator which could operate at high speed and Grebene engineered it.

“That product the first IC PLL was the NE560. I didn’t quite like the outcome of the work so I decided to find a better VCO on my own.

“I used a voltage-to-current converter with the current set by an external resistor. This current charged and discharged an external capacitor.

‘Both the current source and comparator thresholds were dependent on the supply voltage but the two effects compensated making output frequency supply-independent.

“Using this principle Jack Mattis and I designed the 565 PLL and the 566 waveform generator both of which are available which the 560 isn’t!

“In 1970 I took a 4-month break to write a book. Instead of returning as an employee I asked the company for a 1-year contract as a consultant to use the VCO principles to design a timer a circuit which could oscillate or run for just one cycle.

“Signetics’ engineering department didn’t think much of the idea but fortunately Art Fury the marketing man did.

“He had the gut feel that it would sell and broke with the the tradition of introducing a new IC at a high price and then letting the price drop. He set the price of the 555 at 75 cents right from the start.

“I got the contract and completed the circuit-design portion in six months.

&I had already written a report on the design when I realised that I had been incredibly stupid. I had assumed that the compensation principle would only work with the linear voltage-to-current converter and would fall apart if an exponentially charging RC time constant was used.

“For some reason I decided to check that out and found that my assumption was not true at all.

“An RC time constant worked exactly the same way.

“This greatly simplified my design and reduced the number of pins from 14 to 8. Ironically an employee at Signetics quit his job shortly after I delivered the report and took the design to a start-up company.

“They beat Signetics to the market – with the wrong design. Their circuit lasted exactly three months.”

Other versions of the 555 have been seen since the original. Camenzind said: “In 25 years there has never been an improvement on the venerable old 555 apart from a CMOS version by Dave Bingham at Intersil. I have just finished redesigning the circuit from ground up. The new version [designed for Zetex] is much improved. It works down to 1 Volt retains the speed has better accuracy but only draws one-tenth the current.”

And added wryly: “Experience helps but so do smaller device geometries and computer-aided design!”


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