Friday, August 27, 2010

GPS Puzzle Box

Last year the Reverse Geocache Puzzle was created by Mikal Hart. The general concept is that of a box that is locked and will only open when taken to a specific location. All of the implementations I have seen to date are based around using an LCD to show the game player how far they are from the final location. I thought I would try and come up with something a little more cryptic.

I've always been a fan of the 'Myst' series of games. There you are dropped in an environment where you have no idea of whats to be done and how things work. So with this in mind I decided to make a locking puzzle box with a similar amount of instruction for the player. None.

So instead of a LCD displaying distance to the final location I went for an analog needle without a scale;

Analog Output
(showing about 120 degrees)
Turning the box on resets the needle to zero and once a GPS signal has been established the needle is positioned. The analog nature of this made it more of a challenge to solve. For an initial version I made it relatively simple by setting the scale to be 1 degree equal to 1km. Setting the maximum range to 180km.

There are plenty of resources online about how to construct such a device. Mine was a Arduino Duemilanove, GPS shield and EM-406 GPS receiver, 2 servos (1 for the lock and 1 for the needle) and a simple switch. All powered by a 9V battery.

So I gave the box, obviously locked, to my wife Myrto with no instruction on how it worked. All she was told was that it was a GPS puzzle and that I have a love of Geocaching. I also told her not to leave it turned on when it wasn't in use to save battery. (I decided against a polulu switch as I wanted the industrial feel of a toggle switch)

A few hours driving around Athens and she came home with a set of readings from different locations;

Armed with this info and Google Earth it took her a good few hours to finally put it all together and work out that the readings were distance. (Setting the scale of 1 degree to 1km definitely made it easier). Using a KML circle generator (there are a few online, cant remember which one she used though) she loaded the data into Google Earth;

And zooming into the intersection;

Given the analog nature of the needle and the distance from the final location it wasn't possible to get an exact destination. Even without any errors due to parallax reading the needle the level of accuracy was only to within 1km. But even so, the possible band of destinations contained the mountain Parnasos and Delphi. Not enough to be sure, but enough to give it a go on a spare day as it was about 180km drive.

So off we went, I was designated driver, and was keeping shtum about the final destination. Myrto took a couple of reading as we went along to confirm that her theory about the game was correct and as the needle moved less and less all was well. For the final 5 km we just left the device turned on and watched as the needle slowly dropped down to zero about 500m before we entered Delphi. The servo whined and the box opened releasing the gift within. But the main gift was the challenge and the visit to the Temple of Apollo which followed;

Since then I've made a few more puzzles like this, which I'll post soon, but this was by far the most fun to build, plan and solve.


  1. This is a beautiful, beautiful piece of work, Richard. Thanks for sharing.

    My own designs were partially informed by Myst and Riven (and quite frankly, the various little machines in the Nancy Drew mystery games) too. As you have obviously discovered, there is no reason that these need to be forever confined to the digital world. Let's build them in this one! :)

    Congratulations. And thanks for sharing.

    I like that it has nearly no obviously electronic component. Along these lines, I envision a box that has two wooden dials with mysterious letters and numbers. Together these tell the distance. Perhaps on a geometric scale. Or perhaps they simply indicate direction.

    And what about a capacitive sensor switch to turn it on? You'd have to use the Pololu switch in this case, but then there would be no visible electronics whatsoever.

    Very nice. Thanks again!

    Mikal Hart

  2. OK, now I am quite curious how to build this little gem of frustration. I am a locksmith in Colorado and have a client who would love to get his hands on this.
    Not to mention I need one for myself. (anniversary gift) I am semi challenged electronically but creative in woodworking so I would love to get the components and build an incredible box to go with the theme.
    My email is locksmithjoe
    Please email

  3. Guys, this stuff is brilliant! I want to get one of these but designed in a steampunk theme. Mikal's idea of crytpic letting or numbers to tell distance is a great idea.

    I wish I could get a hold of you guys to talk more.

  4. What a great puzzlebox. I'm trying to make one myself but I'm not into programming that much ... is there a way to look into the code?
    Maybe you can mail me? Phreakyhamster at