Even with teachers with names like Kirchhoff and Helmholtz, old Heinrich Hertz himself likely didn’t have the slightest idea that his name would one day become an SI unit. Less likely still would have been the idea that Hackaday would honor him with the 2025 One-Hertz Challenge.

The challenge was deliberately — dare we say, fiendishly? — simple: Do something, anything, but do it once a second. Flash a light, ring a bell, click a relay, or even spam comments on a website other than Hackaday; anything at all, but do it at as close to one Hertz as possible. These are our favorite kinds of contests, because the simplicity affords a huge canvas for the creative mind to paint upon while still providing an interesting technical constraint that’s just difficult enough to make things spicy.

And boy, did you respond! We’ve received over a hundred entries since we announced the contest back in June, meaning that many of you spent 4,662,000 seconds of your summer (at least those of you above the equator) rising to the challenge. The time was well spent, with projects that pushed the limits of what we even expected.

While we loved ’em all, we had to winnow them down to the top three, each of which receives a $150 gift certificate from our sponsor, DigiKey. Let’s take a look at them, along with our favorite runners-up.

Our Top Three

At the top of our judges’ list was “the electromechanicalanalogdigitalclock”, a project that clearly didn’t know what it wanted to be but nevertheless did it with a lot of style. [Christian]’s contraption pushes a lot of design buttons, starting with the mains-powered stepper motor generating a 1-Hz signal with a photochopper, which drives a 12-bit counter made from some CMOS logic chips and a digital-to-analog converter that drives some vintage moving-coil meters to display the time. There’s even a bit of circuit sculpture thrown in, with a brass frame supporting and isolating the noisy stepper motor on a spring suspension. Extra points were no doubt earned thanks to the Space:1999 and Star Trek models in the photos.

Next up we have BEZICRON by [ekaggarat singh kalsi]. If this one looks familiar, it’s probably because we featured it back in January, when we had a difficult time describing exactly what this is. It’s a clock, sure, but its display is vastly different from anything we’ve ever seen before, based as it is on hair bands, of all things, that are bent and stretched into numerals by a series of intricate cams and levers. The idea is unique, the mechanism is complex, the design is striking, and the sinuous 1-Hz pulse of the colon is mesmerizing.

Our final gift certificate goes to [Tim], who managed to use candle flames as a time base. You’ve probably noticed candles guttering and flickering thanks to uneven wax melting or even drafts blowing the flame column around and thought they were fairly random. But [Tim] noticed that these oscillations were actually more stable and predictable than they appear, and used a wire sticking into the flame to trigger the capacitive sensor input on a CH32xxx microcontroller to measure the frequency, which was then divided down to flash an LED at 1-Hz. It’s the perfect combination of physics and electronics that extracts order from a seemingly stochastic in a weird and wonderful way.

Awesome Honorable Mentions

What’s always fun about Hackaday contests is the categories we come up with, which are sort of mini-games within the main challenge. And this time around didn’t disappoint, with projects that explored these side quests in fun and interesting ways.

Our “Ridiculous” category was all about tapping your inner Rube Goldberg and finding the least practical way to generate your 1-pps pulse train. Runners-up in this category included [Brian Stuparyk]’s electromechanical function generator, a pitchblende-powered “atomic” clock by [alnwlsn], and [Sean B]’s “Nothing but NAND” Nixie clock.

For the “Timelords” category, we were looking for the projects that pulled out all the stops to get as many zeroes as possible after the decimal point, and the entries didn’t disappoint. Check out this vintage atomic clock restoration by [CuriousMarc] and his merry band, [Lauri Pirttiaho]’s cheap and simple GPS sync for quartz wall clocks, or this GPS-disciplined crystal-oven oscillator by [Will Carver].

The horologically inclined were the target audience for the “Clockwork” category, which invited you to turn your one-per-second timebase into a unique and interesting timepiece. See [Simon Newhouse]’s Nixie-based frequency counter clock, the DCF77 clock [hayday] made from the 2022 Supercon Badge, or the beautiful bubble displays of [Andrew Tudoroi]’s RPi TinynumberHat9 clock.

And finally, what would a One-Hertz challenge be without the venerable 555 timer chip? Entries we liked from the “Coulda Used a 555” category include [Tom Goff]’s Bletchley-inspired Logic Bombe, this mind-bending, capacitor-free timer that [Mark Valentine] put together, and [Paul Gallagher]’s super annoying “One Hurts” clock — it’s worse than a cuckoo clock!

Everyone’s a Winner!

We’d love to give everyone a prize, but we’d be hard-pressed to manage that with so many cool and unusual projects. As they say, everyone’s a winner just for entering, and we think that’s especially true with contests like this, which bring out the best in everyone. Thanks to everyone who entered, the judges for sorting through everything and making the hard choices, and to our sponsor DigiKey. We’ll see you all again next time around!