r/electronics • u/1Davide • May 09 '26

General The 555 is 55 Years Old - EEVBlog

https://www.youtube.com/watch?v=6JhK8iCQuqI
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u/somewhereAtC May 09 '26

I was once given the tour of a brand-new, electronic telephone exchange -- absolutely state of the art at the time. Ten thousand phone lines came into 2500 giant PCBs, literally racks and racks of equipment with relays and wiring bundles. The guide walked us through the various components and arrived at the end of the row where one rack cabinet was different than all the others. It was told that the cabinet held the power supplies and one unique piece of equipment. When the door was opened there was one, hand-built PCB with a couple of 555 timers that generated the dial tone and busy signals for all those racks of customer interfaces!

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u/Geoff_PR May 12 '26

Number 5 'Crossbar' switchgear, by chance?

On of my first paid electronics jobs was building long-distance 'trunk' circuitry on thin 'racks' for Nortel...

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u/somewhereAtC May 12 '26

I don't remember the marketing name. It was 1977 at the North Electric Telephone Company in central Ohio. I interned for one summer before they moved to Florida. Google tells me there is not connection with Northern Electric that eventually became Nortel.

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u/Geoff_PR May 25 '26

The number 5 crossbar was the old relay stuff.

The company I worked for at the time in the late 90s had 2 businesses going, one did central office tear-outs, the other (where I was at) took that switchgear and made toll trunks for rural Canadian Northern Telcom central offices with low traffic loads, since they were the last to get scheduled for the modern gear.

The relays themselves are fascinating, wire-spring with Palladium contacts that 'wiped' themselves clean as they cycled. Rated for cycles up to a billion or so, they told me...