Fig. 9 – The interior showing chopstick latch

The internal latch is built around a Hitec servo, three eyelet screws, and, as a sentimental salute to the little Chinese restaurant my friend and I used to eat at, a dowel made from one of its chopsticks. One chopstick end is attached to the servo arm, and the other threaded through the two eyelet screws mounted into the box bottom. The servo operates in one of two positions – “open” and “closed”. When instructed to close, it drives the chopstick laterally through a third eyelet screw hanging from the box lid. At this point, the box is latched. Once the puzzle is finally solved, the servo pulls the chopstick back, and the box can again be opened.

Software

The box is running the Arduino version 0016 framework. The sketch I wrote uses native libraries Serial, Servo, and LiquidCrystal, as well as several of my own: TinyGPS, NewSoftSerial, PString, Streaming, and Flash. My libraries are freely available for download at Arduiniana.org.

Conceptually, the software is fairly straightforward. At power up, the system hunts for a GPS signal. If it detects that the box is within 2 kilometers of the target spot — in this case the center of the little island — it unlocks the latch. Otherwise, it displays the somewhat cryptic “Distance: nnn km” and keeps the latch firmly locked.

How the software works

Fig. 10 – A fake voltage overload

The Back Door

It may seem risky to create a device that can only be activated a limited number of times before it locks permanently. What happens if the bride’s four-year-old gets hold of it? What if the batteries unexpectedly run out? What if, despite all my care and attention, there are still bugs in the implementation? Will the treasure inside be forever sealed? To address these concerns, I built a “back door” into the design. This is where the mysterious 2.5mm coaxial connector to the left in Figure 1 comes in.

If you apply some current here at a reasonable voltage, the system will power up just as if you pressed the button. But since this circuit bypasses the Pololu switch, when the box claims to be “Powering down”, it is actually lying. Instead, after a two-minute delay, the latch opens, regardless of where the box is actually located. This gives the sheepish designer (me) emergency access and one more chance to replace the batteries or repair the latest design flaw.

You might question whether it’s a good idea to leave such a gaping back door. After all, it’s a lot easier for my friend to open the box by simply plugging it in than driving it the 391 kilometers to Bréhat. Quite true. So to dissuade him from snooping too far down this path, I built in a few frightening messages. Remember, he has no idea how to use this “back door” or what voltage and current to apply. If he does figure out how to power the box this way, I feel fairly confident that whatever satisfaction he gains will rapidly disappear when the LCD starts flashing the completely fake warnings

Excess Voltage!
Remove Power!

And if these aren’t discouraging enough, they are followed by a blast of random “garbage”, suggesting that a critical failure is underway. (See Figure 10.) Whoever opens the box this way will have to stare at these ominous characters for a full two minutes.

Last minute disasters

Just a couple of days before I left for France I made a pretty serious mistake. At the time I had completed the physical construction and was just beginning the first full-scale closed-lid tests. Everything was going swimmingly – so smoothly, in fact, that I became a bit cavalier about my methodology. To address an intermittent problem, I made the last-minute decision to upgrade my system software. Reflecting back, I should have done plenty of preliminary “open box” tests before sealing it up; instead, riding the wave of my enthusiasm into disaster, I uploaded the new version, confidently closed the lid, and pressed the button. The latch engaged normally, but then the whole system went haywire. The motor began jerking crazily and the LCD limped along, displaying dimly only the top halves of each message. Soon I discovered that I had locked myself out of my own box and that disassembling it was the only way to recover. Because I had designed it to be somewhat tamper proof, this was far from easy. Ultimately, I had to break a bracket to get a screwdriver far enough into the interior to pry open the latch.

In France I had left myself just one short day to reassemble the box (which had been taken apart to increase my chances with airport security), run some last-second tests, and perform the final reset. When it came time to rearm it, something you do by connecting power to the “back door” for four minutes, I found that I had brought the wrong power supply with me. I fruitlessly combed the house I was staying in for a telephone or router with a 2.5mm connector, and then tried desperately to locate the French equivalent of a Radio Shack. After my panic subsided I realized I could just jimmy up what I needed with a couple of paper clips. It wasn’t pleasant or easy to hold those clips in place for four minutes, but I finally did manage to rearm the box with mere hours to go.

Frequently Asked Questions

What happened when they unwrapped the box?

The day after the wedding, the bride’s family hosted a brunch in the back lawn of their beautiful country house. I hadn’t expected to be present at the opening, but at one point I noticed some furtive activity in the corner of the lawn and saw that the box was being passed around, accompanied by much head scratching and gesturing in my direction. My friend, who had apparently already consumed three “attempts”, was darting back and forth exhorting everyone in halting French “Ne poussez pas le bouton!” (“Don’t push the button!”). To his dismay, his new father-in-law didn’t understand and pushed it anyway. Several people approached me to ask questions about the box, which I apologetically declined to answer. At one point a friend of the family, a charming gentleman from up the street, said to me with a thick accent, “You are — how you say in English? — a BASTARD!” That one comment made this whole project worthwhile. As of this writing I do not think they have yet managed to open the box.

What did you put inside?

With all the electronics, there wasn’t much room inside the box for anything too substantial. I put in a few local (US) gift cards to entice them to visit soon, a set of Kazuo Ishiguro audio books (on a USB key), and an overly sentimental card. And of course, as I pointed out in the card, if either of them fancies doing a little Arduino development, there’s a perfectly good Duemilanove to play with, not to mention an LCD, a servo, and a GPS.

How did you choose the target location?

Île-de-Bréhat, a charming island where motorized vehicles are prohibited, is a special place for this couple. It was while my friend was filming a movie here that he first began to realize his feelings for the young lady starring in it. And she for him, if I understand correctly. To help matters, the bride’s family owns a house here, making it a doubly attractive destination. (I may be a bastard, but I’m not going to make them travel 400 kilometers to some random, isolated location just to open a few gift cards.)

How did you get it through airport security?

I worried a lot about this. At first, when I was not certain whether I would be able get to France to attend the wedding, I had thought to present them the fully “armed” box while they were visiting the US. Fortunately, someone pointed out the foolishness of this idea. Can you imagine the interview at airport security when the hand soldered electronics show up in the X-ray?

Sir, did you pack this yourself?— No.

What’s inside?— I have no idea.

Can you open it please?— Nope.

Eventually, I decided that the best strategy was to disassemble it as much as practically possible and hope that none of the individual pieces were sufficiently suspicious to warrant being detonated on the tarmac. This seemed to work. I brought the disassembled box on two major trips without a hint of trouble.

Why limit the number of attempts to 50?

Two reasons. First, a puzzle is just more exciting if it is bounded. If it’s fun to solve a Sudoku, how much more fun is it to try and do it in under ten minutes?

The second reason is a practical one. I needed some way to make sure that power consumption would not end up being a problem. How disappointing would it be if, after all the work I put into it, the batteries ran out and the puzzle was ruined? Constraining the number of attempts helps ensure that this won’t be an issue.

The number 50 has no special significance. 25 or 20 would probably have worked just as well.

Why the 2000-meter tolerance?

There is always a little bit of error in a GPS reading, so some tolerance is necessary. The island is about 4 kilometers across, so setting the tolerance at 2000 meters forces them to actually ferry across to it without constraining them to any particular part.

Conclusion

The box is unwrapped

Building this project was a joy. It was rewarding to conceive of a relatively unique GPS application and to work through the various problems as they arose. Designing the latch, for example, was a challenge I spent a good deal of time on. My experience solving mechanical problems is fairly limited, but I’d like to think that the next time I build something with a mechanical component I’ll be able to approach it with a little more savvy. Meanwhile, when you think about all the cheap sensors, displays, lights, motors, modules, radios, and buttons you can attach to an Arduino, you realize that imagination is the only constraint on the kinds of projects that can be built. This little box is a novel way of combining a GPS, a servo, a button and a display. How many more marvelous combinations lurk out there, if only someone has the imagination to put them together the right way?

[Make sure you read the sequel to this story when you’re finished here.]