Blinkenlights

Originally, Blinkenlights were simply diagnostic lights on electronic devices. In some places, they still are. But that's boring.

Thanks to signs in "mock German" that appeared in various computer rooms in the 1950s, Blinkenlights became something for non-technical people to look at, instead of touching something they really shouldn't touch. From there, it was only a matter of time – less than a decade – for Blinkenlights to become a visual shorthand for high technology in general, not just computers ... and, in Hollywood, they were always blinking.

As computers became more ubiquitous, the trope faded from the public consciousness. (Real Life 21st-century mainframes don't even have diagnostic lights any more, at least not where people can see them.) Nowadays it's used in works that purposefully invoke Zeerust, always paired with Beeping Computers.

Wikipedia has a page about Blinkenlights. The Jargon File has a page about them, too. And so does the Urban Dictionary.

Film

 * The Q-Bomb in The Mouse That Roared was covered in Blinkenlights.

Live-Action TV

 * Shown in The Prisoner episode "The General". is a plot point.
 * Star Trek: The Original Series faked Blinkenlights in Engineering, by sliding cutout screens back and forth behind backlit wall transparencies.
 * Knight Rider's KITT had a simplified set of Blinkenlights inset into his front bumper.