This solar impulse motor is based off the Solarbotics Sun Swinger Pendulum. See description of that project. Even though this motor could run with only one coil, this motor incorporates 3 coils, with a driver circuit for every coil. The reason for the extra coils is to make a faster, more powerful motor, and one that can operate in outdoor shade due to smoother operation at low speed.
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Step 1: Gathering Materials
Commonly available materials were used to keep cost down.
1 ea. perfboard snapped into 3 ea. 1-1/4 in x 3/4 in boards
3 ea. 2N2222 npn general purpose transistors
3 ea. 2N3906 pnp general purpose transistors
3 ea. leds (different colors)
3 ea. homemade flat coils 1" diameter using #26awg enamel coated wire
3 ea. 150K resistors
3 ea. 270K resistors
3 ea. 1000uF capacitors
1 ea. 3300uF capacitor
3 ea. BYV26E diode (but any fast switching type diode would work)
3 ea. miniature SPDT slide switch for on/off (optional)
6 ea. solar cells, 52mm x 52 mm (AliExpress, 50 for $10)
1 ea. 5-3/4 in x 7 in. x 1/8 in birch plywood mount for solar cells
4 ea. 1 in. x 6-1/2 in x 1/8 in birch plywood spacers for solar cells
1 ea. 1/4-20 x 2-1/2 in. screw and nut for coil winding
2 ea. fender washers, 3/16 in id x 1-1/2 in od for coil winding
2 ea. 1/4 in washers for coil winding
1 ea. craft box 8.3 in. X 8 in. X 1.7 in. (Michaels ARTminds 3357774, $3 on sale)
4 ea. miniature nuts and bolts to hold solar panel to box
2 ea. wood screws to hold motor to box
1 ea. 5-1/4 in 6 in x 5/8 in scrap redwood board for motor base (any wood will do)
1 ea. shaft and bearing salvaged from a hard drive sector arm (check the bearing turns smoothly)
2 ea. Neodymium Magnet 1/2 in dia. to 1 in. dia x 1/4 in. (1 in. dia is better, stronger is better)
2 ea. brass rods 3/32 in x 3-3/4 in
5 feet ultra flexible stranded silicone insulated #24 wire (makes it easier to work with)
1-1/8 in wood drill to partially recess coils in base
drill (for recessing coils in base and to wind coils)
vise to hold fender washer during tapping
Step 2: Assembly
Cut (or snap in a vise along a row of holes) the perfboard into 3 smaller boards (1-1/4 in x 3/4 in boards). Solder transistors and components to small perfboards and connect per the Solarbotics link here . They used 2 ea. 100K resistors for the bias of the first npn transistor, but I found that a 270K (top) and 150K (bottom) worked better for me. The bias adjustment is critical, because it sets the voltage at which the magnet is sensed. Too little voltage to the transistor base, and the magnet is not detected. To much voltage and the transistor is on all the time. If it doesn't work you might have to temporarily substitute a 200K potientiometer for the resistors, set it so it triggers the led and coil when a magnet of the right polarity comes quickly over the coil. Then remove the pot and read the resistance of each leg from the wiper terminal and use those values.
To make the coils, start by making the coil former. Hold a fender washer in a vise, and tap with a 1/4-20 tap. Keep the tap perpendicular to the washer so that when the washer is threaded onto the 1/4-20 screw, the washer doesn't wobble. When both washers are threaded, assemble the coil former by putting a standard 1/4 in washer on first, then screw the threaded fender washer on all the way. Thread the second fender washer on, but leave 1/4 in. between washers. This will set the width of the coil. Install another regular washer and the 1/4-20 nut. Wrap 6 in. of the wire around the outside of the coil former (on the screw), then tape to the outside of the washer to prevent the wire from slipping during the first few turns of putting wire between the washers. Lightly oil the inside of the fender washers and threads to make coil removal easier, and prevent glue (from a later step) from sticking. Wind the first few layers by hand, then use the drill to speed up the process of coil winding. Measure the coil width with calipers periodically, and stop at a coil diameter of 1 in., leaving 6 in. of wire free. Apply thick Super Glue and allow to dry, then remove the nut, next unscrew the fender washers and coil from the screw as a unit. Carefully separate the fender washers from the glued coil. Apply additional super glue to the top and bottom of the coil and let dry. Make 2 more coils, keeping the coil winding direction the same for all coils.
Make the rotor assembly by epoxying the 2 brass rods across either side of the bearing. To protect the bearing fro epoxy, carefully apply grease to the area above the bearing seals and shaft, but don't allow to get on the bearing where the rod will be glued. Better might be to fit a plywood piece with a hole to fit the bearing, and just extend the plywood out to support the bearings. Some magnets, like the ones I used, have a concave profile on one side that would allow a wood screw to secure it to plywood (I did not try this). Glue the magnets on, the radius from shaft center to magnet center was 3-1/4 in. Wear safety glasses while the motor is in operation, as the magnets could fly off at high speed an injure you or a bystander.
Drill a hole in the motor base for the rotor shaft, keeping it as perpendicular as possible. Mark the base along a 3-1/4 radius from the center using a compass, then use a protractor to mark spots for the coils 120 degrees apart. Drill the 3 ea. 1-1/8 holes for recessing the coils about 1/4 in deep, and drill an additional 3/16 in. hole all the way through for the coil wires. After marking the coil wires as to which is the center wire versus which is the outer wire, insert the coil wires through the base. Insert a short section of 3/16 in dowel alongside the wire to keep the coils centered in the recess of the base. Apply hot melt glue to keep the coils secured.
Solder the solar cells in series as shown in the pictures using super flexible wire, being careful not to chip the fragile uncoated solar cells (you could just buy a fully completed solar cell of 3 to 5 volts at 100 mA). Test the solar cell output, should be a bit more than 3.5 volts open circuit. Mark the positive and negative leads. Epoxy the 4 plywood spacers to the plywood solar cell base, leaving room so the wires in the middle of the cell will be between spacers so the cells will lay flat. Epoxy the cells to the spacers.
Position the completed solar cell panel inside the box, and secure with miniature screws and nuts. Solder the coil wires to the electronics boards, connect to the solar cell power supply in full sun, and spin the rotor. If the led does not flash as the rotor spins across the coil, reverse the coil leads to the electronics and try again. If all works do the same connections for the other 2 coils. Secure the motor base into the other half of the box with 2 wood screws. Mount the electronics on a bit of wood or foam core after connecting each power supply in parallel to the other (with individual on/off switches if desired). The original Solarbotics circuit calls for a 3300uF capacitor for each board, but I found that wasn't necessary, so I installed only one 3300uF cap for all three circuits (upright cap in the picture near the hinge). Carefully inspect the clearance upon closing the box so that the solar cells don't touch the rotor or other components in the other half of the box.
Step 3: Testing and Results
This was a fun project. I found that the motor spins equally well either direction. Starting required a brisk spin, probably indicating that my choice of resistors was not ideal. A potientiometer instead of resistors would allow more low speed control; my motor turns a minimum of 800 rpm and a maximum of 3000 rpm. The motor is very smooth and operates well in patio shade throughout the day, at about 1000 rpm. The switches I installed for powering down each coil electronics was interesting, it allows slower speed operation in bright sun. In full sun with all 3 coils working, the motor provides considerable torque at around 2000 rpm. The craft box makes a nice enclosure, and protects it from damage during storage.
Things to try in the future: possibly a motor speed control based on the old hit and miss gas engines of the early 20th century, with speed monitored by an Arduino and coils cut in or out to maintain a set speed.
Another idea would be to store the excess electricity generated during the day to operate the motor at night, with the box lid closed even!