Solar USB Box SUBcomandante

As soon as we here at Revolt Lab heard that the Occupy Wall Street movement had come to our city of Boston, we immediately thought of how to put our maker skills in the hands of the revolution. Our first attempt has produced our Solar USB Box or SUBcomandante.

It consists of an array of solar panels mounted on a sturdy military surplus case, a 5 volt regulator, some capacitors, a switch, and a USB hub. The great thing about USB-charged devices and most cell phones is that they all take 5 volts! With this simple project, you can make yourself an emergency charger for phones, mp3 players, digital cameras, and other tools crucial to the revolution. Best of all, the simplicity of the design means beginners can understand how it works and learn to make their own!

I started with a case purchased from the army surplus store. This case was apparently used by Dutch forces to test air for poison gas and signal when it was safe to remove one’s gas mask. I found this instructional video of it in action:

The SUB circuit is very very simple. The 7805 reguator needs between 7 and 12 volts to put out a steady stream of 5 volts. Make sure that the voltages of your solar panels add up to between 7 and 12! When you have the proper amount of panels, wire them in series (positive to negative) until you have just one positive and one negative wire remaining. These wires will be your main power and ground wires.

My 5 panels were 2 volts each for a total of 10 possible volts.

On a piece of perfboard, place your regulator and capacitors according to the electrical diagram. Connect the capacitors to the regulator. Be sure to point the striped or negative side of the capacitors toward the middle pin of the regulator.

Use the diode (any 1n00x will do; I chose a 1n007) to connect the positive end of the solar array to a switch. Connect the other end of that switch to the left most terminal of the regulator. Make sure the stripe on the diode is closer to the regulator and farther from the solar array. This stripe indicates the ground or negative side of the diode. If you connect it backwards, no power will reach your device!

Connect the solar array ground wire with any remaining loose grounds on the board including capacitors and the middle pin of the regulator.

Finally, strip your usb hub or female usb cable and locate the red and black wires. Connect the red wire to the rightmost pin of the voltage regulator. Connect the black wire to ground.

You will notice that there are two regulators on my board. One is a 7808 8-volt regulator I had planned to use for recharging 4 AA batteries. Turned out all three regulators I had were duds! I did however get a chance to try out my new awesome desoldering pump graciously donated to the cause by the good folks over at Farnell (they run the element 14 site).

This tool would have saved me hours making the plantduino control board. It wisps away solder like magic, doesn’t burn or melt, and I can operate it with one hand! If you don’t have some kind of desoldering tool, get one. It is totally worth it.

When you have finished the wiring, wait for the sun to come out and check that everything is working!

I found that this set up will only charge my smart phone when it is off. As that is the state of my phone when I most need to charge it, I was happy that it could work in emergency situations. Try different panels and regulators (ALWAYS FIVE VOLTS!) to see what results you can get!

I used 5 minute epoxy to secure my solar panels, perfboard, and USB hub. Hot glue would work in a pinch.

To prevent my electronic devices form bouncing around, I attached a zipper pocket from an old pair of pants to the inside of the case.

While one of these isn’t much help to an occupation, tens of these could seriously curb demand for outlets and AC power, at least on sunny days.

Please leave any feedback you may have on how to improve the design or if you want to know more about building your own!

Get out there and MAKE REVOLUTION



35 responses to “Solar USB Box SUBcomandante

  1. Pingback: Solar charger for Occupy Boston - Hack a Day

  2. Pingback: Solar charger for Occupy Boston | You've been blogged!

  3. This is a great idea, but I’m bothered by the diode between regulator and load. The internet suggests that the forward voltage drop across a 1N4007 should be .7v, which means you’re only feeding 4.3v to your charged devices. Do they still seem to work at that low voltage?

    It strikes me that an alternative would be to eliminate the diode altogether. That means you’d have to keep an eye on things, but the charging would be a lot more efficient when the sun is out. I pondered what would happen if you put the diode on the solar panel side of the regulator, but I suspect that the regulator will impose as much load on a battery as the solar panels would (I’m just guessing on that one). That would be one alternative, if the regulator doesn’t draw too much from the device being charged.

    You might also improve the efficiency of the circuit by eliminating the hub (which has unknown power handling efficiency), and just terminate the +5v wire in an appropriately wired female USB plug. It’s unlikely you’re getting more than a watt or so out of each panel, so you’ll never be able to charge more than one phone at a time in any case.

    In any case, this is a fine idea, and well done. I love the mounting solution. Army surplus stuff is an amazing value.

    • Ian J,

      Thank you for the feedback! It’s always great to get the community involved in development. The diode does indeed cause some voltage loss, but this is before the solar voltage reaches the regulator. The 10 volt array will still be supplying sufficient voltage for the regulator to work effectively.

      I will definitely eliminate the usb hub in the next design. The voltage coming out of the hub is identical to the regulator output, but intuitively that led and additional circuitry must be eating up some power.

  4. Can you give a more detailed picture of the board, as well as a parts list? I am interested in making one of these, and I would love to get some more detail on this build.

    • I will be making a second iteration with a more detailed look at the board if the schematics are not clear enough. May take a few weeks though. In the mean time I suggest playing around with voltage regulators on a breadboard. They are simple and fun!

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  7. CJ

    Great project using off the shelf stuff…..good in a pinch, but lacks efficiency, your probably around 50% at 1 Amp the rest is dissipated as heat across the 7805. In version 2 if you decide to build another and if you want more bang for your buck….use a step down buck regulator. A real simple one is the LM2592HV, runs about 80% efficient, has a huge input range, up to 2 amps and only 5 pins (TO-220 case). Just need two caps a diode and inductor. BOM under $4. There are better switchers out there about 90% efficiency but more complicated design and higher price. Free samples if you have a corporate email address.

    • CJ,

      You are right! It does lack efficiency. The data sheet for the LM2592HV seems simple enough, Where do you get yours? Would our revolt lab email be fancy enough to warrant a free sample?


    • Also, any suggestions on the inductor ratings? I think I’m gonna go with the suggested 33uH but i don’t know if I need an inductor rated for 2 amps or not. Any ideas?

  8. CJ

    Yes the 33uh should be at least 2 Amps or more. here is a BOM from Digikey for a price of a quantity of one, the more you make the cheaper it gets.

    33uH 2.7A Digikey 811-2102-ND $0.85
    SB530 Schottky Digikey SB530-E3/54GICT-ND $0.75
    LM2592HV5.0 Digikey LM2592HVT-5.0-ND $6.83
    680uF 50V Digikey 565-1613-ND $0.85
    220uF 25V Digikey 493-1062-ND $0.32

    if you notice the 1N5824 is not available but I substituted to a SB530 same current and PIV. yea you should be able to get a samples just call up national semiconductor, there pretty good about giving samples. If you have time most of these parts you can get as “samples” it just takes time and phone calls. Let me know if you can’t get samples, I’m going to order a bunch of these in the next few weeks for another project I’m doing.

    • I have made the new charging circuit and it is much more efficient! Even in cloudy skies it charges the phone while the phone is on! However, after a few minutes the phone charge light turns green and the phone thinks it is already fully charged. It leaves power save mode and everything. This ends up wasting more power and I suspect the phone disengages the charging. Do you have any ideas on the cuase of this?

      • To diagnose the cause of your problem, if you haven’t done so, read the following article at:

        Note the section that nicely summarizes how the various USB delivery current specifications. It states:

        “In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); in USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1500mA (1.5A). ”

        So although USB charging is supposed to follow these standards, some companies build USB chargers that aren’t always compliant with the standards ; seems that you may have done the same yourself and that may be why your phone doesn’t charge properly with this “more efficient” design.

        The LM2592HV that “CJ” suggested that you use has a rated (max) current output of 2 AMPS — and unless I missed something, you don’t mention adding anything to CJ’s proposed design that would limit the output current to meet the aforementioned USB standards.

        Furthermore, most (if not all) 7805 regulators (used in your original design) that are in production today have a max output current of 1 to 1.5 AMPS, so that’s likely why your phone had no issues when connected to that design,

        Rather than me spend more time speculating as to the cause (sic. “cuase” ) of your problem, how about you post a schematic showing the details of the LM2592HV-based version of your charger?

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  11. Ryan

    Would it be a problem to use different sized solar panels, I found a bunch of solar paneled garden things on sale at the shop, and have bought them and ripped them. I was just wondering if I add them all in series, I should get enough juice to make this work. They all give between 1.2V and 2.3V, not a huge difference.

  12. d

    It’s great to see your USB charging effort for Occupy Boston, we’re trying to compile details of different USB chargers here, feel welcome to contribute:

  13. Tamsyn

    Is there a new updated version of this using the new chips? It’d be a great project – thanks! I love the lockable box too – adds a bit of security for those charging their phones. A version that’s switchable for 12V and 5V (or can do both) would be awesome too. (I’m thinking for use with a Raspberry Pi and a 12V Screen.

    • We are currently working on the new version. It is much more efficient but for some reason our phones are getting tricked into thinking they are charged shortly after we plug them in. When we resolve this problem we will post all the schematics and pictures! If you have any ideas to help us troubleshoot please leave a comment =)

      Your raspberry pi idea sounds awesome.

  14. Ted

    How much current do you have running through your system? I’m trying to build a similar USB charger , using solar panels rated 5.5v 320ma.

    • Hi Ted. The answer is not enough. I have a new circuit I have been testing out that I will release this week. There are still some bugs but it delivers much more current than the inefficient 7805 regulator. Maybe we can solve the bugs together! Keep an eye out for the post later in the week.


  15. immanuel joseph

    This was really great , definitely i will build one in next 2 weeks!!!

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  19. Chuck Weiss

    I would love to see the revised circuit schematic if you have found the time to draw one up. I would love to build this to keep my GPS charged up when I’m away from home. Thanks!

  20. William

    Is there an update for this yet? I noticed that an update was planned in March.

  21. Pingback: Bagaimana membuat charger laptop dari solar sel « muhammadfajar046

  22. ErgonAgathon

    This is a great project. I was thinking something like this could really help out in emergencies like Hurricane Sandy. But I have a question (I’m very new to electronics so forgive me if this is a stupid question): Would it be possible to rig something like this up so that it terminated in a standard 3-prong wall outlet, rather than a USB? I’m thinking that way you could charge other devices, like iPhones or anything that had an AC adapter. Is it a matter of the voltage? Again I’m very new to this stuff so apologies if it’s an obvious question. Thanks!

    • Thanks! To answer your question, appliances that plug into the wall need 120 volts of Alternating Current. A USB device uses only 5 volts of Direct Current. Hope that answers it for you. Feel free to ask any and all electronics related questions.

  23. Pingback: Step By Step Build a DIY Solar Laptop Charger

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