Ok, I finally got my hands on Gakken kit #28 - the Edo-era lunar clock (2900 yen). At least, I think it's "lunar". The clock face represents the 12 lunar animals from the Chinese calendar, but at one point, Japan did break the hours up to "hour of the snake", "hour of the rabbit" and so on, so it's a little hard to tell (since I haven't sat down to translate the mook word for word). Anyway, I have it. Recommended construction time is 90 minutes, and it took me about that long to put it together, including the time needed to mess with the threads, and to take the face back off to put the leaf switch in.
If you built the Slow Clock (kit #8), then you know roughly what to expect. A clock like this has a drive shaft that is connected to a series of gears that ultimately cause the clock's hands to sweep around the clock face. For both #8 and #28, there is only one hand (an hour hand for the Slow Clock), which keeps the gearing system simple, but does make the fully constructed kit kind of boring. There has to be a way to control the timing of the gears, and this is where the pendulum comes in.
Both clocks have a rotating pendulum connected to the escapement - a sawtooth gear that advances the rest of the clock. Two plates on the pendulum shaft press against the sawtooth gear and only allow the sawtooth to advance once the pendulum swings far enough to one side or the other. The rotating speed of the pendulum can be adjusted by two threaded weights on the arms, which then fine tunes the timing of the clock.
What sets kit #28 apart from the Slow Clock is that while the Slow Clock got its energy from a hanging weight attached to a string wound around the drive shaft, #28 uses a wind-up spring. Depending on how high up from the floor the Slow Clock was, it would run maybe 8 hours until needing rewinding. The wind-up spring can run longer than that (at the moment, I'm trying to find out how much longer). What this means is that #28 doesn't need to be hung from a wall - you can set it on a table or on the floor if you like. Second, #28 has two rotating pendulum arms, but only 1 runs at a time. Inside the gear mechanism itself is a cam shaft. This cam shaft raises and lowers the pendulum arms so that only one arm is in line with the sawtooth gear at a time. I'm not exactly sure what this means in terms of the operation of the clock's hand. I do know that after 2 hours, the shorter pendulum was the only one moving, and that the clock hand had a little less than 2 hour's worth on the clock face. (Note: The cam shaft might need to be set manually, meaning you can choose which pendulum to use, but I still don't know the point of the taller one.)
There really aren't that many pieces to the clock, maybe 8 parts making up the main body, 5-6 gears and shafts, and another 10 pieces acting as holder plates and stuff. The spring mechanism, and a couple of the gears arrive fully assembled, simplifying assembly even further. Even so, there are a couple places where assembly is kind of tricky and if you're not paying close attention you may have to take things apart and start over. One such section is the central pendulum arm assembly. There are two plastic plates that hold the 2 pendulum arms. One pendulum arm is longer than the other, and each plate is mated to the appropriate arm. That is, the pendulum arm with the long metal rod goes in front of the clock, and is mounted on the longer plate. The pendulum arm with the short rod goes to the shorter plate at the back of the clock. Then, when you attach the threads to suspend the arms from the mounting plates, you need to have just enough slack to let you move the mounting plates both up and down in order to adjust the arms so that the jamming plates on the rods align with the teeth of the sawtooth gear. Keeping in mind that the cams on the cam shaft need to be moved so you can check the alignment of both pendulum arms. (You'll know what I'm talking about when you build the clock).
If you do have to take things apart to do troubleshooting, be careful. The plastic the screws bolt into has a tendency to strip. Don't remove the screws more than you have to.
The fully assembled kit is about 6" tall, and with a 3"x3" foot print. And it weighs a little less than a pound. The ticking is a bit noisy, so you probably won't want this in your room when you're trying to sleep. I've just started letting it run, so I haven't verified how long it can go until needing rewinding. And I'm not sure yet what the taller rotating pendulum controls yet (it might be related to activating the leaf switch). Still, it is cool that a clock like this that I've built does run right "out of the box", so to speak.
This brings me to the leaf switch. One of the advertising points for kit #28 is that there's a leaf switch that allows you to connect the Japanino microcontroller to the clock. Don't worry if you don't have the Japanino, this clock will work just fine without it. And, you don't need to screw in the leaf switch if you don't want it. Basically, the leaf switch is just two pieces of metal screwed onto a plastic mounting block, which is in turn screwed down to the inner body of the clock (you can attach the switch to the body at any time, although the mook suggests doing it right away when you start out). One of the gears has a little plastic tooth sticking up inside it that presses against the switch momentarily once per rotation. You can then use this change in switch status to tell the Japanino to do something (see below).
As always, the mook contains suggestions for modifying the kit. This time, one such mod is to mount a steel bell to the top of the clock, and to connect the Japanino to the leaf switch and to a small servo motor. The servo can drive a small metal bolt to hit the bell to make it chime once a day, or whatever you prefer. Other mods include using the clock to control a motor-driven car, and to put stickers on the face of the clock.
There's a strong nostalgia element to this mook. The first few pages are pictures from around Japan, then there's a quick overview of Edo-era (1600's to early 1800's) clocks, followed by an essay of what Edo life used to be like. There's also an article on Little Edo, a small town north of Tokyo that recreates life from that time (Edo is the original name of what is now Tokyo). Some of the other articles show different styles of antique clocks, celestial clock mechanisms, and even wind-up timers for flint-lock pistols. The original clock that kit #28 is based on did have the bell mounted on it. Other articles show celestial clocks (for predicting the locations of the sun, moon and the inner planets) from around the world, plus some ancient star maps. There's an overview of the effect of the speed of light and gravity on the perception of time, again, and a piece on the human body's own "time keeping" mechanisms (for controlling heart rate, sleep cycles and so on). The rest of the mook is on reader-built science kits (what looks like a home-made satellite and a marble machine), and there's a manga showing what happens when a body dies and decays (might be a bit too much for younger readers, and "sensitive" adults. However, if you watch CSI, this is nothing.)
Overall, this is a better kit than the Slow Clock, because it's quieter and runs longer between rewindings. It's cool to see a clock that you've built running correctly, and you can glue on LEDs and have them light based on a Japanino sketch if you like. At 2900 yen ($35 USD) Japanese price, the import price may be a bit high for what you're getting. Otherwise, recommended.
Next up, the 3-color light lamp, #29, (2800 yen) due out in October.