Intro: Arduino Solar Shield - a DIY Solar Source for Your Projects Without Waiting for PCBs
Myself and the other Solar Pocketeers* are actually waiting for a new set of PCBs for an advanced version of this solar shield to arrive on Wednesday Sept 5 (in 4 days), but being impatient, here is a version of a direct-power solar shield that doesn't require a PCB. You read that right -- PCB-free shields! Any rigid backing -- a thin piece of acrylic, a breadboard, an old empty circuitboard, a discarded Muji plastic pencil holder, even-- any of these will work as the backing for the shield. Some drill holes, a bit of copper tape, and some supergluing and you should be on your way to a lovely solar shield that is sure to win you lots of friends. And a minor amount of lost fingerprint fidelity.
I've included a basic set of templates for cutting the backing and the copper tape (for the two electrical connections you will need to make between the pins of the shield and the positive and negative of the series-connected solar cells (also known as "solettes", since they are tiny and cute lasercut solar shards)). Those two items and a supply of solettes and you can be off to the solar races.
Time to complete: an hour
A wee bit of soldering required
Difficulty level: Pretty easy, but you will likely break 4-5 solettes at first. It requires a bit of a delicate touch.
Usefulness: To be determined
*First, I promise to never use the term "Solar Pocketeers" again -- but better me than someone else. Second, this is the 6th entry in the Solar Pocket Series of projects that Alex (this guy) and I are posting over the coming weeks. For more about Solar Pocket Factories and DIY solar check out our in-progress Kickstarter campaign (Aug 15 - Sept 14): http://www.kickstarter.com/projects/alex9000/the-solar-pocket-factory-an-invention-adventure/posts/303802
Here's what it's all about in just over a minute:
Step 1: What You Need
++ Soldering iron and solder for just a couple joints.
++ A drill press with a ~1.5mm diameter bit
++ A rigid backing of some sort; I used an old non-functional circuitboard, but anything a few mm thick and rigid will do.
++ Copper tape (single-sided adhesive): This is easy to find at most hardware or electronics shops.
++ Printed templates (next step)
++ 11 solettes: these are small pieces of monocrystalline or polycrystalline PV silicon that are typically hidden under an epoxy blob in off-the-shelf panels. You need the raw stuff for this instructable. Available a few places on eBay or via our Kickstarter page here: http://www.kickstarter.com/projects/alex9000/the-solar-pocket-factory-an-invention-adventure
++ High temperature superglue aka cyano: the thin stuff, like crazy glue. The gels don't work well. The off-the-shelf super glues will work, but for a more reliable panel you will need high temperature cyano, since most basic cyanos breakdown at 80 degrees C (which is a bit on the threshold of what your panel could experience in a bottle sitting on your porch). I used Aron Alpha 401XZ, which gets to 120C.
++ Four headers, available at many online arduino-friendly shops. Like, here:
++ Encapsulant: This is not necessary if you are just experimenting, but if you are going to put your solar shield outside or plan on using it for a number of projects, you should encapsulate the panel. Bartop or 5-minute epoxy will work for this, but better encapsulants are PU designed for doming and high-end solar applications (like solar cars); and another great method is EVA laminate with glass.
Step 2: Print the Templates
These two templates are designed to help with the header drill holes and the copper tape placement. Print them out, and stick the Header Drill Pattern template on your rigid backing with some adhesive. They also should help with the solette (small pieces of PV polycrystalline silicon) placement in a few steps.
These templates are designed for solettes that are 52mm x 13mm. If you have solettes that are a different size, you will need to do a small amount of adjustment to the template.
Step 3: Drill the Header Holes and Cut the Copper Tape
Just drill straight through the paper template to get the header hole placement right.
To cut the copper tape, put it shiny side up, under the tape template, and use a sharp blade to slide out the two tape patterns.
Step 4: Add the Copper Tape to the Backing
The copper tape simply needs to connect the underside of the first solette (this is the (+) of the series arrangement of solettes) with the 5V pin, and then a second piece of tape connects the underside of the last solette (this is the (-) of the series arrangement) with one or two of the GND pins of your power header.
Step 5: Add the Analog and Power Headers
This is the trickiest part of the assembly. Since the solettes I used were too wide to fit between headers in a standard shield, I needed to bury the power and analog pins beneath the solettes.
To do this, I snipped the headers down a bit, to make sure the tops did not protrude from the upper surface of my backing. And then I slipped in two blobs of solder on the underside of the backing, at the 5V and GND pins. This turned out to be easier than I expected, as the solder wicks right along the copper tape nicely. I needed to reheat the solder a second time to get the headers properly aligned, but it worked out fine.
Then I completed the header affixing but adding a few dabs of superglue from the upper surface of the backing, and then just hit the surface with a file to make sure all was smooth. I also covered up the top surface over the header nubs with some electrical tape, to make sure there wouldn't be a stray connection with the bottom of any solettes.
Step 6: Superglue + Stack the Solettes
Ok, I lied. This is actually the tricky part.
This step is the key -- combine solettes in series with a superglue-shingling technique. Each solette, or any chunk of mono or polycrystalline PV silicon for that matter, outputs around 0.5 - 0.6VDC, which is not enough voltage to do very many useful things. So, we need to combine enough of these solettes together in series so that their voltage outputs add up. In the case of this Solar Shield, we need to get to between 3V - 5.5V. I shot for 5.5V, to ensure the shield could operate in lower light conditions.
This means we will need a minimum of 10 solettes in series (or, 5.0Vopen). The solettes in this instructable will output Im (or, the max current at the maximum power point of the cells - about max power point here: http://en.wikipedia.org/wiki/Maximum_power_point_tracking) of around 200 mA per solette. So, since we are combining the solettes in series, the voltages add up, but the current does not. Or, to put it another way, 10 of our solettes in series will output 5.0VDC and 200mA on a nice day in Land O' Lakes. Except we are shadowing quite a bit of the solettes with a shingle-connection technique, so the current will be lower.
Back to the solettes: The (+) output is the grey underbelly of the first solette in your shingled stack. The (-) output of the series connected shingled lineup can be accessed either at the bus bar or white silver ink runners on the blue top surface of the final solette in your stack, or by using a "false" solette that doesn't produce electricity but just serves to bring do the top surface connections to a solette underbelly. This is the easiest and cleanest approach, and is worth the sacrificial solette. So, ignore what I wrote in the paragraph above -- you need 11 solettes if you are using one as a basic conductor.
What to do:
A few dabs of superglue (again, regular Crazyglue will work, but breaks down at temperatures of 80C, so I recommend you use high temp cyanoacrylate available in the Solar Pocket Kits or online a number of places, including here), a few seconds of pressure, and you've got yourself a solar series connection! Repeat 11 times. Make sure to use this same connection technique with the two copper connections. If one of the connections fails (and I tested the voltage and short-circuit current after I placed each solettes to make sure there wasn't a faulty solette or connection), you can usually repair it with some soldering, particular if the failed connection is at the copper tape - solette junction. I had to make one such repair at the GND junction.
Step 7: Check the Output With a Multimeter Before Encapsulating
I used a roughly calibrated 500W halogen lamp for a quick test (believe it or not, every microsolar factory I've been to uses a halogen as the test light source for millions of panels - it's not terrible, just surprising). Noonday sun works too.
Step 8: Encapsulate
If you want to encapsulate your panel - which does make sense for the Solar Shield, even though I didn't do it for this instructable -- you can use an off-the-shelf 2-part 5-minute epoxy or UV resistant bartop -- just pour it on with your panel suspended on a couple pencils to allow drip-off, and let it set. Depending on the UV resistance of your epoxy, this will be pretty for perhaps 2-5 years before it yellows and output plummets. You can also use EVA + glass lamination (with a blowdryer and vacuum bag), and sophisticated doming polyurethanes. More on these options in a follow-on instructable.
Step 9: Plug It Into Your Arduino! Solar Shield On!
Stay tuned for a more sophisticated, albeit less DIY, version of the Solar Shield with LiPo charging functionality next week. And if you aren't that patient, make a solar shield with that old Muji tray today!