Introduction: Electromagnetic Pendulum

About: Retired Lockheed Martin Electrical Engineer (BSEE Texas A&M University 1982). Love to design and build things. Craftsman, hunter, angler, pretty darn good cook, prolific consumer of beer and barbeque, aspiring…

Back in the late 1980’s I decided that I would like to build a clock entirely out of wood. At the time there was no internet so it was much more difficult to do research than it is today... though I did manage to cobble together a very crude wheel and pendulum escapement. Run time was limited and it was rather fiddly but it would click along for a few minutes before the weight would touch the floor. Also limited were my resources… tools, money, woodworking skills… which made working on the project rather frustrating. So, for the time, the wooden clock dream was abandoned. Fast forward 30 plus years. I'm retired now, I have a lot of really great tools, and my woodworking skills have improved dramatically. I also have access to computers, amazing computer aided design (CAD) software, and the internet. So the clock project is back on. I’ve decided to write about the process as I work my way through the design. Just seems like a fun thing to do.

Initially I wanted to build a clock that was driven by gravity and regulated by a pendulum. Recently, as I was randomly digging around on the internet, I came across a fellow on the island of Kauai who designs wooden clocks and other types of “kinetic art”. His name is Clayton Boyer. It was the discovery of Mr. Boyer’s clock designs that inspired me to continue my own clock project. One of his designs that fascinated me was called the “Toucan”. The walking escapement used on the clock resembled the bill of the bird with the same name. It was a fun clock to watch and the design was very whimsical but what ultimately caught my attention was how it was driven. There were no weights or springs. The pendulum seemed to magically swing to and fro with no loss of energy. The secret was an electromagnetic drive system hidden within the base of the clock and a magnet on the end of the pendulum. Being an electrical engineer I thought that this was really cool and I decided to figure out how this all worked and build my own version of Mr. Boyer’s Toucan. To be sure… I could have just purchased the plans for the clock since they were available for about $35 but where’s the fun in that?

After a little more digging around on the internet I found that the concept dated back to the early 1960’s with the Kundo Anniversary Clocks. They were powered by a dry cell battery and would run for a year or so before you had to change the battery (thus the name, I suppose). The simplicity of the drive circuit intrigued me. There were two coils (one wound on top of another), a germanium transistor, and a battery. That’s all! I love simple stuff that works and this couldn’t get much simpler. One of the coils is connected to the base input of the transistor and the other coil is in the output side of the transistor in series with the battery. The other piece of the puzzle was a magnet mounted on the end of a pendulum. As the pendulum swings by the coils the magnet induces a current within the coil driving the base of the transistor. This causes the transistor to turn on and current flows in the output circuit from the battery through the coil that is in series with it. There is also a transformer effect that causes more current to be induced in the input coil to the point where the transistor saturates. The maximum amount of current is now flowing in the output side of the transistor and the coil in that circuit is fully energized by the battery thus creating an electromagnet with the same polarity as the magnet in the pendulum. The timing is such that the magnetic field generated by the electromagnet repels the magnet in the pendulum as it swings by and gives it a little kick. Once the pendulum moves past the coils current stops flowing in the base of the transistor and it turns off. This process is repeated every time the pendulum swings by the coils… supplying the additional energy required to overcome the losses within the system and keeping everything in motion. Neat huh? What’s really great about this is that it consumes very little power and the battery will last a long time. Wooden clocks that are driven by springs or weights will only run for a day or so before they have to be rewound. They have their own appeal but winding the clock every day seemed like a pain to me. I still may build one of these someday (I love Arnfield escapements) but for now it’s going to be electronics instead of gravity.

So the first leg of this journey is to figure out how to build the electromagnetically impulsed pendulum as this will not only regulate the clock but also be the engine that drives it. Ultimately in addition to this tutorial on the pendulum I will publish a number of tutorials covering clockwork design in general, gear design, frame construction, and then put it all together to complete a working clock. So strap in... here we go with the design process for the pendulum...


The main component of the electromagnetically impulsed pendulum is the coil circuit. I used a 10d common nail (available at your average hardware store) as the ferrite core. The wiring for the coils is 35 AWG magnet wire. This is a very fine wire coated with a thin nonconductive material. A 2N4401 NPN bipolar junction transistor is used to control current flow through the circuit. Kapton tape covers the nail and the completed core but you can use pretty much any kind of tape. The end caps of the coil are 1/16 inch acrylic sheet as well as a cylindrical piece of oak to house the transistor and coil wiring. Various bits and pieces of scrap wood were used for the rest of the prototype assembly along with dowel rods in a number of diameters. I love working with dowel rods... it reminds me of one of my favorite childhood toys... Tinker Toys! I find they lend themselves quite well to prototype development. The power supply is a plug in wall module that converts AC 110 to 9 volts DC. Ultimately the clock will end up being battery powered but for now the plug in module is very convenient and consistant. Another key component is a neodymium magnet that is imbedded in the end of the pendulum. The magnet I used is 1/2 inch in diameter and a quarter inch thick.

Step 1: Coil Core Assembly

As I was doing my research for the coil I ran across a clock repair forum where one of the threads was discussing the details of the coil design. They had some great pictures that gave me the idea for how to conceal the transistor and associated wiring within the base of the coil. Another key detail was that they mentioned the coils containing 4000 turns. Wow, that sounded like alot and created a bit of a concern in the back of my mind of how reasonable it was going to be to wrap the coil but I pressed on nonetheless.

I thought about how big I wanted the finished coil to be and settled on an inch in diameter and an inch and a quarter long. I cut 1 inch diameter circles out of 1/16th inch acrylic sheet to use for the end caps and another 1 inch diameter disk from a 1/2 inch thick piece of oak for the base. I milled a quarter inch channel in the oak disk as well as drilling a 3/16 inch diameter hole to accomodate the transistor. I also drilled small holes to be able to route the wiring into the channel in the base. See the pictures for details. Initially I cut a section out of the bottom acrylic piece to make it easier to run the wires into the base. In retrospect, I should have just drilled small holes to match the ones in the base. But no big deal. Holes were also drilled in the acrylic pieces and the oak piece for a snug fit over the nail. Assembly was as follows: Place the un-notched acrylic disk onto the nail. Wrap a 1-1/4 inch piece of tape around the nail as shown and then add the notched acylic disk. I applied epoxy to the oak disk and then slid it onto the nail such that it was bonded to the acrylic disk.

Before I moved on to the coil wrapping process I did some quick and dirty calculations to get a rough idea of how big the finished wiring would be and the electrical resistance of the two coils. It appeared that I would be able to fit all of the wire onto my core assembly so I was happy.

Step 2: Coil Winding Jig

I decided that wrapping the wire around the core totally by hand would be a huge pain so inspired by Tinker Toy technology I cobbled together a jig out of dowels and scrap pieces of plywood and MDF. I found that I had to put a dab of hot glue on the oak disk of the coil core to hold it snugly in place. Otherwise there was a little too much friction in the assembly and the core wouldn't move when I turned the crank. So with a little more sanding to further reduce the friction and the dab of hot glue the jig was operational.

Step 3: Winding the Coils

The wire is a special type of wire called magnet wire. It is a very fine single strand wire that is coated with a thin insulative material. I used 35 AWG. It is very common and just like just about everything else you can get it from Amazon. I rescued the spool you see in the first picture from the trash at work after a lab clean out event. Not sure how old it is but it looks to have been purchased many decades ago. LOL.

We will be wrapping two coils, one on top of the other, over the nail in the core assembly. It is essential that both coils be wrapped in the same direction around the assembly... otherwise it won't work. Each coil will have approximately 4000 wraps around the nail. Now it is not that big of a deal if you don't end up with exactly 4000 turns on each coil so you don't need to sweat that detail but I did have a notepad that I used to keep track. It did take a few hours to complete the wrapping process but I just turned on a football game to watch so I didn't get bored. I could make about 50 turns around the nail every pass so I would make a couple passes to get a hundred wraps and make note of that on my note pad and kept going until I got to 4000 wraps.

Here's the process for wrapping: Start wrapping the inner coil by threading 2 or 3 inches of wire into the oak base piece. Label the end of this wire "1". Complete your 4000 wraps and make sure you end up back at the oak base end of the core. Cut the wire and leave about 2 or 3 inches of additional length so that you can thread that back into the oak base. Label this end "2". Start the outer coil the same way by threading 2 or 3 inches of wire into the oak base. Label this end "3". Make another 4000 turns, cut the wire, and thread the end into the base the same as before. Label this end "4". Pictures 4 and 5 show the final result of the wrapping process. Again... Make darn sure you wrap both the inner and the outer coils in the same direction!!!

Step 4: Completing the Circuit

As you can see in the schematic the circuit is extremely simple which makes this device so incredibly cool. I've seen similar projects that used processors instead... which to me is like using a sledge hammer to kill a fly. I don't mean to knock those types of projects but I'm just a real big fan of designs that get the job done with the lowest level of complexity.

In the second picture I was playing around with different routing strategies for the wiring. I probably made a bigger deal out of it than I should. There are only a couple of key points... just wire it like the schematic but since the power supply is going to be external to the coil assembly you need to have the wires that will connect to the power source sticking out the bottom of the assembly. In other words: The V+ wire goes to the collector of the transistor and the V- wire goes to the wire labeled "2" on your coil assembly. So bottom line your coil assembly will have a positive and a negative terminal. It's a good idea to label these as such when you're done so you don't forget which one is which. Ah... I almost forgot. You will need to use a piece of fine sandpaper to remove the insulative coating on the magnet wire before you solder it! For clarity on the schematic... "Lo" is the outer coil and "Li" is the inner coil and also take note that I've labeled the ends of the coil wires 1, 2, 3, and 4 to match how we did it when we wrapped the coils.

I did test the coil before I potted it with epoxy... good thing since I had made a mistake! Ha, I jinxed myself by talking about how simple everything was. So make sure you test your assembly before potting it.

To test the completed assembly I taped a rare earth magnet to a length of thread and dangled it just over the head of the nail in the coil. Then hook up power to the coil and swing the magnet past the nail head. It should take off on its own. There is a sweet spot for the distance between the magnet and the nail head. Too close and the motion is jerky... too far and it won't work.

The last picture shows the completed coil as well as the rare earth (neodymium) magnet that I used.

Step 5: Pendulum Components

Once I had a known good working design for the coil assembly I needed to build up a prototype pendulum so that I could assess its performance characteristics. I was most curious to find out how much power the device used and I also needed to know how great of an arc the pendulum would swing as this would affect how I proceeded with my clock design.

I packaged my coil assembly inside a little wooden box and added a switch and power connection. The box fit inside a cutout on the bottom of the base assembly shown in picture two. Everything was a friction fit so that I could make adjustments along the way to get optimum performance. I added a brass tube to the upright in picture 3 to help reduce the friction. I used a 10d nail for the pin to connect the pendulum to the upright piece. In picture 5 you can see the rare earth magnet in the end of the pendulum. I never found anything that said magnet polarity was important. It doesn't seem to matter.... which kind of bugs me because intuitively somehow I think it should. But I've never paid any attention to it and it always seems to work so I guess not. The last pic shows the 9 volt DC power source. The 1 amp current capacity is overkill... it doesn't need to be anywhere close to that as I found out later.

Step 6: Assembling the Pendulum

The base is a two inch thick chunk of pine. I wanted it to be heavy to keep the assembly from tipping over when the pendulum was swinging. Even though this was a prototype I still decided to dress it up a little bit and trimmed it out with thin pieces of red cedar. Couldn't help myself! :)

The coil module plugs into the bottom side of the base (picture 2) and the whole thing is flipped right side up (picture 3). The upright is inserted into the top of the base (picture 4). It is a friction fit. Insert the nail through the brass tube in the upright (picture 5). And finally press the pendulum onto the nail (final pic).

I adjusted the pendulum so that there was a slight gap between it and the base.

Step 7: Prototype Performance Results

By taking a look at the chart that I placed behind the working pendulum in the video you can see that the pendulum swings past the middle line but doesn't quite make it past the last line. This places the entire arc that the pendulum swings between 72 and 80 degrees... I'm estimating around 75 degrees. This is valuable information when it is time to design the walking escapement for the clock.

I also connected a current probe to the power line and monitored the current draw during operation. I was extremely pleased to find out that the average current draw was a little over 2 milli-amps!!! What's really cool about that is I will be able to make the clock battery powered. If I use C cell batteries I'll get over 5 months of run time before I have to change the batteries. Not too bad!

The reason that I'm excited about using batteries is that I don't want to have a power cable running to the clock giving away the secret of how it operates. I'll hide the batteries in the base of the clock. Plus I'll be able to put it anywhere.

Step 8: Coming Up Next...

As you can see I've been busy with the next steps of my clock design. I did get burned out on cutting the gear teeth. Oh my gosh is that a tedious process. If I ever decide to build a bunch of these clocks I believe that I will be investing in a nice CNC router!!!

So while taking a break from sawing out gear teeth I cut out the hands and started working on the clock frame. So far so good!

As I think ahead to the next instructable in this series I believe I'll talk about the process I went through to design and build the gears so stand by on that one.

See you then!


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