Rubidium Frequency Standard Explained

August 6, 2026 by Al Williams No comments

You’ve probably heard of rubidium frequency standards, which are used where you need an extremely accurate time or frequency reference. [IMSAI] guy has a good explainer video about what’s actually going on inside one of these standards. Much of the basic idea also applies to cesium standards.

The explainer starts with the periodic table. Rubidium and cesium are both alkali metals, with a single electron in their outermost electron shell. Rubidium has 37 electrons, with the outermost one relatively loosely bound. Naturally occurring rubidium consists mainly of two isotopes, rubidium-85 and rubidium-87, which have the same number of protons and electrons but different numbers of neutrons.

A rubidium standard typically has three gas cells that have a bit of rubidium in them. An RF-excited rubidium-87 discharge lamp produces light at very specific wavelengths. The RF energy excites rubidium atoms into higher electronic states, and when their electrons fall back to lower-energy states, the atoms emit photons.

That light passes through a filter cell containing rubidium-85. The filter preferentially absorbs part of the lamp’s spectrum, leaving light that optically pumps the rubidium-87 atoms in the second resonance cell into one of two closely spaced hyperfine states of the atom’s ground state.

Those two states differ because of the interaction between the magnetic moment of the outer electron and that of the rubidium-87 nucleus. Their energy separation corresponds to a microwave frequency of about 6.835 GHz.

The resonance cell is illuminated by the filtered light while also being exposed to microwave energy from a local oscillator. When the microwave frequency is exactly equal to the rubidium-87 hyperfine transition frequency, it transfers atoms between the two ground-state hyperfine levels. That changes how strongly the cell absorbs the optical pumping light, producing a detectable dip in the light reaching a photodetector.

Electronics then servo the microwave oscillator onto the center of that absorption dip, using a feedback technique somewhat analogous to a phase-locked loop. Once locked, the oscillator is effectively referenced to an atomic transition rather than to the dimensions or mechanical properties of a crystal, giving you an extremely stable frequency standard.

Continue reading “Rubidium Frequency Standard Explained” →

Calculus-Free PID (Almost) In A Spreadsheet

August 6, 2026 by Al Williams 20 Comments

PID controllers are everywhere. They regulate temperature, motor speed, power supplies, positioning systems, process equipment, and probably a dozen things within arm’s reach of you right now.

They’re also frequently explained with enough calculus to make them seem more mysterious than they really are. Granted, the I and D in PID stand for calculus terms, but they are easy enough to build into a spreadsheet. Grab a copy and keep it open while you read this post.

The Google Sheet implements a simple simulated PID controller along with a simulated process — the thing we’re trying to control. You can change the controller gains, alter the process, introduce disturbances, and watch what happens without compiling anything or wiring up a heater that might accidentally become a toaster. Continue reading “Calculus-Free PID (Almost) In A Spreadsheet” →

FitzRoy’s Glass: Victorian Weather Marvel Or Glorified Thermometer?

August 6, 2026 by Al Williams 21 Comments

Everyone talks about the weather. This is doubly true for sailors, where bad weather could mean a very bad day. So it isn’t surprising that navies around the world have had a keen interest in weather forecasting. But how did you predict the weather before modern instruments, radar, and satellite images? Vice Admiral Robert FitzRoy had great faith in “storm glasses,” a glass chamber containing some chemicals that he didn’t invent, but did document and promote heavily during the 1860s.

Did it work? Apparently not, but the device is still interesting in its own right. FitzRoy was a pioneer of meteorology, replacing folklore with actual observations and attempts at scientific rigor. While he did arm observation stations with conventional things like thermometers and barometers, he was also a proponent of the weather glass. Continue reading “FitzRoy’s Glass: Victorian Weather Marvel Or Glorified Thermometer?” →

At Last! CP/M For Protected Mode

August 4, 2026 by Al Williams 34 Comments

If you used a serious computer pre-IBM PC, there was a fair chance its operating system was CP/M. CP/M was a staple among 8080 and Z80 computers and while there were other versions, we’ll always associate CP/M with the Z-80. There was a CP/M made for the PC which used an 8088 (a hybrid 8-bit bus with a 16-bit 8086 core), but it was overwhelmed by MSDOS. However, there was another interesting version made for the 68000, and now [johnsonjh] has ported that over to create an early version of CP/M for 80386 protected mode.

The Z-80 only had a 16-bit address bus, so it could only handle 64K of memory. It was common to “bank switch” some memory, and CP/M Plus could be made to understand that (for example, you might have 32K of common memory and three banks of 32K memory; you could address one bank at a time). However, the 386 had a full-blown memory management unit that could remap physical 4K memory pages to anywhere in a program’s virtual address space.

Ordinary CP/M couldn’t handle that, but the Motorola 68000 had a similar page management model, so it makes sense it might be easier to port CP/M-68K to the 80386 than starting from the original, even though the instruction set for the Z-80 is conceptually more similar to the 80386.

What can you do with it? We don’t know. Presumably, it will allow you to use lots of memory. Historically, CP/M software from one variant would not run on another, so you’ll have to build anything you want to use. Of course, the real killer for lots of CP/M memory was multitasking, but that takes MP/M, and only about half of that is currently working. But we won’t be surprised to see it completed soon.

While CP/M skills won’t land you many jobs these days, it is a pretty good way to get mentioned on Hackaday.

The 16K Display That Ate Las Vegas

August 3, 2026 by Al Williams 36 Comments

You may have a 4K television. Perhaps you have even bought an 8K screen, despite the shortage of things worth watching in 8K. A 16K display is, today, a rarity. But even when those eventually become commonplace, yours probably will not cover 14,900 square meters, rise 73 meters into the air, or wrap over your head and behind your peripheral vision.

That is approximately what happens inside Sphere in Las Vegas. The venue’s interior display is quoted as having a resolution of 16K by 16K and an area of 160,000 square feet, or about 3.7 acres. Unlike most enormous movie screens, it is not illuminated by a projector. The entire surface is a direct-view LED display: an immense, curved video wall assembled from tens of thousands of smaller pieces.

After seeing The Wizard of Oz at Sphere, however, the most interesting part was not simply the screen’s size. It was how thoroughly the screen could disguise itself.

Where Did The Theater Go?

Radio City or the Sphere? (It is the Sphere; photo courtesy [DP])

Before the presentation began, the auditorium appeared to have a conventional architectural ceiling. Great orange ribs curved over the seating, while ventilation grilles, suspended loudspeakers, lighting fixtures, curtains, and video monitors completed the illusion. It looked like the Radio City Music Hall’s proscenium. Then the show started — and the apparent theater completely disappeared. The speakers, the TVs, even the stage.

The obvious first conclusion was that the LED surface must be optically transparent, allowing the audience to see the real roof behind it until the pixels illuminated. That explanation was attractive because Sphere’s audio system really is installed behind the display, and the surface must allow sound through it.

It was also, apparently, wrong. The only explanation that makes sense is that the ceiling, ribs, grilles, speakers, and monitors were already being displayed by the screen. It was like a holodeck impersonating a physical theater interior. When the Oz material began, the system simply replaced one complete visual environment with another.

That’s what happens when a display fills nearly all of your useful visual field. A normal screen announces itself with a bezel, a wall, or at least a clearly visible edge. Sphere’s display extends upward and around the audience, removing many of those references. Give the image credible perspective, texture, shadows, and familiar architectural details, and the brain accepts the pixels as a room.

The same effect makes the Oz landscapes seem less like scenes displayed in front of the audience and more like places into which the auditorium has been inserted. Of course, there are more special effects. For The Wizard of Oz, there is wind and smoke, along with paper leaves, flower petals, and foam-rubber apples that fall from the sky. All of this makes it even more immersive.

Continue reading “The 16K Display That Ate Las Vegas” →

Hackaday Podcast Episode 380: 3D Printing The Rainbow, IR And IP Camera Hacks, And Americium 241 On The Loose

July 31, 2026 by Al Williams No comments

Elliot Williams and Al Williams got together to compare notes on the most interesting posts this week on the site. As usual, there are just too many choices, so you’ll have to settle for just the few that can fit in a podcast. The guys were excited about 3D printing — both FDM and SLA — as well as a few camera projects. Ever wanted your own starship? They do, too, and you’ll hear about it along with portable radar and more.

Want to make flexible PCBs? Fill up a carbon dioxide tank? Or play Doom via regular expressions? Tune in, and you’ll find out about those stories and more.

Follow along with the links, and as always, tell us what you think about this episode in the comments! Better still, drop us a note in the mailbag, and you might hear your question on a future episode. You can record audio or send us a message, and one of the hosts will read it on your behalf.

Direct download in IR color-corrected DRM-free MP3.

Continue reading “Hackaday Podcast Episode 380: 3D Printing The Rainbow, IR And IP Camera Hacks, And Americium 241 On The Loose” →

BASICally, Its Retro Machine Language

July 23, 2026 by Al Williams 29 Comments

We enjoyed [Beej’s] trip down memory lane looking at a BASIC game, The Wizard’s Castle, written for the Exidy Sorcerer. It appeared in a 1980 magazine that included the title graphic above. It reminded us how, back in those days, we did things with BASIC that you shouldn’t be able to do and it often looks, today, rather cryptic.

In particular, even if you know modern BASIC, these few lines might give you a pause:

10 REM"_(C2SLFF4
40 POKE 260,218: POKE 261,1: T = USR(0): T = PEEK(-2049)
80 Q = RND(-(2*T+1))

Line 10 is a comment, but a strange one. Certainly that doesn’t matter, right? Actually, it is a key part of the action. On line 40, you can see some pokes to write directly to memory and a peek to read some memory value back. The USR function calls some machine language program. You may realize the whole thing is to get some value T to seed the random number generator in line 80.

This leads to a few obvious questions. First, how does USR know what to call? Second, where is the machine language program? The details varied by system, of course, but in this case, the program knows that location 259 has a jump instruction that USR called. So poking an address into 260 and 261 was telling USR where it should go.

But what’s at that address? Keep in mind that an old computer like the Sorcerer didn’t have megabytes of memory being swapped about by an operating system. That means that things tended to be in known places and that BASIC had to be judicious about storing source code.

Continue reading “BASICally, Its Retro Machine Language” →