Computer Nerd Kev


Projects > Xmas Swing

The Christmas Swinger

What are you doing looking at still images? The whole point of this is that it swings! (4MB video).

One Christmas I attempted to build a levitating Christmas bauble which would hover under a circuit and electromagnet hung from a Christmas tree. In spite of determined attempts, the best that I could manage with my final device was a perceptibly delayed fall when the bauble was let go under the magnet. So when I happened across the "perpetual swing pendulum" page at Bowden's Hobby Circuits, and its corresponding link to the original project's article in the August 2012 issue of "Nuts and Volts" magazine, I could see the path to my Christmas bauble redemption.

Using electronic components that I had to hand, bits of Neodymium magnet salvaged from a dead computer Hard Disk for the levitator project, a cardboard Christmas "gift box", and some soft (easy to bend) fencing wire, the swinger came into being. I substituted the 2N4403 and 2N4401 transistors for Q1 and Q2 with models C8550 and C8050 which I had available.

Redrawn schematic including series resistor and and component substitutions.

I blame the failure of the levitator project (perhaps unfairly, as I also heavily modified the circuit) on the electromagnet which I wound with enamelled wire on a large bolt using a bench drill. I'd hoped that I'd be able to guide the wire back and forth as it wound its way onto the bolt like a proper wire winding machine. I failed repeatedly to get it right and ended up resigning myself to a hopeless mess of a winding. Either due to this, or the poor magnetic permeability of the bolt, I felt that the electromagnet just wasn't powerful enough to put up a fair fight against the Earth's gravity.

This time, again without the specified winding wire or core, I decided to use a different approach. I used a ferrite RF suppression loop designed to fit around a ribbon cable, which I got from an old flatbed scanner that I pulled apart, as a more magnetically permeable core than a regular steel bolt. A tooth pick was split in half and glued into each end of the gap for the ribbon cable, to hold the wire in place. This proved to be just strong enough to do the job. Finally I gave up on the easy route and wound the core by hand. It still ended up a bit of a mess, but not as bad as last time (hopefully fewer shorted turns). The wire used was approximately AWG 20, and very approximately 110ft (worked out highly inaccurately using the resistance reading on a standard multimeter and a chart of wire gauge ohms/1000ft, I soon lost count of the turns and just wound on as many as would fit).

The finished electromagnet made from enamelled wire, a bit of flatbed scanner, and a toothpick.

Cardboard gift boxes are a favourite of mine for housing electronics projects. They're insulated, much easier to cut/drill holes into than plastic or metal, and can be bought for cents if you look in the right places (Australian readers: have a look in your local branch of "The Reject Shop" a few weeks after Christmas for some especially cheap buys). For once, this time the decoration even looks right. The original project used threaded rods to form the supports for the swinger. I decided on a more striking design, described in the (good, to my standards) sketch below. Not shown is that the wires supporting the bauble are actually secured at screw terminals glued into the ends of the case. This allows (fairly) easy adjustment of their length by allowing the excess to hang out of the terminal.

Rough initial sketch. I never ended up putting the bumps into the wires to stop them moving up and down, and the solar panel was too big to fit on the case.

Internal layout, including otherwise unmentioned power switch top left.

Because the coil is not suspended by the frame as in the magazine design, it was secured by gluing inside a block of wood with a hole drilled through it, then stacking thick cardboard on top of it until the distance between the electromagnet and the top of the case when it was placed on top of this tower was minimal. At that point, the electromagnet was secured on top with cable ties threaded through the hole in the block of wood, with the cardboard spacers sandwiched in between. I did check that the electromagnet doesn't get too hot around all this flammable material.

The circuit was constructed on VeroBoard. I usually use VeroDes design software to design my layout, but I wanted to get this done in a day so, seeing as the circuit is very simple, I simply sketched it out on paper. The result is shown below, viewed from the top with "X" meaning broken track. Sorry if you can't make sense of it. Instead of putting the LED on the circuit board, I decided to be more adventurous and put it on the swinging bauble itself. To do this, I used a thin wire instead of string to hold the swinging bauble, and connected the screw terminals securing it to the LED's electrical connections. Then the 3mm LED was fitted in a hole in the cap at the top of the bauble (which simply pulled off) with its leads holding onto the wires looped around them. Even though the circuit relies on the LED to function, this proved to be a good enough electrical connection to work reliably, and the LED produces a clear flash every time it passes above the hidden electromagnet.

My quickly sketched VeroBoard layout. The tracks run horizontally along the holes (dots). Hopefully you can make something out from it.

My electromagnet read 1.1ohms on my multimeter, the specified one was apparently 32ohms. In fear that the standard circuit wouldn't be able to pump enough power into this low resistance, I doubled the value of C1 to 4400uF. Luckily there was plenty of room in the large case to fit a couple of axial electrolytic capacitors wired in parallel.

I had a solar panel, picked up somewhere or other, to use for power as in the original project. For testing though, I found that it could be powered from the regulated 5VDC output of my bench power supply using a 150ohm 1/4W resistor in series. Therefore I put this resistor permanently in series with the DC socket that also connects the solar panel. Now either the solar panel or a regulated 5VDC plug pack can be connected to power the swinger. Both work well, however in either case the auto-start function often doesn't seem to be able to start the bauble swinging, which means that under solar power it often gets stuck after the sun has passed under cloud for a time and the pendulum has slowed to a stop.

The result is finally a bauble-based Christmas novelty using the power of magnetics, all built in about a day. I also think the design is quite striking, and an improvement over other builds, but I'm yet to receive reinforcement of this appraisal from anyone else...


Content Copyright Kevin Koster 2018