GPS PPS Watch

Summary

The GPS PPS Watch is an open-source custom wristwatch project created by Sean Hodgins that serves an unconventional purpose: rather than displaying time in the traditional sense, it uses LEDs that blink in perfect synchronization with GPS satellites' Pulse Per Second (PPS) signals. As Sean Hodgins explained in "GPS Has a Secret Job," the watch leverages the atomic clock system built into GPS satellites to demonstrate extremely accurate timing across any distance on Earth.

The watch features multiple BNC connectors that allow users to compare timing accuracy between different GPS sources. One LED displays the PPS output from the watch's onboard GPS, another can receive input from an external PPS source via BNC, and a third LED acts as a sync indicator—only lighting when the two signals are out of alignment. The design uses a MAX-M10 GPS Module (selected for its compact size suitable for wrist-mounting), two buffers for signal splitting, an XOR gate, and notably includes no microcontroller beyond what's already inside the GPS module itself. Sean Hodgins created this project specifically for Jeff Geerling's visit with only two weeks to design the schematic, order boards from PCBWay, source components from Digi-Key, and prepare a 3D-printed case.

What Was Said

In the episode, Sean Hodgins built the GPS PPS Watch as a hands-on project for Jeff Geerling's first surface mount soldering experience. The watch became the centerpiece of their discussion about GPS timing technology and precision synchronization.

Sean Hodgins emphasized the watch's unique synchronization capability: "And the weirdly cool part is that if two people are wearing one of these and they both have a clear view of the sky, the watches should blink in sync. They don't need to be near each other. One person could be here and the other could be on the other side of the planet. As long as both watches can see GPS satellites, they're both listening to the same clock system. So, the LEDs blink together. It's time that connects them."

When demonstrating the sync-checking functionality, Sean Hodgins explained: "So, the point was that you can take two of these, or if there's two people... And you can sync them up with like a BNC cable. So, we're going to try that right now, and we'll see how off the signal is, cuz they should be pretty accurate." He further clarified the third LED's purpose: "the middle blink is actually will only come on when there is a signal missing and one is going. So, the idea is that you will see a quick blink if there's any sort of offset on it."

After successfully testing the watches, Sean Hodgins noted their accuracy, stating "technically to us they are perfectly in sync."

The show highlighted the open-source nature of the project, with GitHub files, Fusion360 files, and Instructables documentation available for anyone wanting to build their own. Sean Hodgins mentioned practical considerations including the need for a 30-minute 3D print for the case, a power switch to keep the battery installed, and a quick connector for accessing data from the GPS module for potential future expansions like adding a display screen. He also noted the challenge of GPS reception indoors, explaining that the final assembly needs to be placed near a window to acquire a GPS fix.

History

GPS Has a Secret Job

The GPS PPS Watch was the central project of this episode, designed by Sean Hodgins on a tight two-week timeline specifically for Jeff Geerling's visit to the shop. Sean Hodgins guided Jeff Geerling through soldering his first surface mount circuit board, building one of the watches from start to finish.

The episode featured detailed discussion of the watch's technical design, created in KiCad and manufactured by PCBWay. The watches were tested successfully, demonstrating their synchronized timing capabilities when connected via BNC connectors. Both Sean Hodgins and Jeff Geerling observed the precision of the GPS-based timing system, with the watches blinking in perfect sync according to their visual assessment.

Sean Hodgins promoted the project's connection to Open Sauce, encouraging viewers to get tickets to the event where he would be attending. The complete project files were made available as open-source resources through GitHub and Fusion360, with sponsorship support from PCBWay and components sourced from Digi-Key.