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An Arduino hollow clock uses two stacked, toothed rings instead of a solid dial: the rear ring shows minutes, while a gear train reduces motion by 60:1 for the hour indication. To build one, print and align the mechanism, couple the rings as specified by the design, connect a 5V geared stepper through a ULN2003 driver, then set and calibrate the time.
How the hollow clock mechanism works
The clock’s open rings and concealed drive create the impression that the hands or indicators float. Arduino’s project description specifies two horizontally stacked rotating rings lined with teeth. The rear ring carries the minute indication; gearing reduces the minute drive by 60:1 for the hour indication. In designs using magnetic coupling, magnets transfer motion between parts without making the visible assembly look like a conventional clock face.
The floating effect is mechanical, not a transparent or digital display. The exact ring arrangement and magnetic coupling depend on the STL set and firmware variant, so use the assembly instructions and wiring intended for the same design revision.
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- Controller: an Arduino Nano, Uno, or Pro Mini supported by the chosen firmware. Match the board’s voltage and clock configuration to the firmware settings.
- Motor and driver: a 5V 28BYJ-48 geared stepper and a ULN2003 driver board. Use another 5V stepper only if the project documents that variant and its wiring.
- Power and wiring: a USB power adapter or suitable regulated supply, plus ordinary hookup wire and headers.
- Printed components: the project’s STL files for the rings, gears, hands, base, rear panel, and any decorative or index parts.
- Magnets: one documented design uses three 8 mm × 3 mm neodymium magnets; another option in the design documentation is six 8 mm × 1 mm magnets. Follow the pockets and magnet count for your selected version.
- Fasteners and lubricant: a newer guide specifies eight 2 mm × 10 mm self-tapping screws. Use high-viscosity grease on gear interfaces.
- Fabrication supplies: a 3D printer, suitable filament, and tools for removing supports and carefully deburring printed gear teeth.
Print and assemble the clock
1. Choose a matching design revision
Download the STL files, firmware, wiring diagram, and assembly documentation for one version of the clock. Do not mix parts or pin mappings from different revisions unless you verify that they are compatible. Slice the parts for your printer. A 2026 guide says most parts fit a 200 × 200 mm bed, but its decorative index is about 203 × 203 mm; that piece needs a larger bed or careful resizing.
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2. Print and check the moving parts
Print slowly enough to preserve small gear teeth, and use supports wherever the selected STL or guide calls for them. Remove supports and deburr the worm gear and small center gear. Before installing electronics, check that each ring is round, concentric, and able to move without binding. Poor print quality or misalignment can increase friction and disrupt movement.
3. Build the ring-and-gear stack
Assemble the rings and gears in the documented order. Fit magnets into their designed pockets, securing them with adhesive when the instructions specify it. Check the ring movement before attaching the hand and index pieces; they should turn freely rather than scrape or catch. Apply high-viscosity grease to the gear interfaces as directed by the design.
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4. Install the motor and electronics
Mount the 28BYJ-48 and ULN2003 board in the base or rear panel. The basic circuit uses four Arduino output pins connected to the driver’s four input pins, along with 5V and ground. Use the pin mapping in your selected firmware documentation rather than assuming a universal pin order. Keep the motor shaft and power connector clear of the printed frame.
5. Upload the matching firmware
In the Arduino IDE, select the board and clock configuration expected by the firmware, then upload the project’s stepper code. Project notes warn that choosing the wrong 8 MHz/16 MHz clock or 3.3V/5V configuration can alter timing by roughly a factor of two. Confirm the settings before trying to correct a large timing error by changing the calibration constant.
Rank #3
- This DIY assemble kits is equipped with 6 digits LED module, it's fun to build and also help you learn more professional knowledge about electronics.
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- The kit contains a 150MM single-head power supply line, the end with the terminal can be directly plugged into the PCB power supply socket. The other end has no terminal, and can be connected to the power supply after stripping the wire.
Set the hands and calibrate the rate
Set the time mechanically
One documented assembly sets the time by lifting the clock face from the base, positioning the hands, and reinserting the face. Follow the procedure for your version and avoid forcing a ring against the gears. Some firmware variants advance the mechanism by one minute on startup, so account for that initial tick when positioning the hands.
Adjust milliseconds per minute
The firmware’s nominal interval is 60,000 milliseconds per minute. Observe the clock against a reliable reference over a sufficiently long interval, note whether it gains or loses time, and adjust the interval in small steps. Recheck after each change, narrowing the adjustment like a binary search rather than making a large guess. A documented Pro Mini build reached 59,853 ms per minute after repeated comparisons; this is an example for that particular build, not a universal value.
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Rate depends on the controller’s oscillator, motor load, friction, gear backlash, and print tolerances. If the clock runs inconsistently, first check for rubbing, misaligned rings, poor gear engagement, and wiring or power problems; calibration cannot compensate reliably for mechanical binding.
What accuracy to expect
The basic design keeps time from the Arduino’s internal oscillator and stepper timing, not from a real-time clock (RTC). Acamar MCU Projects notes that “The clock relies on the internal oscillator of the Arduino, so it is not as precise as a RTC module.” Calibration can improve the observed rate of an individual build, but it does not make oscillator-based timekeeping equivalent to an RTC. The cited base circuit does not include an RTC or a network time source; either would be a separate design enhancement for users who need more stable or synchronized time.
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Choosing between documented variants
Before printing, compare the controller and firmware, motor pin mapping, printer-bed requirements, magnet arrangement, and mechanical time-setting method. Confirm whether the version adds an RTC; the base circuit described here does not. Keep the STL files, wiring diagram, and firmware constants together as one revision wherever possible, because seemingly small differences can affect fit, wiring, and timing.
Quick Recap
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