2025 One Hertz Challenge: An Animated Ferrofluid Display

August 6, 2025 by Lewin Day 12 Comments

Ferrofluid is fun. You’ve probably seen all kinds of demos with it bouncing around in response to magnetic fields, or dancing near a speaker. [beastie417] decided to turn the entertaining fluid into a display.

The basic concept of the ferrofluid display. Note the header image of this article shows the electromagnet array without the ferrofluid pane in place.

The concept is straightforward enough. First, construct a tank of ferrofluid with a white panel behind it for contrast. Then, place it in front of a grid of electromagnets. Now you have many “pixels” you can turn on and off. You turn a magnet on to attract ferrofluid to that point, and turn it off to let it fall away. Since the ferrofluid contrasts with the white background, you have a viable display!

[beastie417] notes that while the concept is simple, the execution is hard. Ferrofluid can be very difficult to work with, instantly staining many materials like acrylic and even glass that isn’t properly prepared. It can also be quite expensive to construct a display like this, with [beastie417] noting their 16×12 pixel design costing approximately $700 thus far. Then you have to figure out how to drive all the pixels—this project uses DRV8908 coil driver ICs running off a microcontroller which controls the display and handles animations.

We’ve seen some great ferrofluid displays before, like this neat build that could even create readable glyphs. Meanwhile, if you’re doing rad things with the coolest fluid of the new millennium, don’t hesitate to let us know!

Flex PCB Underlies The Watch Of The Future

August 5, 2025 by Tyler August 8 Comments

If you were at OpenSauce, you may have seen new Youtuber [Sahko] waltzing about with a retrofuturistic peice of jewelery that revealed itself as a very cool watch. If you weren’t, he’s his very first video on YouTube detailing the design and construction of this piece. We’ve embedded it below, and it’s worth a watch. (Pun intended, as always.)

The build was inspired by the delightful amber LED dot-matrix display modules that circle the band of the watch. They go by HCMS2901, but [Sahko] recommends using the HCMS3901 as it’s both more 3.3V-tolerant and easier to find now. A challenge in mounting so many displays was the voltage on the supply rail dropping below the logic level; presumably the newer version does not have this problem to the same degree. Either way we love the look of these little displays and are pondering projects of our own that might include them.

He’s got quite a few wrapped around his wrist, so at full brightness, all these displays draw one amp. That explains why like the LED watches of the 1970s, the default state of the displays is “OFF”. Even with a LiPo pouch salvaged from a disposable vape, the runtime would only be half an hour at full brightness without that periodicity. Luckily [Sahko] included buttons on the band of the watch to activate it and control the brightness so it isn’t always blasting at full. There are also different modes available, including a really cool waterfall effect you can see in the video.

The band is an interesting choice, too: it’s just a flex PCB. There’s nothing backing it, aside from its own stiffeners, which makes us very curious how well this watch would hold up to daily use. There’s no clasp in the traditional sense, either: the band is closed by a 4-pin connector that doubles as both charge and the USB programmer for the stm32u08 microcontroller that runs the displays. Conveniently for a watch, this version of the stm32 has an RTC, so it keeps time as well. We dig the minimalism of this design; it’s a great contrast to the maximalism of wrapping your wrist in displays.

We’ve seen very similar displays on an edge-viewed watch, but a tiny amber LED matrix never gets old. If you wrapping your wrist in all those tiny LEDs is too impractically power-hungry, try using Nixie tubes.

We’re always watching for projects– wrist mounted clocks or otherwise– so if you’ve got the time, please drop us a tip.

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Some renderings of shapes made from lines including triangles and a circle.

2025 One Hertz Challenge: Analog Clock For Microsoft Windows

August 4, 2025 by John Elliot V No comments

Our hacker [glgorman] sent in their submission for the One Hertz Challenge: an analog software clock for Microsoft Windows.

I guess we’d have to say that this particular project is a work-in-progress. There is no final clock, yet. But a number of yak’s have been shaved. For instance, we have code for computing geometric objects without using branch instructions, including points and lines and circles and such.

The notes dive deep into various rabbit holes. At one point we find ourselves computing the angle to the sun in the sky, that we may be able to cast the shadow of the clock hands on our clock face. The notes include miscellaneous source code snippets and various screenshots of geometric renderings which have been achieved so far.

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2025 One-Hertz Challenge: The Flip Disc Clock

August 1, 2025 by Lewin Day 1 Comment

Do you like buses, or do you just like the flippy-flappy displays they use to show route information? Either way, you’ll probably love the flip-disc clock created by [David Plass].

The build is based around four seven-segment flip disc displays. The modules in question are from Flipo.io. They use a hefty 0.5 amp pulse to create a magnetic field strong enough to flip the discs from one side to the other with coils placed underneath the fluro/black flipdots themselves. The modules are controlled by a Wemos D1, which uses Wi-Fi to query a NTP server to keep accurate time. It then drives the necessary segments to display the current time. The whole thing is assembled in what appears to be some kind of kitchen storage tub.

Notably, the clock flips a couple dots once every second to meet the requirements of our One-Hertz Challenge. This also makes it obvious that the clock is working when it would otherwise be static. However, [David] notes commenting out that part of the code at times, as it can be quite loud!

This clock has got fluro dots, it’s well-executed, and it’s a fine entry to the 2025 One-Hertz Challenge. We’ve also previously explored how these beautiful displays work in detail, too. Meanwhile, if you’re busy repurposing some other kind of mechanical display technology, don’t hesitate to let us know!

A sine wave and triangle wave on a black background

2025 One Hertz Challenge: Op-Amp Madness

July 31, 2025 by Adam Zeloof 7 Comments

Sometimes, there are too many choices in this world. My benchtop function generator can output a sine, square, or saw wave anywhere from 0.01 Hz up to 60 MHz? Way too many choices. At least, that’s what we suspect [Phil Weasel] was thinking when he built this Analog 1 Hz Sinewave Generator.

Rendering of a PCB — A KiCad rendering of [Phil]’s design

[Phil]’s AWG (which in this case stands for Anything as long as it’s a 1 Hz sine Wave Generator) has another unique feature — it’s built (almost) entirely with op-amps. A lot of op-amps (37, by our count of the initial schematic he posted). His design is similar to a Phased Locked Loop (PLL) and boils down to a triangle wave oscillator. While a 1 Hz triangle wave would absolutely satisfy judges of the One Hertz Challenge, [Phil] had set out to make a sine wave. Using a feedback loop and some shaping/smoothing tricks (and more op-amps), he rounded off the sharp peaks into a nice smooth sine wave.

Sometimes we make things much more complicated than we need to, just to see if we can. This is one of those times. Are there much simpler ways to generate a sine wave? Yes — but not exclusively using op-amps! This entry brings stiff competition to the “Ridiculous” category of the 2025 One Hertz Challenge.

A photo of the project on a breadboard in a briefcase.

2025 One Hertz Challenge: Precise Time Ref Via 1 Pulse-Per-Second GPS Signal

July 29, 2025 by John Elliot V 5 Comments

Our hacker [Wil Carver] has sent in his submission for the One Hertz Challenge: Precise Time Ref via 1 Pulse-Per-Second GPS Signal.

This GPS Disciplined Oscillator (GPSDO) project uses a Piezo 2940210 10 MHz crystal oscillator which is both oven-controlled (OCXO) and voltage-controlled (VCXO). The GPSDO takes the precision 1 Pulse-Per-Second (PPS) GPS signal and uses it to adjust the 10 MHz crystal oscillator until it repeatedly produces 10,000,000 cycles within one second.

[Wil] had trouble finding all the specs for the 2940210, particularly the EFC sensitivity (S), so after doing some research he did some experiments to fill in the blanks. You can get the gory details in his notes linked above.

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2025 One-Hertz Challenge: Fixing The Clock That Once Synced The World

July 29, 2025 by Lewin Day 5 Comments

The HP 115BR is not one of the most well-known products from Hewlett-Packard. And yet, it was remarkably important nonetheless. This hardware once synced time around the world. Now, for our 2025 One-Hertz Challenge, [curiousmarc] has taken on the job of restoring it.

The HP 115BR itself was not used alone, but in concert with the HP5060A atomic clock. The latter would output a 100 KHz reference output. It was the job of the HP 115BR to divide this frequency down to provide a superbly accurate 1-second tick.

The example on [curiousmarc]’s bench showed up in poor shape. It was “very broken,” and he reported that it had also previously been hacked to some degree. However, he has been able to restore it to proper functionality, including the special modification for continuous tick adjustment, as used in the 1964 flying atomic clock experiment. He was even able to sync it to NIST’s current atomic clock signal from Fort Collins using the WWW radio signal.

We’ve seen plenty of old HP metal restored over the years; it’s always pleasant to see how well things were built back in the day. Video after the break.

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