The MintyBoost and MightyMintyBoost (MMB) are both very cool instructables. I just had to build one and since I am into solar I wanted to go with the MMB project. I decided to use the larger solar cell for my MMB project because I would like to be able to charge my iPhone every day with my MMB and would like to go pure solar if I could. If you have an iPhone you know they are pretty power hungry. I typically need to top off my battery during the day, and recharge every night. The MMB with its large 2000 mAhr battery allows you to do that. It contains over 1 1/2 charges for the iPhone 3GS, which has a 1150 mAh battery. That's enough to get through all but the toughest days with both daily top-offs and an over-night full charge.

The smaller solar cell recommended in the MMB project is great because its nice and compact. But its small size limits the amount of power it can generate. It has an output of about 100mA, which would take about 20 full-sun hours to charge the MMBs 2000mAh battery. A sunny winter day produces about 4.5 full-sun hours and a summer day about 8.5. An average day year-round is about 5 full-sun hours, including cloudy weather. Those numbers depend on the local climate and the figures given are for my area, which is near San Francisco right on the coast. Therefore the small solar cell will take on average 4 days to fully charge the MMB battery. That is fine performance if you only intend to use it for topping off and will charge you iPhone from the grid over-night. But if you want to go pure solar like I do, you need a bit more juice from your solar cell

The big solar cell produces 310 mAh, which can fully charge the MMB battery in just 6.5 full-sun hours. Thats pretty good and should satisfy your iPhone's power appetite on a daily basis much of the year except maybe in the winter when you are playing lots of games or something. I was able to verify that charge time the first time I charged my brand new MightyMintyBoost. But to get that full on performance it helps to have the cell held at the optimal angle to the sun and to turn in occasionally so that it tracks the sun during the day. So I designed a support frame for my solar cell that has three different tilt angles to optimize the cell's angle for different times of the year.


Enough power generated to fully charge and top-off an iPhone 3GS on solar power alone.
Voltage regulated so you can use cells with more than 7 volts output.
Three tilt angles optimized for Winter, Equinox (spring and fall), and Summer sun angles.

Refer to the MightyMintyBoost Project for the construction of the MightyMintyBoost battery and charger part, which is the stuff in the Altoids tin. This project covers how to make a frame for and how to wire a larger solar cell.

Step 1: Tools and Materials

Just a reminder. This Instructable only talks about making the frame and installing the large solar cell. See MightyMintyBoost for instructions about how to make the rest of the project.

An Analemma (see second picture)
Soldering iron
Wire cutters
Wire Stripper
Hot melt glue gun
Wood saw (hand or power)
Miter box (if you use a hand saw)
Carpenter's square (if you are cutting by hand)
Drill motor
7/64" drill bit
Sand Paper

Large Solar Cell
Voltage Regulator, 5v
3/4 No. 8 wood screws (qty 8)
Hot melt glue
Rubber cement
Super Glue (optional)
Brush on electrical tape
Finished lumber 1x2 x 18 long (poplar is very nice, but any wood will do)
Birch Plywood, 1/4 thick, 2 pieces at least 5 x 4 each

The MightMintyBoost charger will accept up to 7 volts input, but this solar cell puts out 9 volts so it needs to be regulated down to keep from frying the charger. Because the cell produces smooth DC voltage, capacitors that are sometimes used with the 7805 voltage regulator are not required. The voltage regulator will only dissipate about 1.5 watts so a heat sink is not required.

I have both a cabinetmakers table saw and a crosscut saw, which makes this project super simple, quick and very accurate. However if you dont have all that stuff dont worry, it can easily be done with hand tools if you are careful and take your time and maybe make a practice cut or two if you havent done much wood work before. There are only a few cuts to make so you can take all the time you like and still finish the project quickly. Making accurate 90 degree cuts, especially when cutting the 1x2 to length, are important to making the project fit properly so do make use of a miter box. The more precise the lengths are the better the final product, but it will work just fine even if things are not perfect so don't sweat it.

I have been itching to get a chance to use the word "analemma" in text ;-). I used it to estimate when to use the winter, summer and equinox tilts. More on that later.

Step 2: Calculate Your Tilt Angles for You Latitude

The first thing you need to establish is your latitude, because that determines the angle of the sun rays in your area. You can usually find it very easily by doing a google search of a large city near you using the search string: "CityName Latitude". Here is San Francisco's latitude for example.

You need to calculate three angles:

Equinox Angle = 90 - Latitude

Summer Angle = Equinox + 23

Winter Angle = Equinox - 23

That's pretty simple. In the next step, the drawing shows where to use those angles. They allow you to just set the frame down on one of three edges that are preset to the optimum angles for each season. In the picture below my frame is sitting on its summer angle. Each angle is labeled on the drawing.

Step 3: Cut the Frame Parts, Rails

Print out at full size the drawing attached in the PDF file.

The frame is made up of four parts. There are two end plates and two rails. The drawing attached below shows the exact dimensions I used based on the size of my solar cell. The cells might vary a little bit so you might need to adjust the dimensions to match your cell.

So the first thing to do is to measure your cell and write those dimensions down.

You will notice that I have made a notch for my cell to sit in on the two rails. That's pretty easy to do if you have a table saw, however it would be tough if you are cutting out parts by hand so if that's the case for you I would just skip making those 1/8" notches. However, if you do want to make them, do that milling operation before you cut your 1x2 to length. Note that a 1x2 has actual dimensions of 3/4" x 1 1/2", which is what I show on the drawing.

Cut your 1x2 into 2 pieces the same length as, or just a tiny bit longer than, your solar cell long dimension. Use your miter box for these cuts. Getting them square is important to things fitting properly later. Do not do any sanding on these parts at this point.

Double check your cut rails for length against the solar cell. They need to be the same length as the cell or a bit longer. If they are too short, you need to cut new ones that are long enough.

Step 4: Cut the Two Side Panels

You should start by cutting two side panel pieces from your plywood that are 1/2" wider than your solar cell. That should be about 5" wide and 4-6" long. Make that cut as square as you can. This is easier if done with a cross cut or table saw, but you can also do a good job using a hand saw and your square to draw a square cut line. Unfortunately many miter boxes are too small for a part this wide, but if yours is big enough use it.

Use the rubber cement to glue the two panels together with the edges well aligned. Then use the rubber cement to glue the drawing side frame view to the two panels to use as cutting template. Make sure the rubber cement has a chance to dry well so that the panels don't slip around when you are cutting. If you have a vice to hold them that will help with the cutting step.

Start cutting. Since the panels are glued together you will cut out both panels at the same time following the cutting lines on the drawing. The finished panels should look like the picture below. Keep your cuts as square as you can so that the panels will be the same size.

Pry apart the cut panels. Rub the rubber cement off. Give them a light sanding both on the edges and faces to make them look nice and to remove splinters.

Step 5: Assembling the Frame.

Line up the edges of a rail with a panel and run a light bead of hot melt glue along the back side joint between the parts. This is only temporary to hold the parts in alignment while the screws are installed. Do the same for all four joints between the rails and panels. Take care to allow the glue too cool for strength and be careful not to break these temporary joints.

Once the frame is glued together. Set it on end on a flat stable work surface and carefully drill pilot holes for your No. 8 screws using the 7/64" drill bit. Drill the hole about the same depth as the screw length. Don't skip this step or you will have a heck of a time with splitting and driving the screws. You can eyeball the screw locations or measure them, which ever you are more comfortable with. Make sure you hold your drill nice and square so the holes are not at an angle. Take care not to break the temporary glue joints of the frame as you do this step.

Drill one hole at a time and then drive its screw in to minimize the problems with the frame glue joints breaking.

Drive the screws in until the head is flush with the wood surface.

Finish sand the piece now. If you want to paint or otherwise apply a finish, now is the best time.

Step 6: Prepare the Solar Cell for Mounting.

At this step you will add the voltage regulator to the cell and shorten the cells cable if you want to.

The solid red lead is positive on the cell suggested. The negative lead is black with a red stripe. If you use another brand of cell, make sure you understand the lead polarity on that cell.

Cut the positive lead about 2-3" from the connection point on the cell. Strip the insulation back on both ends by about 1/2".

Cut just the insulation of the negative lead, but not the wire, and pull it back about 1/2" to expose wire. If you cannot do that you can cut and strip both ends also, but you will need to cut about 1/2" from the positive lead and restrip it so the lengths will match.

Bend the two outer leads away from the center lead of the 7805 Voltage Regulator and super glue or hot melt glue it to the back of the solar cell as shown in the picture below. Make sure the leads can reach it with enough slack to wrap the stripped wire around the leads of the 7805. Make sure it is oriented like shown in the picture.

Solder the positive lead to the two bent outer leads of the 7805, wrap the leads with the wire to make a secure solder joint. Make sure it is oriented like shown in the picture below. If you get it backwards the voltage regulator will not work.

Solder the negative lead, both leads if you had to cut it, to the center lead.

Test the cell output. I should be 5 volts in full sun. If not double check your leads and correct the hookup.

Insulate the exposed leads with Brush-On Electrical Tape.

Shorten the cell cable (cut strip and solder) and insulate that splice now if you want a shorter cable.

Step 7: Mount the Cell in the Frame.

Position the cell on the frame and attach it with a few beads of hot melt glue on the back side of the cell to the rails. Don't over do it. That way you can remove the cell again later if you want to by cutting the hot melt glue beads.

Use hot melt glue to glue the cable to the back of the cell. Use a nice big glob to act as a strain relief on the cable. See the picture. That will prevent the leads of the voltage regulator from being yanked and damaged later.

Step 8: Using the Frame.

The Frame has the three angles that can support it at three different tilt angles. In the pictures below you can see the Summer, Equinox, and Winter tilts being used.

In the summer, May through July, you will want to use the summer angle in the middle of the day. In the spring and fall, which would be March, April, August, September and October use the equinox angle. In the winter, November through February use the winter angle. I determined these months and angles using the analemma shown below. They are only approximate and using the analemma you can determine more accurately the best dates to switch from one angle to the next.

The winter angle is good for early morning and late afternoon all year around when the sun is low in the sky.

You can collect a lot more power if you can rotate the frame every few hours during the day so that it follows the sun. That is especially true in the winter.

You can also harvest a quite bit more power in the winter time, you need all you can get when the days are short, if you set the frame on a large piece of cardboard covered with aluminum foil with most of that to the south of the cell. The foil captures sun light hitting in front of the cell and reflects it up to the cell increasing the current output by as much as 50%.
Do you need to be concerned that your panel supplies 310mA to the LiPo charger which only "accepts" 280mA from the DC plug? Does the LiPo charger just convert the 30 or so mA's to heat? Any idea how much current you can throw at the charger before something bad happens?
I'm not expert, but I doubt it would be good to over current the charger. However there are a couple of things to consider. The charger probably is voltage controlled and if the panel is not outputting enough voltage it cannot output that much current either. If it does need to limited, an inline resistor of an appropriate value will act as a current limiter, similar to how they are used with LEDs. I hope that helps.
So I got a solar cell around the same size as the one your using. On a sunny but overcast day it put out 7.06v when connected to the voltmeter. On a clear day I don't know. It is supposed to have a Voc of 6v so I don't know where the 7 is coming from. I guess the safest thing I need know is how much over 7v can the charger take. Thanks David
The overcast-day voltage you measure should be a accurate value for Voc, unless the light level is very very low. The Voc measure should be independent of illumination intensity if its reasonably bright out. I am guessing here but I think you probably can get away with using that cell without a voltage regulator because of the quiescent load the charger would present to the cell. Plus it seems very unlikely that the charger is going to be fried by going 60mV over its max input limit. If you want to make very sure that your cell will not fry your charger you could add a resistor wired across the cell leads to load it down just a tiny bit. I am guessing here but I expect a 1K ohm resister would be enough to drop the max cell voltage below 7V. It would draw about 7mA at 7V. That small current draw from the cell should pull its voltage down a bit from Voc and protect the charging circuit. You can check cell voltage, which must be done in full sun, with a 1K resistor wired across the leads, that will tell you if this resistor value will work. If not, you can adjust the resistor value. A lower resistance value will pull the voltage down more and vice-versa. So fiddle around until you find a resistor that pulls the voltage down to just under 7V in full sun. Then add that resistor across the cell leads in your circuit. It basically will act as a poor-man's voltage regulator.
So if I have a solar cell that puts out 6v 200mA and it bright sunlight it hits, for a brief moment 7.2 v. Will this automatically fry the charger? Or should I use the voltage regulator to make sure that doesn't happen? Thanks David
These are great questions. It has made me think about this a bit and really improved my understanding of how this gizmo works. If the cell is rated for Voc (voltage open circuit) of 6V and Isc (current short circuit) of 200mA, then it will not produce more than 6V no matter how bright the sun. You could even use mirrors to increase light intensity to greater than a full sun and the Voc voltage would not increase (current does increase in that case and can exceed Isc). Voc voltage is related to how the cell is wired internally and the wavelength of the photons it converts, but not light brightness. Note that under a real load, actual cell voltage will increase with increased light and can approach Voc, but Voc is the maximum possible voltage the cell can produce. So if you Voc is <7V it would be safe to use that cell without a regulator. However if the Voc rating is much greater than 7V, I would consider using a regulator even though the cell may actually operate a few volts under the Voc rating when connected to the charging circuit - and when there is charging load. If you stop charging, either by disconnecting the battery, or switching to the trickle charge mode when the battery is fully charged, the voltage may float up since the load is reduced, which may cause damage. My cell has a Voc rating of about 9V, which is why I used the regulator. Note that voltage regulators need a couple of volts higher input to work properly. The Fairchild data sheet for there 7805 has a minimum input voltage requirement of 7V. So don't try to hook a 6Voc cell to a 5V regulator. You need a cell with at least a 7V output, preferably a couple of volts more than that to accommodate the real cell voltage droop under load. The 9V cell in this project is a near perfect match for the 5V regulator for that reason. It has enough voltage headroom to produce near maximum current under load and drive the voltage regulator. The 9V cell should in fact operate pretty near its Mpp (maximum power point) running into the 5V regulator. If the cell rating were closer to the regulator's minimum voltage input requirement, I expect the cell voltage would be forced up to the regulator’s minimum voltage input, which would cause the cell current to droop. That might be pretty substantial actually if the Voc rating were near the minimum input voltage for the regulator. That implies that a cell with say a Voc of 7.3 V might be a poor choice for this project using a 7805 regulator since the cell might not put out anywhere near its Isc current rating in that configuration. This relates back to you first question. Which the short answer for my cell is there is not much current reduction, probably less than 10mA, but in the case of a cell with a Voc rating nearer to say 7V, the regulator may considerably reduce cell current. Thanks for asking, Scott
Thanks, So if I went out and used a volt meter and it was greater than 6v then that would mean it would have to actually be a Voc > 6 since I got up to 6.8x which is not high enough for a 5v regulator. So the next question where do I get a solar cell like the one your using?:) Thanks David
David, You should be OK with a cell that has a Voc of 6.8V. The Sparkfun LiPoly Charger used in the MightMintyBoost project will take a 7.0V input. So that cell should be fine without a voltage regulator. If you want to get the same cell I used. Its the Large Solar Cell from Sparkfun. There is a link to its product page on the Step 1 page of my instructable. Scott
Hi, I see how to install the voltage regulator. My question is does it still push the same amps even though the voltage dropped down to 5v? Thanks David
According to the data sheet the device consumes 5mA typical with a max of 8mA. The charging load will have more effect. Solar cells have a characteristc IV curve, so the circuit impedance the cell is connected to will determine its output voltage and current. The data given for the cell are the open circuit voltage and the short circuit current. The probable operating point will be closer to the short circuit current end of the IV curve in this case. The following link on page 10 of the document shows a typical IV link. This cell may be running at 6-7 volts in this circuit. Note also that the IV curve is only accurate for full sun illumination. The endpoints don't change with reduced light but the middle of the curve droops. <a rel="nofollow" href="http://www.fsec.ucf.edu/En/education/k-12/curricula/use/documents/USE_13_%20PVPowerOutput.pdf">IV Curve</a><br/><br/>

About This Instructable




Bio: I built the MightyMintyBoost. Very nice.
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