You will find that having the armature balanced is much more critical with horizontal mounting than with a vertical suspension system. Actually my armature is out of balance due to one of the solar cells shifting off center after the motor was finished. I didn't do any balancing on it for a test run and found that the out of balance doesn't really affect the running of the motor. The worse that can happen with my motor is that the bottom of the armature swings around in a small range of low RPM. On a levitated motor the out of balance can cause the armature to bounce out of the levitating magnetic field.  Also, a low power motor may not have enough power to speed up beyond the out of balance low RPM resonance speed.

Using field magnets on either side of the armature reduces the out of balance swinging to a minimum and this motor easily accelerates through the out of balance RPMs. So keep that in mind when deciding which suspension system to use. Or try vertical and then horizontal levitation with the same armature.

If you plan on building this motor please read this article several times until you understand what needs to be done.  Once the solar cells are mounted it is very hard to remove them from the armature.

The motor takes very little light to run. I have the motor placed near a north facing window. With the curtain closed each night the motor is sometimes already running in the morning at a very slow speed due to the light leakage around the curtain. The motor normally still runs 5 minutes after the published "sun set" times while inside the house.

The design of the motor is a stationary magnetic field created by magnets attached to the side of the 2 liter soda bottle and a styrofoam armature with four solar cells and two coils wound on the armature. The timing of  the current flow through each coil is done by pair of two opposing solar cells that are mounted on the armature. The two coils and their connected solar cells are mounted 90° from the other set of solar cells and their connected coil.

Each pair of solar cells are electrically connected "back to back". That is; the positive terminal of each cell is connected to the negative terminal of the opposite solar cell of the pair. Each of the two solar cell pairs and their coils are not electrically connected to the other cells or coil.

As the armature rotates the voltage polarity applied to one connected coil is reversed every 180° of rotation. This reverses the direction of the magnetic field generated by the switched coil. The fixed external magnetic field surrounding the armature repulses the magnetic field generated by the coils. This repulsive force causes the armature to rotate.

The direction of rotation can be reversed by simply rotating the bottle ~180°. Rotating the bottle reverses the external magnetic field polarity in relation to the direction of sunlight striking the solar cell in use as the armature rotates.

The CD is used to have a flat surface for the bottle to sit on. The bubble plastic foam is used as a shock absorber in case the armature drops away from the suspension magnets. The thin solar cells are very brittle and it doesn't take much to break them.

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The idea is to have just enough magnetic attraction between the small bearing and the ball point pen to keep the armature attached to the small ball bearing. This will reduce the mechanical friction between the ball point pen and the small bearing.  A little Marvel Mystery oil is used to lubricate the ball point pen ball bearing.

By using the ball bearings, the armature can hang vertical even if the bottle is tilted.  If a flat disk magnet were used as the contact point for the ball point pen and the the magnet was not perfectly horizontal while running, the pen would tend to walk off towards the lower side of the magnet and eventually it would jamb on the side of the disk magnet which would stop the motor from running.

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Here is how to actually build the motor.

Since the cells are very fragile I decided to not run the long jumper wires from each cell down to the end of the armature and back up to the opposite solar cell. Instead I wired the cells with the jumper wires going directly around the outside of the cells to shorten the length of the jumpers and perhaps provide a little protection to the cells. When handling the solar cells try to not get finger prints on them.

The jumpers should be pre-formed to pass over the adjacent cells without touching them before mounting them on the armature.  The following pictures will show the shape of the jumpers.

First problem:
The tab plating on the front and back of the solar cells is very thin and you can not use ordinary rosin flux cored solder to tin or solder a wire to a solar cell as you would ordinarily do. Perhaps solder with silver content will work OK but I don't have any.

Trying to use flux cored solder just coats the solar cell tab area with flux and for one reason or another the solder won't flow onto the tab. If you try rubbing the tab with the iron tip the tab plating gets ruined. I tried some pre tinned tab wire and it wouldn't stick either.

After thinking about why my solder wouldn't stick to the solar cells I realized that I have a very old 5 lb. roll of rosin cored tin-lead solder. New solder now days is usually made with water-soluble flux and doesn't use use lead (Pb).  My advice is to NOT use old tin-lead solder with rosin flux like I did!  But all my warnings about the cells being very fragile still hold.

I finally broke down and bought a proper solar cell flux pen through eBay which works fine.  It only takes the slightest drop of the stuff to allow you to solder the wires to the cell.  Don't apply too much of it because the heat of the iron causes the excess to form a very hard blob.  If you try to scratch or scrape  it off you will more than likely crack the solar cell.

My method of soldering jumpers to the solar cells:
If you don't have the proper flux pen try this:  place the tinned tip of a piece of ~2" long #28 or thinner magnet jumper wire on the rubbing alcohol cleaned cell tab and then apply a bit of solder to the soldering iron tip; wait until the smoke is almost gone (leaving a minimum amount of flux on the tip of the soldering iron) and then quickly place the iron on the end of the jumper wire and with luck the solder would flow onto the wire which in turn would transfer a bit of solder to the tab.  It ends up looking like a cold solder joint but it does hold.  But the solder flux pen is the way to go.

In trying to figure out the sequence of mounting the cells and attaching the coil wires I came up with the following method. Attach one of of the jumper wires to the positive tab point (on the back of the solar cell) near the edge of each cell. Position two of the cell jumper wires so they aim slightly to the right of the short axis of two cells and two more jumper wires to aim slightly to the left on the other two cells. Next tin the negative tabs on the top of the solar cells at the edge of the cell opposite the positive connection wire. The solar cells will be mounted on the armature so that as you rotate the armature the jumper wires alternate angling from left to right as they come into view.

Once a cell is mounted on the armature you will not be able to get to the positive tab to do any soldering so make sure the tab wire is really soldered to the cell before mounting it.  I checked them by very slightly wiggling the free end of the wire from side to side.

Hint:
Since the silicon used to make a solar cell is a good heat sink you may need some sort of an insulator under the cell while soldering the tabbing wire. I used a piece of notched in the center of the long edge hard flat wood to lay the solar cells on while trying to solder the tab wire to them.  You need three hands to do this, one to hold the wire, one hand to hold the soldering iron and the last hand to keep the solar cell from moving. And a forth hand to apply the solder would help too.  Not having even the third hand I just placed a thick rubber eraser on each end of the solar cell.

Once you get the wire attached you have to be careful when you flip the solar cell over to tin the negative tab area so the blob of solder and jumper wire hangs over the side of the wood in the notch or you will certainly break the solar cell with any force you put on it.

Tin the negative tab area on the opposite side of the solar cell from where the tab wire is attached.  Place the cells aside for later use.

The sequence of how I built the armature is as follows;

1. Install axle:  After I cut the 1" square styrofoam armature out I drew cross lines on each end from one corner to the other to find the exact center at each end.  I then used an ice pick to make a hole down the length of the armature form so that the point of the pick came out at the center of the other end of the armature. I enlarged the hole with various diameter Phillips screw drivers until the ball point pen would fit snuggly into the armature. Do this slowly so you don't split the foam. Leave the pen in the foam from now on as a handle to hold the armature.
2. Make wire grooves:  I used a small soldering iron to make the wire grooves down the length of the armature.
3. Draw a marker line around the armature at the mid section of the styrofoam. Use your soldering iron to make a shallow pit in the styrofoam where the jumper wires will leave each positive cell connection so the wire will have a little clearance to the styrofoam.  If you don't do this the solar cell is likely to crack when you press it in place on the scotch tape.
   
4. Winding the coils: I wound the coils so the start/end end wires come out at one end of the armature and the other coil start/end wires come out the other end. I did this so it wouldn't get confusing which wire goes with which coil. Leave 2" to 3" of  wire leads on the coils to make the connections later.  After the first coil was wound I wrapped the extra wire around the pen and used a piece of scotch tape to hold it out of the way while winding the 2nd coil.
5. After both coils were wound I wrapped short lengths of the thin wire around the coil bundles to keep the wire bundle together in the grooves.  I used 4 wire ties on each long leg of the coils. The tie wires are not electrically connected to any part of the motor.
6. Pre bend the jumper wires so they will end up over the proper tinned negative tab point of the opposite solar cell before mounting the cells. 
7. Hold each solar cell in turn on the armature, cut each jumper so it can be soldered to the opposite cell's negative tab. Tin ~1.8" of the free end of the jumpers.
8. Apply tape to hold solar cells:  Rather than try to find a glue which wouldn't dissolve the foam I just wrapped some scotch tape around each end of the armature with the sticky side out to hold the solar cells in place. It would probably be better to use double side sticky tape to do this.
9. Carefully mount the solar cells.  Offset one pair of solar cells to the other pair about the width of the negative tab width.
10. Solder each free end of the jumpers to the tinned negative cell tab of the 180° offset solar cell.
11. Attach each of the solar cells to the sticky tape with the jumper wires aimed so that they are aligned Left-Right-Left-Right to the negative tab area of the cell they pass over. Use extreme care so you don't break a solar cell!  Once mounted it is almost impossible to remove a cell without breaking it.
12. Pick one of the solar cells to be S1A. This will automatically determine the part symbol numbers for the rest of the parts. It really doesn't matter which end of the pen you use as the reference direction. But do remember which cell is S1A for the next steps.
13. Position the coil wires so they can be soldered to the pre tinned negative area of the proper cell.  
14. Cut the start wire of coil L1 to length, tin it and solder it to the negative tab of S1A.
15. Cut the end wire of coil  L1 to length, tin it and solder it to the negative tab of S1B.  Trust me on this.  ;-)
16. At this point the motor should run as a one coil motor. I would test it to make sure you have this set of parts wired correctly before connecting the L2 wires. If the motor doesn't run check the trouble shooting paragraph for help.
17. Cut the start wire of coil L2 to length, tin it and solder it to the negative tab of S2A.
18. Cut the end wire  of coil L2 to length, tin it and solder it to the negative tab of S2B.

The motor should now start by itself and run much faster than when running a one coil motor,  Have fun with your new toy!  ;-)

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