The Most Accurate Watch On Your Wrist Has No Display At All

Most watch projects start with a display and work backwards. Sean Hodgins started with atomic-grade timing accuracy and ended up with something that cannot tell you what time it is.

What They Built

PPS stands for pulse-per-second — a single electrical pulse fired exactly once every second, synchronized to GPS satellites and accurate to within nanoseconds of UTC. It’s the timing signal that laboratories, radio stations, and network infrastructure use when “close enough” isn’t close enough. Normally it lives in a rack-mounted instrument. Sean Hodgins put it on a watch band.

The PPS Watch uses a u-blox MAX-M10S GPS module to lock onto satellites, then outputs that 1Hz reference signal through a pair of BNC connectors mounted directly on the PCB. The idea is that you could strap it to your wrist, walk up to any piece of timing-sensitive equipment, and plug in a genuinely GPS-accurate reference signal — no rack, no mains power, no lab required. LED indicators show GPS lock status and PPS presence. There is no display. There is no way to read the time off this watch.

How They Did It

The design is deliberately minimal: no microcontroller, no firmware. The GPS module’s hardware TIMEPULSE pin drives the LEDs and BNC outputs directly through a pair of 74HC125 quad logic buffers. A 74AHC1G86 XOR gate acts as a phase comparator, letting the watch accept an external PPS reference and compare its phase offset against the GPS-derived signal. The whole thing runs off a CR2450 coin cell. It’s a lab instrument, shrunk down to fit a 22mm watch band and a 3D-printed case.

Jeff Geerling — who stops by the shop in the accompanying video and solders his first surface-mount board — is listed as a collaborator on the project, which is fully open-source: KiCad schematic and PCB files, Fusion 360 mechanical files, and a complete BOM are all on GitHub.

Why It’s Worth Your Time

The joke is the point, but so is the engineering. Stripping a timing reference down to pure hardware logic — no processor, no software, no abstraction layer between the GPS module and the output pin — is a genuine design philosophy, not just a gimmick. And producing a wearable-grade form factor around it, with a proper enclosure and a watch strap, takes it from bench instrument to wearable conversation piece.

Go See It

Sean Hodgins documented the full build in a YouTube video, and the hardware and CAD files are on GitHub under the Idle Hands Dev banner.