Kit 28: Edo-era Lunar Clock

Gakken kit #28 – the Edo-era lunar clock (2900 yen). The clock face represents the 12 animals from the old Edo-era timekeeping system (i.e. – “hour of the snake”, “hour of the rabbit” and so on). 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 up to 2 or 3 days. 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.

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″ x 3″ 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. 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.


The background:
According to multiple sources, the primary one being the Japanese Clock entry on wikipedia, early time keeping in Japan consisted of burning columns of incense, and marking off the time in units of 6. Numbering for time started at “9” at the top of the new incense column and burned down to “4” at the bottom (numbers 3, 2 and 1 were reserved for Buddhist calls to prayer). What was important at that time was the amount of daylight available for farming. So, a “day” started with sunrise (when the stars were no longer visible in the sky) and ended with sunset (when the first stars became visible again). This led to two different counting systems – the 6 time periods during “day” and the 6 during “night”.

Because the length of the day changes with the seasons (the longest “days” being in the Summer and the shortest in the winter), time keeping had to constantly be adjusted every few days to keep up with the actual occurrences of sunrise and sunset. As an example, on August 21st, 2010, sunrise in Tokyo occurred at 5:04 AM. Sunset at 6:24 PM. This gives a “day” of 800 minutes, and a “daytime hour” (800/6) of 133.3 minutes. Plus, a “night” of 640 minutes, with a “nighttime hour” that is 106.6 minutes long. Just 1 week later, on the 27th, sunrise was at 5:09 AM and sunset at 6:16 PM, with the “hours” changing to 131.2 and 108.8 minutes respectively.

“6” was at sunrise and then again at sunset, so a farmer would get up at “6” in the morning, work from “5” to “4” to “9” (when a new column of incense was started) and then back down to “6”. It would now be sunset and the end of the “day”. “Night” would run from “6” down to “4”, then jump back up to “9” again with new incense and work back down. In addition, the hours were given the names of the animals from the Chinese lunar calendar, with “6” being the “hour of the rabbit”, “5” being the “hour of the Dragon”, etc. (the others being snake, horse, ram and monkey).

Then, in the 1500’s, Dutch and Portuguese traders arrived in Japan, bringing with them the technology for western-style clocks. In 1641, Japan entered its isolationist phase, and Japanese clock makers were left on their own to come up with their own timepiece mechanisms. This led to the mechanization of the earlier incense-based time keeping system.

The What:
The Edo-era kit from Gakken has a clock face divided up into 12 equally-spaced units, running from 6 at our “9:00” position on the left, with 9 at “12:00”, and 6 again at “3:00”. The bottom half of the clock is the same, with 9 at the “6:00” position. The top half represents daytime, and the bottom half is night. The clock only has the “hour” hand, making timekeeping kind of approximate at best. In order to convert Edo time to modern western time, we need to know when sunrise and sunset are. If the clock is at roughly “day” 4.2 on August 21, then that would be 5:04 AM + (6 – 4 + 0.2) * 133.3 = 10:40 AM.

Again, the clock would need to be readjusted every 2 weeks or so to match up with sunrise and sunset as they changed with the seasons (this would usually be done after the clock ran down and needed rewinding).

The How:
Effectively, we need two clocks. One that is calibrated to advance at one rate from sunrise to sunset, and the second calibrated to advance from sunset to sunrise. For the design we’re interested in right now, we want to use a horizontally-rotating pendulum, meaning that our clock is going to have two horizontal pendulums. (Stand-mounted clocks were weight-driven; smaller designs for sitting on a shelf or table were wind-up spring-driven).

The trick then is to switch between the two pendulums. This is accomplished by adding a gear that advances whenever the hour hand hits “6”. This gear causes a cam rod to rotate 90 degrees. There are two cams on this rod. One cam raises its pendulum bar up out of the way of the escapement, while the second cam lowers its pendulum down into place.

(Side view of the clock showing the two horizontal pendulums. Underneath them you can see the cylindrical sawtooth gear. The cam ensures that only one pendulum engages the sawtooth gear at a time.)

(Closeup of the sawtooth gear with the “daytime” pendulum engaged.)

Only one pendulum interfaces with the escapement at any given time. You then adjust the time by turning the threaded weights on the two pendulums to make them rotate faster (weights closer to the center of rotation) or slower (farther from the center). The original Edo-era clocks used standardized weights at fixed locations on the pendulum arms.

The final enhancement is to add a gear mechanism to ring a bell “on the hour”, which on the Gakken kit is implemented by connecting the leaf switch to the Japanino (the Japanino then drives a servo which swings a brass rod at the bell).

I hope this is clear enough. For a better understanding of what the actual components look like, check out the Gakken site’s assembly instructions.


I’ve kind of been busy with work, the Garo magazine write-ups, and so on, so I haven’t spent all that much time on working with Gakken’s Edo-era clock (kit #28). However, there’s also been another complication that delayed me – the color LCD shield from Sparkfun.

I got this shield for my birthday, and it arrived without documentation. Although the Sparkfun site had some example code for running it, that code was written for the Arduino Mega, which has a lot more memory than the Japanino does. So, I had to rip out the Mandlebrot and raw RGB display routines, and slowly piece together what I needed and what I didn’t. Another complication is that there are two driver sets for the Nokia LCD panel – Epson and Phillips – and I had to figure out which driver I needed (Epson). Additionally, when I posted a request for help on both the Sparkfun and Arduino forums I got ignored, initially. Fortunately, I decided to check out the Analog Clock demo on the Fun with Microcontrollers (FwM) site and the sketch I found there gave me code for drawing lines, rectangles and text on the LCD shield. Naturally, the Analog clock circuit included a 1-wire real time clock, which I don’t have, so I had to pull that code out also.

Anyway, I got the barebones code to work for drawing the Edo analog clock on the LCD Shield, which I think looks pretty cool. This also gave me the code for writing debug text to the shield, which made life a lot easier from that point on, too. The next step was figuring out which pins were still available for connecting to the real Edo clock (I decided to double up on pins 3 and 5, which are assigned to 2 of the 3 pushbuttons on the LCD shield) and to find out just how bouncy the Edo clock leaf switch is. Actually, it’s really badly bouncy. It takes about 8 minutes for the connection to become steady. Meaning that I had to throw in some debounce delays in the Japanino sketch. Fortunately, the final code is just about 12K, and the Japanino has 14K of code space, which is fine for my needs.

Link to the video.

Gakken suggested mounting a bell on top of the Edo clock, and then using the Japanino to drive a servo with a brass bar tied to it to ring the bell when the leaf switch closes. I decided to be a little more silly. A year ago I got a birthday card with a built-in sound chip and speaker – the kind where when you open the card it plays “Tequila”. Turns out that you can run the sound chip off one of the Japanino output pins. Just pull the button batteries out of the card, and connect the card’s power pads to the Japanino output pin and ground. Write a “HIGH” to the appropriate pin and the card plays just fine. That meant that all I had to do was add a small delay to the sketch and then have it write a “LOW” to the pin when I wanted the music to stop. Alternatively, if you don’t want to drive yourself insane listening to Tequila all the time, you can replace the sound chip with an LED.

Side notes:

Initially, the cams didn’t seem to be rotating for me. Turns out that when I took the faceplate off the clock in order to install the leaf switch, the cam spring fell out and disappeared. After I discovered where the problem was, I replaced the lost spring with 1/2 of a spring from a ballpoint pen. Works fine.

There’s no particular reason to use the leaf switch supplied by Gakken. It’s really nothing more than 2 pieces of bent metal screwed down to a separator block. If you plan on connecting the Edo clock to an alarm circuit, you’re better off using a commercial roller-type microswitch and hot gluing it in place in the clock. That way you’ll avoid the problem of having to debounce the switch.

Since the Edo clock doesn’t have a fixed rate for sweeping out 1 “hour” of time, it’s not that important that the kit doesn’t have a minute hand. You’re looking at rough approximates for the current time of the day no matter what you do. Still, it’s a fun look at Japanese technology history.

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