First introduced to the market in 1991, the Casio F-91W watch has become one of the most iconic watch designs known to man. Coming in at $10.89 a piece, these watches are an iconic symbol of the 90s, a decade that most people claim was either the epitome of civilization or the nadir of the twentieth century. While in Cambridge, MA at Massachusetts Institute of Technology over the weekend, I learned more about this beautiful watch than I ever thought possible.
This timepiece is quite impressive. Casio is so confident that their watches will never run out of battery life that they ship them to you already on. If that display of certainty isn’t impressive enough, 28 of the nefarious Guantanamo Bay’s captives have been captured wearing or possessing one of these watches. While I’m partial to the idea of Al-Qaeda wanting to be stylish and coordinate the accessories of the outfits, terrorism experts have attributed these watches to multiple IEDs and bombs that have mutilated and killed American citizens. I was curious why this watch was the unofficial face of bombs’ timing circuits but a quick look under the hood made it very clear. When we took it apart, we realized that the watch’s alarm function works very simply — the watch has a buzzer inside that shakes very quickly during the alarm’s beeps. When the buzzer shakes, the very large positive terminal connects with the negative terminal, causing the voltage across where they meet to oscillate.
Anyone with rudimentary soldering skills could solder to the watch’s massive plates, which is precisely what we did. However, the charge oscillated instead of staying constant, something that doesn’t sound as cool as the later. Hence, we built a basic circuit that could hold a charge using a a Set-Reset Latch. SR latches are very basic and can be made from two NOR gates. To make a SR latch, The output of the first NOR gate is directed into the input of the second NOR gate and the output of the second NOR gate is directed into the input of the first NOR gate. Depending upon the orientation of the gates, Q and Q-bar are interchangeable.
Using a SR latch requires a separate source of power so what we’re doing is different than rectification, we’re simply triggering something to stay on whenever the alarm goes off. After amplifying the three-volt charge using three MOSFETs we had about nine volts of power, enough to power a basic nine-volt motor. After that, you could do anything you’d like with the charge. To reset the SR latch, all that is needed to be done is simply sending a high value to the reset line of the NOR gates.
After we built this circuit, I looked online to try and find anyone that may have tried to do this. I found the image below, taken from a raid in the early 2000s.