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;
(showing about 120 degrees)
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;
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.