Tell Time With A Reverse-Sundial Watch

[Xose Pérez] set out to make a sundial wristwatch by combining a magnetometer a small nylon bolt for the gnomon, but it doesn’t work like you’d think. Instead of using the magnetometer to point the sundial north, you angle the watch until the bolt’s shadow matches the white line on the PCB, and the ATmega328P computes the azimuth of the sun and determines the time thereby. To display the time he used one of those QDSP-6064 bubble displays, because sundials are retro.

His description of the project build includes a lot of fun anecdotes, like him attempting to solder the LCC connections of the HMC5883 magnetometer before giving up and making use of Seeedstudio’s PCBA service. He got 10 boards back with the ATmega and magnetometer populated while leaving the rest for [Xose] to fill in.

One fun detail of the project? You can’t tell what time it is without the sun, but you can’t read the bubble display in bright sunlight.

If you’re looking for more watch projects we’ve published, check out this wrist-controlled watch, the Chronio DIY watch, and this cool nixie-tube watch.

23 thoughts on “Tell Time With A Reverse-Sundial Watch

  1. “One fun detail of the project? You can’t tell what time it is without the sun, but you can’t read the bubble display in bright sunlight.”

    A corollary to “The Most Useless Machine” ?

    1. From the OP:

      There is a legend (they say a false myth) that tells that a long time ago people from Sant Pol restored an old sun dial that had been corrupted by the action of sun and rain. To prevent that from happening again they decided to cover the sundial with a roof top, effectively driving it useless. Since then people from neighbours town used to laught at people from Sant Pol saying “Sant pol quin’hora és?” (Sant Pol, what time is it?) whenever they happen to meet a local.

      Well, you know what? That bubble display is really cool but it’s also really hard to read outdoors. So after aligning the sun shadow on the white line and pressing the button you better find a dark place to actually read the time. You have 5 seconds to do so.

      1. At more extreme latitudes, a roof need not interfere with the sunlight, as The Sun will be lower on the horizon.
        Yeah, I know, it probably doesn’t apply to Sant Pol…

        1. The watch operates only when the button is pressed. GPS duty cycle would be very low. I haven’t designed GPS into any projects, but, considering GPS collars exist, I have to assume it would work. Of course battery life will be reduced to some degree.

          1. A significant degree, seeing as you’d have to hold the button down for 5-10 minutes each time in order to get a new fix on the GPS satellites and sync the GPS receiver time to the different satellite signals time down to a tiny fraction of a second.
            But after holding the button for awhile then yes, you would know where you are and what time it is, and can discard that highly accurate time information and pass just the location information to the magnetometer to adjust to true north to determine the time based on the inaccurate and gross reading of your heading and…

            Hmm, maybe I’m just confused on your modifications goals.
            You could have simplified the setup by wearing a $5 quartz crystal watch.
            You could have complicated the setup by putting a wrist strap on a 200 foot wide Foucault pendulum clock.
            In either case I’m sure one of us is just misunderstanding some detail here.

          2. Think GPS has time included in it’s message so than the whole sundial idea would be redundant.

            “. The Navigation Message provides all the necessary information to allow the user to perform the positioning service. It includes the Ephemeris parameters, needed to compute the satellite coordinates with enough accuracy, the Time parameters and Clock Corrections, to compute satellite clock offsets and time conversions,”


    1. The OP knows where he is — I assume it’s hard-coded in. Next you’ll want it to take account of timezones!

      But yeah, this is a great opportunity for him (and us!) to learn more about what makes sundials go. Heck, maybe that would be a good Hackaday piece…

  2. It should have just had a light detector at the bottom of a black lined tube, then once the light level peaks the angle of the sun is found. Tubes can run along the edge of the PCB board so you hold it up to the sun, or if using a small mirror/prism at a more convenient angle to the sun. Have a pair of tubes so you can use one for sighting, then the entire thing is more compact, when the tube end lights up you know you are aimed at the sun. The relative length of the tube to it’s width is important as longer means a more accurate reading, but you have to balance that with ease of use and the fact that the sun is not a point source.

    You could make a similar device using tiny crystals of Iceland spar so that it was still usable on overcast days. See “viking Sunstone”

    However the concept is still brilliant and it is a very neat conceptual hack.

    1. That would also alleviate the problem of reading the display in the sunlight. If you have to hold the PCB at 90° to the sun you only need a small barrier to shade the light off to the display.

      But I would instead add a watch crystal. :-)

      1. Why would I want to steal somebody else’s glory, the core idea is very original and not mine. You can stand on another’s shoulders or a 1000 hands can help lift another even further, think about that for a while.

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