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STM32F072 IN-16 Nixie Clock with GPS and RTC Sync

A 6-tube IN-16 Nixie clock with RTC and GPS-based time synchronization, prototyped on an STM32F072 board and later moved to a custom MCU design.

Intermediate Project — This is intermediate because it combines a custom high-voltage Nixie supply, RTC plus GPS time synchronization, and mixed-voltage PCB design, even though the MCU and prototype board are straightforward.
Assumptions:
  • The user wants a mains-powered desktop clock, not a battery-powered portable device.
  • The IN-16 tubes are driven by a separate high-voltage Nixie driver stage that is not yet specified in the database.
  • GPS is only for time sync and location/time reference, not for displaying coordinates or logging position.
  • The STM32F072B-DISCO will be used only for prototyping; the final design will likely use an STM32F072RBT6-class MCU or similar.
  • The clock needs RTC backup so it can keep time when GPS is unavailable.

Bill of Materials

Microcontroller
Top Pick STM32F072RBT6 STMicroelectronics From our database
For the final design, STM32F072RBT6 is the best pick because it matches the user's intended MCU class, has enough peripherals for RTC plus GPS sync, and is easy to carry from prototype to custom PCB.
Mouser $6.79 (645 in stock) Digikey
Dev Board NUCLEO-F031K6 STMicroelectronics
Ready-to-use board for prototyping with this chip
Mouser $32.44 (116 in stock)
STM32F072RB STMicroelectronics From our database
Same core family and resource class as the RBT6, suitable if you want the same firmware target with a different package/ordering option. Good peripheral set for UART GPS input and I2C RTC control.
NUCLEO-F072RB STMicroelectronics From our database
Best for prototyping because it already includes the STM32F072RBT6, USB programming, and onboard support circuitry. It is not a final IC, but it is the fastest way to validate firmware and interfaces before spinning a custom board.
GPS/GNSS Receiver
Top Pick MAX-M10S-00B u-blox From our database
MAX-M10S-00B is the best overall choice because it is compact, low power, and more than accurate enough for GPS-based time synchronization in a Nixie clock.
Mouser $13.33 Digikey
MIA-M10Q-00B-02 u-blox From our database
Also an M10-family GNSS module with multi-constellation support, which improves lock reliability and time-to-first-fix in marginal antenna conditions. Good if you want a module-oriented design with broad GNSS compatibility.
LC29HAAMD Quectel From our database
High-precision multi-GNSS module with L1/L5 support, which is overkill for simple time sync but excellent if you want better reception and future expansion. More capable than needed, but robust.
RTC
Top Pick DS3231SN Analog Devices / Maxim Integrated From our database
DS3231SN is the best pick because it gives the most reliable clock accuracy for a Nixie clock and is easy to prototype with using common breakout modules.
Mouser $16.39 (6,010 in stock)
RV-3028-C7 Micro Crystal AI suggestion - verify availability
Very low-power RTC with good accuracy and a small footprint, useful if you want a compact custom PCB and coin-cell backup. Strong choice for a polished final design.
PCF8563TS NXP
Simple, low-cost RTC with broad ecosystem support and easy I2C integration. Good if you want the cheapest practical RTC and can tolerate lower accuracy than a DS3231-class part.
Mouser $1.42 (10,257 in stock)
Display
Top Pick IN-16 Various AI suggestion - verify availability
IN-16 is the correct choice because it matches the user's stated tube type and the rest of the design should be built around that display geometry and drive requirements.
IN-12A Various AI suggestion - verify availability
Common and well-supported Nixie tube family with lots of hobbyist documentation and driver examples. A practical fallback if IN-16 sourcing becomes difficult.
IN-14 Various AI suggestion - verify availability
Another widely used Nixie tube option with strong hobbyist support and readily available driver examples. Good alternative if you want a different tube size or appearance.
Actuator
Top Pick K155ID1 Various AI suggestion - verify availability
K155ID1 is the most practical traditional choice for a Nixie clock because it aligns well with IN-16-style tube driving and the classic hobbyist implementation style.
HV5522 Microchip
Modern high-voltage shift-register style driver for multiplexed Nixie or VFD-style outputs, reducing the need for many discrete driver parts. Better for a cleaner custom PCB if you want to avoid vintage driver sourcing issues.
Mouser $7.80 (76 in stock)
Power Supply
Top Pick LM2577T-ADJ/NOPB Texas Instruments From our database
LM2577T-ADJ/NOPB is the best overall power choice because a 6-tube Nixie clock needs a real high-voltage boost stage, and this part has the headroom to support that requirement.
Mouser $8.39 (1,859 in stock) Digikey
TPS61040DBVR Texas Instruments From our database
Useful boost converter for generating the Nixie high-voltage rail from a low-voltage input, with output capability up to 28 V and good efficiency for a compact design. Appropriate for the low-power logic side or as part of a higher-voltage supply architecture, though Nixies may need a higher-voltage stage depending on tube and resistor design.
LMZM23600SILR Texas Instruments From our database
Integrated buck module for clean 5 V or 3.3 V logic power from a higher input supply, simplifying the low-voltage rail design. Good companion regulator if the clock is powered from a wall adapter and you want a tidy, low-noise logic supply.
Connectivity
Top Pick USB-UART bridge via STM32F072 USB FS STMicroelectronics AI suggestion - verify availability
Top pick: USB-UART bridge via STM32F072 USB FS (STMicroelectronics). The STM32F072 family includes USB full-speed capability, so you can use the MCU itself for firmware upload, debugging, and optional time-setting communication without adding a separate connectivity chip. This keeps the prototype simpler and cheaper.
Dev Board NUCLEO-F031K6 STMicroelectronics
Ready-to-use board for prototyping with this chip
Mouser $32.44 (116 in stock)
ESP-01S Ai-Thinker AI suggestion - verify availability
Low-cost WiFi module if you later decide to add network time sync or web configuration. Not needed for the stated requirement, but a common expansion path.
RN-42 Microchip
Simple Bluetooth serial module for local configuration from a phone or PC. Useful if you want wireless setup without adding full WiFi complexity.
Mouser $67.08 (10 in stock)

Compatibility Notes

  • The STM32F072 runs at 3.3 V logic, so any GPS module or RTC breakout should be selected for 3.3 V STM32F072RBT6 operation or level shifted if needed.
  • The Nixie tube display requires a high-voltage rail, typically far above MCU logic levels, so the tube driver and HV supply must be isolated from the STM32 GPIO domain by proper driver circuitry.
  • The GPS module UART output should connect directly to the STM32F072 UART if the module is 3.3 V logic; otherwise add level shifting.
  • If you use a DS3231 breakout, confirm whether the breakout includes pullups to 5 V, because that would require either 3.3 V pullups or level translation for the STM32 I2C bus.
  • The prototype can be powered from USB for logic, but the Nixie HV rail will still need a separate boost supply path.

You'll Also Need

  • Nixie tube sockets and mechanical mounting hardware.
  • High-voltage resistors, decoupling capacitors, and the full tube driver transistor/IC stage.
  • GPS antenna and any required RF connector or cable.
  • RTC backup battery holder or supercapacitor.
  • Custom PCB, headers, test points, and enclosure.
  • Mains input protection and an isolated AC/DC adapter if you are not using USB plus a separate HV supply.
Estimated BOM Cost: $95-105 (based on live distributor pricing)

Design Considerations

High-Voltage Nixie Drive
IN-16 tubes need a high-voltage rail and proper current limiting, so the display section is the main electrical risk in the design. Keep the MCU ground and HV switching loop tight, use adequate creepage/clearance, and expect to tune the anode/cathode resistors during bring-up. A noisy boost converter can also inject jitter into the RTC or GPS if layout is poor.
Timekeeping Strategy
Use the GPS as the absolute time reference and the RTC as the holdover clock. On startup, wait for a valid GPS fix before disciplining the RTC, then periodically resync and ignore obviously bad GPS data during cold starts or poor reception. This gives you stable time even when the antenna is indoors or temporarily disconnected.
Antenna and Reception
GPS time sync is only as good as the antenna placement and sky view. For a desktop clock, put the antenna near a window or use an active antenna if the module supports it, because indoor reception can be marginal and cause long time-to-first-fix. Design firmware to detect loss of fix and fall back gracefully to RTC-only operation.
Firmware Architecture
Structure the firmware as separate tasks for tube refresh, GPS parsing, RTC maintenance, and user interface. Avoid blocking delays in the display update path, because Nixie multiplexing and GPS UART parsing both benefit from deterministic timing. A watchdog is worthwhile because a hung high-voltage control loop can leave the display in an unsafe state.
Power Budget and Heat
The logic side is low power, but the Nixie HV supply and tube current dominate the thermal budget. Six tubes can easily make the clock warm, so measure the actual HV rail current and tube segment current during all-digit-on tests. If the enclosure is small, plan for ventilation and avoid placing the GPS antenna near the boost converter or hot tubes.
Prototype-to-Final Transition
The STM32F072RBT6 is fine for firmware development, but the final board should be laid out around the chosen MCU, RTC, GPS connector, and HV stage from the start. Keep the prototype pin mapping close to the intended final PCB so you do not rewrite the firmware pinout later. Also reserve test pads for UART, I2C, reset, and the HV enable line.

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