To simplify construction I used one of those solar yard lights available from OSH, Home Depot or Lowe's for about $5. These contain a simple circuit to recharge a battery along with a Joule thief type circuit and use that to drive a super bright LED after dark. Sundown is detected by a photocell located on the face of the device.
This circuit has many drawbacks but it eliminates the need to build a circuit or run down to Radio Shack, although obviously for a science fair project one would presumably build a simple joule thief/recharger circuit.
The design can be scaled up with additional cells wired in series or in parallel to increase power output and the output power can be fed to any suitable low voltage circuit. Do a web search for "solar powered light circuit" and substitute the appropriate al/air fuel cell array for the solar panel(s).
Now let's take a look at what we're going to need for this project.
Step 1: Bill of Materials
You're going to need some non-conductive tray containers. I used some carryout containers from a local rib joint but almost any tray may be used. Since there only two dinners I used two bases and one lid. The common glass 9X11 glass cooking trays would be perfect.
3 paper towels to act as the porous layer.
A box of carbon aquarium filters. I used the Fluval brand of carbon pad replacement filters available at PetSmart for about $4. These come 4 to a box, I used the "4 Plus" size. These are used as the carbon electrodes in our assembly and function in this role quite nicely.
Some salt to use as an electrolyte. Ordinary table salt is fine, you'll need about 5g to start with. That's approximately 1/2 TBSP. In a liter of water that should produce approximately a 5% solution. Feel free to vary electrolyte strength.
A solar yard light. (see picture). These are available at most hardware stores for about $4. This is a self contained solar powered light with a rechargeable battery. it works by using a joule thief type circuit to maximize energy stored in the battery, the difference being that this circuit uses a capacitor rather than a traditional toroid so it doesn't provide the power amplification normally associated with a joule thief. NOTE: In the joule thief circuit the toroid provides both power amplification and capacitance.
4 electrical leads with clips, these are available from Radio Shack in a package of 12. These are used to make all connections.
A scissors, ruler and ball point pen along with a small screwdriver for dissassembling the solar light.
Now let's get started...
Step 2: Assembling the Cells
Each cell contains a sandwich composed of an aluminum foil electrode, a porous non-conductive layer (we use paper towels but almost any material may be used) and a carbon electrode.
As noted in the bill of materials almost any non-conductive shallow container may be used, so I measured the bottom of my pans and cut an electrode from the foil, leaving a significant lead approximately 1" wide and several inches long (see picture). This is folded over the end of the pan and is used to secure the lead cable for the cell.
In my case the pans measured 6"x10" across the bottom so I chose to make the foil electrode 5" wide and 9" long with a 1" lead to be trimmed after assembly . Then I cut out the porous layer from the paper towels and made them 5.5"x9.5" long. This insures that the foil will never come into direct contact the carbon brush and short out the power supply.
Even though the lid was slightly larger than the two bottom pieces I chose to keep the same dimensions in all cells. With the same surface area and electrolyte concentration each cell should produce approximately the same output.
I found it easiest to layout the electrode on the foil with a ball point pen and ruler then carefully tear the foil along the edge of the ruler. Alternatively scissors can be used to cut out the final shape. Make sure the power lead is long enough to reach easily over the edge of the pan and bend over the lip to firmly secure the foil. If using the ruler method always place the ruler so that it protects the electrode portion during the tear.
Once the foil has been cut out assemble the cells by placing the aluminum foil in the bottom anchored to one edge by the foil lead which is gently formed around the lip. Place an insulating layer of paper towel on top of the foil so that it is completely covered. Place one of the Fluval carbon filters on top of the paper towel and the cell assembly is complete.
In a later step we will wire these cells in series and charge them using a saltwater electrolyte.
For now let's set these aside and turn out attention to solar powered yard light and its mysterious inner workings....
Step 3: The Power and Light Assembly
Be very, very gentle and do not tug on the wires. I am not going to attempt to explain the circuit here I will simply explain its function.
The light assembly contains a solar panel and a light detecting diode located on the face. When the sun is up the solar panel provides power which is used to recharge a battery located inside the head of the device. When the sun goes down the photodetector allows power to pass to the light. The light is actually driven primarily by the battery which is recharged by the solar panel
First take apart the light assembly by gently twisting off the base and then the lens. We will use the lens later to house the final light assembly. In the base where the light is located there are 3 small screws, remove these and gently pull the light assembly open ( see picture ).
I found corrosion inside the assembly ( see picture ) but when I cleaned off the leads and replaced the battery it seemed to work okay.
There are 4 leads coming off of the face, two lead to the small photodetector and two other lead to the solar cell. Generally the power leads will be red and black. Clip these leads close to the solar panel which we will not be using. Strip a short length off the end, be careful these are fine wires and its easy to pull out several strands along with the covering. Carefully cut the covering just enough so it slides easily off the wire.
On the other side you will see a small circuit board which is held in place by two plastic posts. GENTLY pop the circuit board off the posts being carefully not to pull on the wires leading to the light. The light is held in its socket by a small blob of thermoplastic glue. This can be carefully scraped off and will come off cleanly allowing the light to be easily removed from it's holder BY PUSHING DOWN ON THE TOP, NOT PULLING THE WIRES AT THE BOTTOM. If you have a hot glue gun you can also heat it up and soften the glue, allowing the light to be removed.
This technique may also allow you to easily remove the photodiode from the face. Then a battery holder from radio shack could be used to hold the batteries and a more finished assembly created.
Now we've extracted a usable charger and light circuit let's take a look at the final assembly...
Step 4: Final Assembly
The cells are wired in series, the yellow wires show the interconnect while the red and black leads represent the power taps. For more information wiring in series see my related instructable:
How to Wire Batteries in Series or in Parallel
The leads are clipped either directly to the carbon electrode or attached to the foil lead and secured to the lip of the cell.
The power taps are attached to the appropriate red/black leads on the power assembly ( see picture ).
At this stage the system is ready to be relocated to a sunny spot (for sundown detection) and charged with electrolyte to begin energy generation.
Step 5: Charging and Operating the Assembly
The power and light assembly needs to be balanced so that the face is exposed to the sunlight and the light positioned so that it is viewable. The light, circuit board and power leads can be CAREFULLY inserted back into the lens to provide a weather resistant housing. You can see this in the intro picture if you look closely, the light and circuit board are both inside the original lens housing. With some thought and a little soldering of the leads the whole assembly should fit completely back into the lens housing which can then be reattached to its original base and the power leads going out through the original hole in the base.
Once the cells are in place they are charged with a mild saline solution. I used a 5% solution ( 5g of salt in 1L of tap water) and got satisfactory results. I charged each cell with approximately 300mL of electrolyte and began to measure performance of the circuit. As you can see I rapidly got 1.3V but this dropped to about 800mV after a few hours.
The device will operate without maintenance for about a week or so. With maintenance it will operate as long as there is fuel (aluminum) and oxygen. Overall I found this assembly more reliable than the solar lights which generally lose power by about 2 or 3 A.M while the al air run reliably from sundown to sunup.
I found it best to circulate the electrolyte a couple of times a day and replace every two or three days. I replaced the foil once a week and got satisfactory results.
Enjoy and good luck.