In a world where a $3 microcontroller can blink an LED once per second before your coffee cools, the 2025 One Hertz Challenge asked makers to do something more interesting: make something happen at exactly one hertz, preferably with a little flair. One entry answered with a gloriously overbuilt, delightfully noisy, deeply charming machine: an electromechanical CMOS clock that keeps time by listening to the heartbeat of the electrical grid.
This is not the sort of clock that hides quietly on a nightstand and politely glows at 2:00 a.m. It is a steampunk-flavored timepiece built from CMOS logic, a mains-driven stepper motor, optical sensing, DAC chips, LEDs, and vintage moving-coil meters. In other words, it is what happens when someone looks at an Arduino, says “too easy,” and reaches for a pile of classic logic ICs instead.
The result is a clock that bridges old and new: a mains-frequency timebase like early electric clocks, CMOS counter logic from the golden age of hobby electronics, and analog needle displays that make the passing of time feel mechanical, visible, and a little theatrical.
What Was the 2025 One Hertz Challenge?
The One Hertz Challenge was built around a beautifully simple rule: create a device where something happens once per second. That “something” could be practical, ridiculous, precise, artistic, mechanical, electronic, or all of the above. The challenge celebrated the humble one-hertz rhythm: one event per second, sixty per minute, and about 31 million per year.
That sounds easy until you start thinking like a builder. A blinking LED can do it. A 555 timer can do it. A microcontroller can do it while also checking the weather, ordering pizza, and judging your solder joints. But the real fun begins when the one-second pulse comes from an unusual source. That is where Christian’s electromechanical analog-digital clock shines.
The project became one of the standout entries because it did not simply produce a pulse. It turned the act of generating, counting, converting, and displaying time into the main event. Instead of hiding the mechanism, the clock celebrates it. The logic boards, LEDs, meters, and motor are not just functional parts; they are the show.
The Core Idea: Let the Grid Set the Beat
The clock’s cleverest idea is using mains frequency as the timing reference. In Christian’s build, the local grid is 230 volts at 50 hertz, as used in Germany. That means the alternating current changes direction 50 times per second. The project uses that predictable AC rhythm to drive a stepper motor.
A 200-step stepper motor normally requires controlled coil sequencing. In this build, the motor is driven in a wonderfully old-school way: a capacitor creates a phase shift between windings, helping produce the rotating magnetic field needed to spin the motor from the AC supply. Because the motor needs four steps per AC cycle, a 50 Hz supply produces 200 steps per second. With a 200-step motor, that works out to exactly one revolution per second.
That rotating shaft is not the time display itself. Instead, a small 3D-printed arm interrupts a slot optical sensor once per revolution. Every interruption becomes one clean one-second pulse. That pulse is then sent into the CMOS counting circuitry. It is a mechanical-to-optical-to-digital handshake, which sounds complicated because it is. That is also why it is delightful.
Why Mains Frequency Can Keep Time
Using grid frequency for clocks is not a new trick. Synchronous electric clocks have relied on power-line frequency for more than a century. The basic concept is simple: if the grid averages 50 Hz or 60 Hz over time, a clock can count cycles and convert them into seconds, minutes, and hours.
Historically, this mattered a lot. Early electric clocks and grid operators were closely connected because accurate frequency regulation helped both timekeeping and power sharing. When interconnected power systems grew, keeping generators synchronized became essential. A stable grid frequency allowed power stations to work together, and clocks became one of the everyday beneficiaries.
However, mains frequency is not the same as an atomic clock. Grid frequency can drift slightly depending on the balance between generation and load. When demand exceeds generation, frequency tends to sag; when generation exceeds demand, it can rise. Over longer periods, grid operators may correct accumulated time error, but modern practices vary by region. For a hobby clock, the grid is often accurate enough to be interesting, but it is not magic. It is more like a very large orchestra trying to keep tempo while everyone plugs in air conditioners.
The CMOS Brain: CD4040 Counters Do the Counting
Once the clock has a one-pulse-per-second signal, it needs to count. That job falls to classic CMOS logic, especially the CD4040. The CD4040 is a 12-stage ripple-carry binary counter/divider. It advances on a clock edge and provides multiple binary outputs representing the count state.
In this clock, the counters are arranged to handle seconds, minutes, and hours. Seconds count up to 60, then reset and trigger the minutes counter. Minutes do the same. Hours count up to 24 before rolling over. This is the digital skeleton behind the analog-looking clock face.
The use of CD4040 counters gives the clock a distinctly educational quality. A modern microcontroller would hide most of this behavior in code. The CMOS approach puts the logic out in the open. You can see bits change. You can watch LEDs blink as binary values advance. It is timekeeping as a visible logic lesson, and frankly, it makes a normal digital clock look a little lazy.
Why Divide by 60 and 24?
Time is inconveniently human. Computers like powers of two; people like 60 seconds, 60 minutes, and 24 hours. The CD4040 naturally counts in binary, so the circuit must detect when the count reaches a target value and reset it. For seconds and minutes, that target is 60. For hours, it is 24.
The project uses logic gates to detect the needed binary states. When the counter reaches the reset condition, the circuit clears the counter and passes a pulse to the next stage. It is a simple idea, but it requires careful wiring, reliable edge behavior, and a good understanding of how ripple counters behave. This is exactly the kind of detail that separates a charming build from a pile of blinking confusion.
From Binary Numbers to Moving Needles
The clock does not display time with seven-segment LEDs or a traditional dial. Instead, it uses vintage moving-coil meters. These are the kind of analog instruments you might expect to see on old lab gear, industrial panels, or equipment that looks like it should be operated by someone wearing a white coat and saying “increase the voltage slowly.”
To make the meters show time, the binary counter outputs must become analog voltages or currents. That is where the DAC0808 comes in. The DAC0808 is an 8-bit digital-to-analog converter, meaning it accepts a digital input value and produces a corresponding analog output current. In this clock, the DACs translate counter values into meter movement.
The lower unused bits are tied down, and the circuit is adapted so the meters can show hours and minutes on custom scales. Christian also modified the meters themselves, replacing resistors so they would work over a 0 to 5 volt range instead of their original 0 to 10 volt range. The result is a display that feels both technical and handmade. The needles do not just tell time; they perform it.
Why This Clock Feels So Satisfying
The electromechanical CMOS clock is satisfying because every layer has a job you can understand. The grid provides the rhythm. The stepper motor turns that rhythm into motion. The optical interrupter converts motion into pulses. CMOS counters accumulate the pulses. DACs convert binary values into analog signals. Moving-coil meters turn those signals into visible time.
That chain of cause and effect is wonderfully physical. Nothing feels abstract. A pulse becomes a count. A count becomes a needle position. A needle position becomes “oh no, I’m late again.”
It also highlights a larger maker philosophy: the best engineering project is not always the shortest path between problem and solution. Sometimes the best project is the one that teaches the most, looks the best, and makes visitors ask, “Wait, how does that thing work?”
Electromechanical Design Choices That Make the Build Stand Out
1. A Stepper Motor as a Timebase
Most digital clocks use a crystal oscillator, a real-time clock module, GPS, network time, or a microcontroller timer. This clock uses a stepper motor spinning from the mains frequency. That choice adds sound, motion, and personality. It also makes the timing source visible rather than hidden in a tiny metal can.
2. A Slot Optical Sensor for the Second Pulse
The motor does not directly clock the logic. A 3D-printed arm breaks a light beam once per revolution, producing the one-second signal. This improves the interface between the mechanical and digital parts. It also gives the build a satisfying photochopper-style mechanism: simple, visual, and reliable when aligned properly.
3. CMOS Logic Instead of a Microcontroller
The project intentionally avoids Arduino-style convenience. That decision makes the circuit larger, more complex, and more interesting. CMOS logic gives the clock a transparent architecture. The builder can troubleshoot with a meter or oscilloscope and follow the signal path from pulse to display.
4. Analog Meters as a Display
Moving-coil meters are not the most compact way to show time, but they are among the most beautiful. Their smooth needle motion adds drama. Custom scales turn ordinary electrical instruments into a clock face. It is the difference between reading time and watching time flex its eyebrows.
The Role of Noise, Hum, and Real-World Imperfection
One of the project’s funniest lessons is that mains-powered electromechanics are not silent. A 50 Hz system can hum. A stepper motor driven in this unconventional way can be loud. The project documentation even notes that the hum and stepper noise became part of the experience.
This is not a flaw so much as a reminder that physical computing lives in the real world. Motors vibrate. Coils heat. Capacitors have tolerances. Buttons bounce. Sensors need clean edges. Wires pick up noise. Breadboards behave beautifully until they decide to become tiny radio antennas with commitment issues.
That is why the clock is more than a clever schematic. It is a build full of practical lessons: debounce your buttons, isolate noisy mechanical parts, protect mains-voltage sections, and never assume that a circuit that works on a bench will behave perfectly once mounted in a beautiful frame.
Safety Matters: This Is Not a Casual Beginner Project
Because the clock interacts with mains voltage, it deserves a serious safety note. Mains-powered projects can be dangerous or fatal when handled incorrectly. Anyone attempting a similar build should understand isolation, fusing, grounding, enclosure design, strain relief, creepage and clearance, and local electrical codes. The high-voltage section should be physically protected, clearly separated from low-voltage logic, and worked on only when de-energized by someone qualified to do so.
For readers who simply love the idea, a safer educational version could use a low-voltage AC source or a signal generator to simulate the mains-derived timebase. You would keep the learning experience while removing the part where your hobby project tries to become a medical emergency.
Why the Project Won the Internet’s Tiny Mechanical Heart
The electromechanical analog-digital clock ultimately became one of the winners of the 2025 One Hertz Challenge because it matched the spirit of the contest so well. It was not just accurate enough. It was inventive, well documented, visually memorable, and full of personality.
It also blended several eras of electronics. The grid-frequency reference points back to early synchronous clocks. The CMOS counters and DACs evoke classic hardware design. The custom meter faces and visible boards give it a maker-era aesthetic. The 3D-printed parts bring in modern fabrication. It is a clock made from multiple decades shaking hands.
That is what makes it more compelling than a simple “clock project.” It is a conversation about time, power, logic, motion, and design. It asks a fun question: what if one second was not just a number on a screen, but a whole chain of physical events?
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The topic also has built-in curiosity. “One hertz” is simple enough for beginners to understand, while the implementation is rich enough for experienced electronics hobbyists. A reader can arrive for the novelty and leave with a better understanding of frequency division, optical sensing, synchronous motion, DAC outputs, and the historical relationship between electric grids and clocks.
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Lessons Makers Can Learn From This Electromechanical CMOS Clock
The first lesson is that constraints create creativity. Asking for “one pulse per second” could lead to boring answers, but the challenge encouraged builders to explore unusual timing sources. The second lesson is that older components still have enormous educational value. A CD4040 may not be trendy, but it makes counting visible in a way firmware does not.
The third lesson is that display choice matters. The moving-coil meters give the clock its soul. They transform a logic project into a physical object with character. The fourth lesson is that documentation matters. A strange build becomes valuable to others when the creator explains the decisions, the mistakes, and the fixes.
Finally, the project proves that “because I can” remains one of the finest engineering motivations known to humanity. Not every project must be commercially practical. Some projects exist to make us smile, learn, and appreciate the strange beauty of a one-second pulse.
Hands-On Experience Notes: Building Around One Hertz in the Real World
Working with one-hertz projects teaches patience in a way faster electronics rarely do. At kilohertz or megahertz speeds, signals blur into oscilloscope traces and timing diagrams. At one hertz, every mistake has the comedic timing of a slow elevator. You press a button, wait, watch, and then realize the counter reset one state too late. The circuit is not broken quickly; it is broken dramatically.
A project like this electromechanical CMOS clock also reminds builders that “simple timing” is rarely simple once hardware is involved. A pulse must have the right edge, the right voltage, the right duration, and enough noise immunity to survive the real world. If a spinning arm passes through an optical interrupter, the signal may need shaping. If a button sets the time, it probably needs debouncing. If a motor sits near logic boards, its electrical and mechanical noise may deserve more respect than your first layout gave it.
The moving-coil meter display adds another layer of experience. Analog meters are wonderfully expressive, but they are not plug-and-play decorations. Their scale, coil sensitivity, series resistance, damping, and calibration all affect the final result. A DAC output that looks mathematically correct may still need buffering, trimming, or scaling before the needle lands exactly where the clock face says it should. That final calibration process can feel like tuning a musical instrument, except the instrument judges you once per second.
Then there is the mechanical side. A 3D-printed interrupter arm must be balanced enough not to wobble, opaque enough to block the sensor, and positioned carefully so the pulse happens cleanly. Mounts need to be rigid where alignment matters and flexible where vibration should be isolated. The prettiest build can become annoying if a stepper motor transfers every buzz into the frame. Suddenly, foam pads, springs, brass spacers, and enclosure design become as important as the schematic.
The biggest experience-based takeaway is that a one-hertz clock makes electronics feel alive. You can hear the motor, see the LEDs, watch the counters advance, and follow the needles. It is slower than software and less convenient than a ready-made clock module, but that is exactly the point. Every second becomes evidence that physics, logic, and craftsmanship are cooperating. When they are not cooperating, you get troubleshooting practice. When they are, you get a clock with personality.
For anyone inspired by this build, the best path is to start safely and modularly. Build a low-voltage one-hertz pulse generator first. Feed it into a counter. Add reset logic. Then add a display stage, whether LEDs, meters, or something completely impractical. Once each section works alone, connect them one at a time. That approach preserves the joy of the original project without turning debugging into an Olympic event.
In the end, the 2025 One Hertz Challenge clock is memorable because it treats one second as something worth building around. It is not merely timekeeping. It is timekeeping with gears in its imagination, CMOS in its brain, and a little mains hum in its singing voice.
Conclusion
The 2025 One Hertz Challenge electromechanical CMOS clock is a wonderful reminder that electronics can be practical, educational, and charmingly excessive at the same time. By using mains frequency to spin a stepper motor, an optical interrupter to create a one-second pulse, CD4040 counters to track time, DAC0808 converters to drive analog meters, and visible construction to show the process, the project turns timekeeping into a full-body performance.
It is not the easiest way to build a clock. It is not the quietest. It is certainly not the smallest. But it may be one of the most satisfying ways to understand what a second really is: a rhythm, a count, a motion, a signal, and, in this case, a beautiful excuse to make CMOS logic dance.
