I bought this watch for $6.80, including the fancy plastic box, at a store that sold mostly makeup and toys, near our hotel in Loja. It's the standard digital watch — stopwatch, an alarm too quiet to wake you up, and keeps track of the day of the week and the date. It's a bit over 3cm in diameter and a bit under 1cm thick.
And it's water-resistant to 30 meters.
It's also damned tough. I stood on it (I weigh substantially over 100 kg) and threw it against concrete, with no effect. The water resistance comes by way of an oiled O-ring in the seal between the back plate (held on with four screws, which compress the O-ring) and the main part of the watch body. I had no success dissecting the button mechanisms to see how they avoid passing water.
It also has an electroluminescent backlight — a piece of white electroluminescent foil behind the (transparent) LCD, which glows blue-green for a few seconds when you press the "light" button. Much of the electronics within appear to be a high-frequency AC power supply for this backlight.
The crystal is thick plastic and slightly convex; it is tightly bonded to the rest of the watch body.
All in all, this watch was by far the most bad-ass ruggedized device I examined during this project, which makes it that much sadder that it was the only device I wasn't able to put back together successfully. The contacts for the watch buttons had to be compressed to reinsert the circuit board into the watch, rather like piston rings — just smaller and more fragile. I took advantage of the opportunity to dig a little bit deeper and crack the epoxy off the chip within, but I wasn't able to see anything interesting on the chip — I couldn't get the epoxy off the front side of the chip.
As with most of the other devices I investigated (Brick Game, the little watch, and the other Brick Game, and possibly the FM radio, but not the calculators (good, crappy)), the LCD display was
connected to pads on the circuit board with zebra strip. The backlight
was connecte to other pads with a similar material, but without the
zebra stripes — EL foil only needs two terminals to light.
The black housing holds the watch battery. The larger SMT components are transistors; the red-and-black through-hole component with axial leads is a (rather large) diode; I assume the two through-hole components with radial leads are capacitors; I don't know what the other SMT components are. The 32.768kHz quartz crystal is hidden under a piece of white foam tape.
I never did find the piezoelectric speaker, and I apparently broke it the first time I took the watch apart — along with, apparently, the circuitry that controls the backlight.
The buttons press little posts inward against bits of springy metal, which are screwed to the edge of the circuit board, and bend inward into little gaps there to make contact with metal plating. This doesn't give a very responsive feel to the buttons, and it was really hard to put back together — I had to compress all four springs at once with my fingernails while reinserting the watch into its housing, after making sure that all four buttons on the housing were all the way out.
The two or three millimeters of depth occupied by the battery housing are otherwise more or less unoccupied, leaving a lot of extra space for packing more electronics into this form factor; also, as discussed later, the chip could do a lot more without being any bigger.
Six tiny screws held the main circuit board onto a plastic frame, which squished the display against the circuit board. Removing two of these screws suffices to replace the battery (an LR1130 button-cell, presumably 3V).
I had worried that my #00 Phillips screwdriver would be too large for these screws, but it was fine, other than shedding little bits of sharp metal from the handle's lousy chrome plating. (It was part of a no-brand set of eleven small screwdrivers that I bought for $1.)
This photo, like some later photos, has 5mm graph paper as a background, which helps to judge the size of the components. The display under the plastic frame contained seven parts: the transparent LCD display itself, the EL foil, two pieces of zebra strip connecting the display to its 27 contacts on the PCB, two conductive bits connecting the EL foil to its two contacts on the board, and a plastic spacer in between. 27 contacts would be enough to operate an 182-pixel display, so it's somewhat disappointing that the display only has 51 elements: six 7-segment digits, seven days, an alarm indicator, and a colon.
I really screwed up the lighting on this photo, and didn't notice until it was too late to fix it.
The chip is about 2.5mm square, which was pretty much the smallest size you can get fabricated through MOSIS, last time I checked. It was hidden under a blob of black epoxy on the main board; since I had already broken the backlight and the speaker, and was not having any success reassembling the display's parts with tweezers, I decided to go the rest of the way and chip the epoxy off the chip, necessarily rendering the board unusable. I never did succeed in getting the epoxy off the front side of the chip. I remember Jeri Ellsworth recommending fuming nitric acid for opening up plastic chip casings; I wonder if that would have worked on the epoxy?
2.5 mm square is really quite large in terms of number of components. By my guesstimate, the watch's current behavior needs only about 64 bits of memory, and the state machine that controls its behavior has a little over ten states, so you could implement the whole watch in a few hundred to a few thousand transistors. But 2.5 mm is about 40 000 lambdas with an 0.060 micron process, so you have room (I think — I really don't know much about this stuff) for tens of millions of transistors, four orders of magnitude more than they're using. The dawn of the PC age has several popular 8-bit CPUs under 10 000 transistors. In the last few years, one of the Motes researchers at Berkeley fabricated, through MOSIS, a 2.5mm-square Mote, including CPU, RF circuitry, and enough RAM to run their sensor network software.
Even if you're using fifteen-year-old third-hand 1.5-micron machinery, that's 1600 lambdas square, enough for hundreds of thousands of transistors.
In short, if you can afford the cost of custom silicon (I'm guessing it currently costs at least $20K per design, even through MOSIS) I think you can fit dramatically more intelligence into this size of device than the watch has, at (again, I'm guessing) no additional per-unit cost.
There's also a lot of unused space inside the case, so you could fit in a substantial number of additional discrete components (for, e.g., RF communication), although not nearly as many as in the CX-61 FM radio. I suspect that mass-market inexpensive computing devices will need to be nearly as tough as this watch to withstand extended use, under the hypothesis that people won't buy padded protective sheaths for them the way they do for $60 cellphones, and that many of the users will be children. From that point of view, it's good news that this tough case costs $6.80 at retail.