Make your own Solar Bag – part two

Today you will make your own Solar Bag complete with the guidelines for the electronic part of our project.

talk2myShirt_DIY_Solarbag.jpgA few words to the design: The following electronic concept is based on the DIY instruction from Popular Science but we added a diode which is recommended by our Solar panel supplier Sundance Solar.

The diode protects the solar panel from damage in case current is flowing from the device back to the solar panel. This can happen when device is connected to your bag but the solar panel is not active, meaning no sunshine to let the solar panel work.

Let’s get started with the shopping list, we recommend using Digikey, a very reliable online shopping source:

1x 78M05 Voltage Regulator
1x 0.47µF 50V Electrolytic Capacitor
1x 0.1µF 50V Tantalum Capacitor
1x 20V 1A DO-41 Schottky Diode
1x 1.3mm DC plug
1x 1.3mm DC socket
1x Copper glad PCB, 3″x4″

1x Plastic box at least 2.8″x1.8″x0.8″, any type you can find will do
1x USB female connector (cut off from an USB extension cable at a length of 20cm)

You will need the following tools: solder, a solder iron and I strongly recommended a multimeter. A simple one is fine, you do not have to buy a high end multimeter for our simple electronic.

The image below shows the connectors we use: on the left side is the DC-socket that goes on the printed circuit board and next to it is the DC-plug that is soldered to the power cable of the solar panel. Connect the positive wire of the solar panel to the center pin of the DC-Plug and the negative wire to the outer part of the DC-Plug. Use the same polarity for the DC-socket on the PCB.


In the center and the right image show the cut off female part of an USB extension cable. Open the cable end and separate the four wires: red, black, green and white. The green and white wires are USB data lines we do not need, cut them off. Strip off a few millimeter of the red and black wire for soldering to the PCB.

Next we take the plastic box (see image 1) that will hold our small PCB and give support to the USB cable and the DC-socket. Drill two holes, one just large enough for the USB cable, slide the cable in and make a knot for a strain relieve to the solder points of the cable (see image 2). The second hole is for the DC-plug to fit (see image 4). In image 3 you see the PCB before and after we cut it down to just fit into the box. The PCB should not move around. If your box is bigger than your PCB, glue it after everything works into the box.

sbe1.jpg sbe2.jpg sbe3.jpgsbe4.jpg

Image 5 shows how all the components have to be connected to each other. Watch out to have the correct polarity! The positive lead of the 0.47mF electrolytic capacitor is soldered directly to the Input of the voltage regulator. The positive lead of the 0.1mF Tantalum capacitor is soldered to the voltage regulator’s Output. Finally, the negative leads of both capacitors are soldered to the Ground of the voltage regulator. Connect the positive lead of the DC-socket to the negative lead of the Diode and the Diode’s positive lead to the input of the voltage regulator. The negative lead of the DC-socket is connected to the Ground of the voltage regulator (see image 6 and 7). We used so called jump wires to make the connections on the PCB.


Solder now the red wire from the USB cable in the box to the Output of the voltage regulator and the black wire to the voltage regulator’s Ground.

Now comes the most important task: measure if all your connections are correct. We have here a ‘secret’ trick to avoid having to go out to the sun with your solar bag to verify your work. We used a battery box (available in many electronic shops) that holds 4 AA batteries (see image 8). We have soldered a DC-plug (same as we used on our solar panel) on the battery box and can now connect the battery box to our Solar-USB-charger electronic. Those 4 AA batteries give 6 volt (4×1.5) and 1400mAh, much more current than our solar panel but the voltage regulator will cut this off at the maximum allowed for USB2.0 at 500mAh.

Connect the Batteries or the solar panel (if you have sun shine right now) to the electronic box and test the voltage with a multimeter at the voltage regulator input and on the USB connector wires. The reading at the first point should be approximately 6 volts. The value at the USB wires should be 5.15 Volts (see image 9). If everything looks correct, you can now connect your iPod or any other USB chargeable device (see image 10).


Our electronic design will charge any device that can be charged via a USB connector and is USB standard compliant. That means USB devices that do not follow the USB standard will not charge (like the Motorola Razr V3). Our recommended solar panel delivers under maximum sun exposure around 9 Volt by 250mAh (the nominal values are 7.2 Volt by 200mAh). The USB1.1 standard specifies 5.15 Volt and 100mAh for power supply to devices. The newer but already very common USB2.0 standard has the same voltage put allows up to 500mAh. That means our solar bag will charge in full sun twice as fast as a USB1.1 connector on a PC and half the speed of the USB2.0 connector on a PC.

sb-box.jpgWe selected this simple concept as our first DIY project because it is easy to make with very common components and low cost. Of course, adding a battery to store energy for rainy days is something we do have in mind.

Our DIY Solar Bag 2.0 is on top of our to-do list, so come back soon and see how we make it.

For now we wish you lot of sunny days and we like to hear from you in our comments how your solar bag is helping you to stay entertained and connected all the time with green, environmental friendly energy.


  1. Hey,
    well its an awesome project, i must admit. the drawback is the 7805. a 7805 is a linear regulator, so everything above ~5.6 is burned off as heat. check out the appnote below. maxim-ic makes a dc-dc converter chip which converts values above and below 5v to 5v! its called a step up/ step down converter. you would get more power to your device, and well it would work for voltages below 5v.

    Dane Kouttron
    Rensselaer Polytechnic

  2. Hi Brian,
    yes, the solar panel energy can be used for battery charging as well. We are working on such a DIY version and look around for simple to build battery charging circuits. I will let you know when we are ready and post this instructions.

  3. this is a nice project….
    but can i need help..i want to build a cellphone charger out of a stepper motor, which produces ac current…can some one help me?

  4. Confusing article….mAh means milliamp hours, which is a measure of charge, not current. I think the author is incorrectly using mAh in place of mA in many cases. At least that is my guess based on the context.

  5. Hi, Have you a design for charging battery yet? Pls let me know how i can incorporate it into my bag.


  6. Hi Joe, we have not worked out a battery charger, to keep it simple we only did direct charging.

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