Circuit: GPS-Synchronized Nixie Clock

What happens when a classic Nixie clock design runs out of I/O? This GPS-synchronized Nixie clock project shows how smart component choices and clean circuit thinking make room for seconds. It’s all done without adding unnecessary complexity.

A Next-Gen Clock

Back in May/June of 2016, we presented Willem Tak’s eye-catching GPS-synchronized Nixie clock. The design’s features included: GPS synchronization, full time display including seconds, an internal reference clock bridges GPS signal dropouts, and automatic switching between summer and winter time.
 
At first glance, extending a GPS-synchronized Nixie clock from four digits to six sounds trivial, until you hit the hard wall of microcontroller I/O. That’s exactly where this design gets interesting.

The original architecture, built around a PIC18F2480, simply didn’t have enough pins to directly drive two additional Nixie tubes for seconds display. The solution was to step up to a PIC18F4420 in a 40-pin package. That decision preserved the original circuit philosophy: direct control, deterministic timing, and minimal abstraction between the microcontroller and the high-voltage display stage.

The Circuitry

The circuit of the original Precise Nixie Clock was been expanded with two additional tubes and the PIC18F4420. The microcontroller The reads the time data from the GPS module and drives the Nixie tubes, Tak explained. “Very little changed in the rest of the circuit aside from the two additional Nixie tubes and associated drive circuitry,” noted Tak. “Port D is new here. Seven pins of this port are used to drive the decoders for the seconds display. The remaining pin is used for an additional neon lamp that separates the minutes from the seconds.”

Tak selected a black PCB to give gives the clock modern appearance.
 

The GPS-Synchronized Nixie Clock Project

The original article, “"New Precise Nixie Clock: A GPS Synchronized Clock with Seconds Display,” appeared in Elektor May/June 2016. Here is the article.

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Editor's Note: The article first appeared in a 2016 issue of Elektor, so some parts and components might not be available. Still, we think the project will inspire you to start a new design.