Precision Clock

Is there anything better than having a big ISO timestamp on your wall?

The Precision Clock is the most precise wall clock you can get. The time is automatically synchronized to the GPS satellites and the display reads out in units of 0.01 seconds. So long as there is GPS reception, the time will always be exactly right, and the display precision is stable enough to be used as a reference for high-speed photography.

There is an ambient light sensor on the clock, and at night the display dims to a faint red glow. There is an adjustment dial on the back to set the maximum brightness.

Size

Each digit is 2.5cm (1 inch) tall, and the entire clock is 48cm (19 inches) wide, 3.4cm (1.34 inches) tall and about 2.5cm (1 inch) deep.

Accuracy

There is no interface on the clock. Simply power it up, and the GPS module will search for satellites to give it the current time.

The time is only accurate when the module is emitting its PPS signal, which will only happen if it has at least three satellites in view. There is an LED wired directly to the PPS line on the back (this is slightly redundant now, as the current GPS module has an LED onboard too). The firmware is set to blink the colons only when the time is accurate. If the colons are not blinking, the time is not guaranteed to be correct.

GPS works by having very accurate clocks on each satellite. The PPS signal is reportedly accurate to within 50 nanoseconds. However, with the overhead of the serially-addressed display drivers, expect the display to be accurate to within about a millisecond.

Daylight saving and timezones

Rules for daylight saving are hard-coded into the clock, along with the assigned timezone. Since there is no interface, in order to change timezone, a programming cable is required. The timezone is also shown on the little window in the middle of the display. When you order this clock kit, please state what timezone you want the clock to be in, and I will set the timezone on the chip and laser-etch the display to match.

Daylight saving is supported for most places, but the rules can be quite complex (particularly in places like Egypt). There is a jumper on the back of the clock which can be used either to disable daylight saving entirely, or if the chip is configured for it, the jumper can used as a simple setting to add one hour to the current timezone, which allows daylight saving to be implemented manually.

Power-up

The clock has a standard 5.5/2.1mm barrel jack for a 5V power source. The kit comes with a USB-to-barrel-jack cable so it can be powered from a USB port (or even a USB power bank if you like).

From cold, the GPS module will take a while to find a signal for the first time. Depending on the local signal strength this can take up to few hours.

There is a supercap on the GPS module which helps it remember the time, so that it doesn't need to do a cold boot every time it's powered up, but in practice the supercap goes flat in anything between a few minutes and a few days. It's possible to replace the supercap with a 3V coin cell, if you want the clock to keep time while powered off.

Assembly

The kit is entirely through-mount parts and is designed to be easy to solder.

In addition to the kit, you will need:

a soldering iron (I used a TS100, set to 320 Celsius)

solder

super glue or something similar for attaching the laser-cut parts

wire cutters

tweezers or pliers

The video at the top of this page gives an assembly tutorial. There are also more detailed, written instructions here.

Order

Update (September 2020): Current lead time on the clock kit is about three weeks.

Precision Clock Kit: £100 GBP

Postage and packing: Free (worldwide)

The laser-cut parts are made to order, so please specify the timezone you want the clock to operate in (E.g. London, New York, etc).

Timezone/Region:

For more information, check out the project page for the precision clock where I explain in detail how the kit was developed.