Solar Powered USB Power Supply and Charger

I've always wanted to go green and use solar energy to charge my power hungry devices. I recently found and purchased a new United Solar US-21 12 Volt 21 Watt solar panel at a garage sale for $20.  What a deal!  So, I decided to construct the necessary components so I can use it to power and charge my gadgets.  This instructables shows how to construct a solar powered USB power supply and charger that can used with a solar panel or large solar cell.

I made it at TechShop.

The following parts are needed to construct for the Solar Powered USB Power Converter (pictures in this step).  Parts can be obtained at Jameco, Digikey, Radio Shack, etc.

• 1 LM7805 5 volt 2.2 amp voltage regulator (Jameco 786138)
• 1 Heat sink for LM7805 (Jameco 158051)
• 1 Heat sink mounting kit for TO-220 heat sinks (Jameco 34121)
• 1 47 uF 50 volt Electrolytic capacitor (Jameco 31114)
• 1 100 uF 50 volt Electrolytic capacitor (Jameco 158394)
• 2 0.1uF 50 volt Ceramic capacitors (Jameco 544921)
• 1 1N4001 50 volt 1 amp Diode (Jameco 35975)
• 1 2 volt 20 mA LED with chrome bezel (Jameco 141129)
• 1 150 ohm 1/4 watt Resistor
• 1 2 position Dual barrier strip (Radio Shack 274-656)
• 1 Single Pole Single Throw (SPST) Switch (Radio Shack 275-602 or similar).  I had a switch with a red safety cover so I used that - the safety cover is optional but it makes it look really cool!
• 1 tube Heat sink compound (Radio Shack 276-1372)
• 1 Plastic case (Velleman G311 or similar) - no smaller than 4.5" x 3.5" x 2.2"
• 1 Printed circuit board (Adafruit Perma-Proto Quarter-sized Breadboard PCB part # 589)
• 4 Rubber feet (available at hardware stores - also called rubber bumpers)
• 2 #6-32 x 1/2" Machine bolts with rounded heads
• 2 #6-32 Nuts
• 2 #4-40 x 3/4" Machine bolts with rounded heads
• 2 #6/6 Nylon washers
• 2 1/4" #4 Nylon spacers (#6 can also be used)
• 2 #4-40 Nuts
• 12" Red wire
• 12" Black wire
• Solar Panel or large solar cell

The following parts are needed to construct the USB charging cable (photos of parts are shown in Step 7):

• 1 USB cable with a male type A connector and a female type A connector
• Electrical tape
• Heat shrink tubing (optional)

The following parts are needed to construct the Apple charging adapter (photos of parts are shown in Step 8):

• 1 USB cable with a male type A connector and a female type A connector
• 1 Printed circuit board for the Apple charging circuit (Radio Shack 276-159)
• 1 Small plastic case no smaller than 3 1/8" long, 2" wide, and 1 3/8" deep
• 1 Small piece of insulating material
• 6" Thin insulated wire
• 2 75K ohm 1/4 watt Resistors
• 2 51K ohm 1/4 watt Resistors

The following parts are needed to construct the test cable used in steps 9, 10, and 11:

• 1 USB cable with a male type A connector
• 1 Four position barrier strip (Radio Shack 274-658)
• 1 Nine volt battery

The following tools are needed:

• Soldering iron and solder
• Wire strippers and cutters
• Small knife
• Small screwdrivers (Philips head and flat head)
• Electric drill and assorted sized drill bits
• Heat gun (optional if heat shrink tubing is used)

The Solar Powered USB Power Supply and Charger consists of a Solar panel, a power converter, a standard USB cable, a USB charging cable, and an Apple Charging Adapter as shown in the first diagram.

• The Power Converter connects to the Solar Panel and reduces the voltage to a regulated 5 volt output suitable for powering and charging USB devices.  The Power Converter will accept up to 35 volts as input if you use the specified voltage regulator.
• The standard USB cable connects the Power Converter to the USB device and is used to power the device and for older USB devices it may be used to charge them.
• The USB Charging cable is a standard USB cable modified to implement the USB charging standard that allows the device to "know" that it can draw enough current to charge the device.  More details on this can be found in Step 7.
• The Apple Charging Adapter contains a circuit that allows newer Apple devices to "know" that they can draw enough current to charge the device.  More details on this can be found in Step 8.

The schematic for the Power Converter is shown in the second diagram.  It's a simple and standard circuit that uses a voltage regulator to convert the input voltage to a regulated 5 volts.  Note that there is a diode in the circuit (next to the switch) that will protect the circuit if the input power leads from the solar panel are connected incorrectly.

The physical layout and wiring for the circuit is shown in the third diagram.

Prepare the case as follows:

1. Drill two mounting holes for the Adafruit printed circuit board in the bottom of the case as shown in the first photo.
2. Drill holes for the switch and the LED in the top of the case as shown in the second photo.
3. Drill a hole in the side of the side of the case for the USB cable as shown in the third photo.
4. Drill the four holes in the other side of the case as shown in the fourth photo. The upper holes are for the barrier strip.  The lower holes are for the power wires coming from the circuit board.
5. Mount the barrier strip as shown in the fifth photo using the #6-32 machine screws and nuts.

The first diagram on this step shows the circuit wiring on a breadboard. I usually breadboard all my circuits before soldering them to a printed circuit board so I can verify the circuit works properly and that none of the components are defective.  If you use the Adafruit Perma-Proto Breadboard printed circuit board, you can use the breadboard diagram as a guide on how to place and solder all the components.

1. Attach the heat sink to the voltage regulator as shown in the second, third, and fourth photos.  Use a dab of heat sink compound between the voltage regulator and the heat sink.
2. Solder the components to the board using the breadboard diagram as a guide as shown in the fifth diagram.

Prepare the external components as follows:

1. Solder the wires to the switch and to the circuit board as shown in first photo.
2. Solder wires to the LED as shown in the second photo.
3. Insulate the exposed connections using heat shrink tubing or electrical tape as shown in the third photo.
4. Cut a female USB type A connector and strip the ends off the power leads as shown in the fourth photo.  Just cut off the data lines (green and white wires).  They will not be used.

Complete the assembly as follows:

1. Attach the LED to the top of the case as shown in the first and second photos.
2. Insert the USB cable through the hole in the side of the case and solder the switch wires, the LED wires, and the USB wires as shown in the third and fourth photos.  Use the breadboard diagram as a guide.
3. Slide the power wires through the holes below the barrier strip on the side of the case and mount the circuit board in the case using the #4-40 machine screws, nuts, nylon spacers, and washers as shown in the fifth and sixth photos.
4. Gently push all the wires into the case, close the case, and screw it shut as shown in the seventh and eighth photos.
5. Strip the power wires and attach them to the barrier strip screws and shown in the ninth and tenth photos.
6. Attach the rubber feet to the bottom of the case as shown in the last photo.

Voila! The Power Converter is now complete!

According to the USB specifications, the technique a USB charger should use to indicate to a device that it can draw full current for charging is by shorting (connecting) the two data lines together.  This works for most devices except Apple devices which are covered in the next step.  The power converter constructed in the previous steps can provide power to a USB device but the data lines are left unconnected so newer devices will not charge with just the power converter and a standard USB cable.  This step describes how to modify a USB cable for charging.

The following parts are needed for this step:

• USB cable with a male type A connector and a female type A connector (shown in the first photo).
• Electrical tape.
• Heat shrink tubing (optional)

Follow these steps to construct the charging cable:

1. Cut the cable in half.  Optionally slide a piece of heat shrink tubing down one end of the cable.  See the second diagram.
2. Using a knife, carefully remove the outer shielding from the USB cables.  Using wire strippers, strip the wires as shown in the third diagram.
3. Connect and solder the various leads together as shown in the fourth diagram.  Note that the data lines are connected on USB cable with the female connector.
4. Insulate the exposed connections with electrical tape as shown in the fifth diagram.
5. Wrap electrical tape over the wires or use the heat shrink tubing to hold the individual wires together as shown in the sixth diagram.

Voila! The finished cable is shown in the last two photos.

Apple devices require that a specific voltage be present on the two data lines in order for the device to "know" that it can draw sufficient current on the power leads in order to charge the device.  An excellent write-up on the mysteries of Apple device charging can be found at http://www.ladyada.net/make/mintyboost/icharge.html.  The circuit used in this step comes from that write-up.

For this step the following parts are required:

• USB cable with a male type A connector and a female type A connector as shown in the first photo.
• Printed circuit board for the Apple charging circuit.  I used the Radio Shack PC board 276-159 as shown in the second photo.  Only one half of the board is used in this project.
• A small plastic case to hold the circuit board as shown in the third and fourth photos. The case I used was 3 1/8" long, 2" wide, and 1 3/8" deep.  I would not recommend using anything smaller as this was a tight squeeze to get everything inside the case.
• A small piece of insulating material - I used a piece of stiff plastic.  See fifth photo.  You only need this if your case has a metal bottom.
• 6" thin insulated wire
• 2 75K ohm 1/4 watt resistors
• 2 51K ohm 1/4 watt resistors

To construct the Apple charging adapter:

1. Drill a hole in each end of the case for the USB cables as shown in the sixth photo.
2. Cut the USB cable in half (if it's a long cable you can cut the leads to a shorter length but I'd recommend not shorter than 5 inches. See the seventh photo.
3. Using a knife, carefully remove the outer shielding from the USB cables and strip the wires.
4. Using the schematic (eighth diagram) as a guide, solder the circuit on the PCB as shown in the ninth photo.  Important: pull the ends of the USB cables through the holes in the case before soldering the USB wires to the circuit board.
5. Push the circuit board and USB wires down into the case as shown in the tenth photo.
6. Place the non-conductive material on top of the circuit board as shown in the eleventh photo.
7. Screw the cover onto the case and attach the rubber feet as shown in the twelfth photo.

Voila! The Apple charging adapter is now complete as shown in the last photo!

Before using the USB power converter, it's important to test it to make sure everything is connected properly so you will not damage your USB device. A simple test cable can be constructed to make testing easy.  This cable will come in handy for your future USB projects.

To construct the test cable, you will need a four position barrier strip (Radio Shack 274-658) and a USB cable with a male type A connector. 

1. Cut the USB cable, remove the outer layer and shielding.
2. Connect the wires to the barrier strip as shown in the first two photos.  I used some heat shrink tubing to protect the wires; electrical tape would also work.

Testing the USB Power Converter:

1. Attach a power source to the power converter using the barrier strip on the side of the case.  For testing purposes, I used a 9 volt battery.
2. Connect the USB test cable to the USB female connector on the power converter as shown in the third diagram.
3. Turn the power switch on the power converter to the "On" position.
4. Using a multimeter, set it to test for DC volts and touch the test probes to the positive (red) and ground (black) leads as shown in the fourth photo.  The voltage should read very close to 5 volts as shown in the fifth photo.
5. Turn off the power using the switch on the case and verify that no voltage is present - the meter should read zero.
6. Set the multimeter to measure resistance and touch the test probes to the two data lines (white and green) as shown in the sixth photo.  The meter should indicate that the resistance is infinite (i.e. not connected) as shown in the seventh photo.

Now that the power converter is working correctly, it's time to test the charging cable.

1. Connect the charging cable to the power converter and the test cable to the charging cable as shown in the first diagram.
2. Turn the power switch on the power converter to the "On" position.
3. Using a multimeter, set it to test for DC volts and touch the test probes to the positive (red) and ground (black) leads as shown in the second photo.  The voltage should read very close to 5 volts as shown in the third photo.
4. Turn off the power using the switch on the case and verify that no voltage is present - the meter should read zero.
5. Set the multimeter to measure resistance and touch the test probes to the two data lines (white and green) as shown in the fourth photo.  The meter should indicate that the resistance as zero ohms (i.e. connected) as shown in the fifth photo.

The last testing step is to verify that the Apple charging adapter is working properly.

1. Connect the Apple charging adapter to the power converter and the test cable to the Apple charging adapter as shown in the first diagram.
2. Test the power lines (red and black) as you did in the previous two steps to verify that 5 volts are present when the power converter is on and no voltage is present when the power converter is off.
3. Turn the power switch on the power converter to the "On" position.
4. Using a multimeter, set it to test for DC volts and touch the test probes to the white data lead and ground (black) lead as shown in the second photo.  The voltage should read very close to 2 volts as shown in the third photo.
5. Touch the test probes to the green data lead and ground (black) lead as shown in the fourth photo.  The voltage should also read very close to 2 volts.
6. Turn off the power using the switch on the case and repeat the last two steps and verify that no voltage is present - the meter should read zero.

Voila! The project is now complete. 

One last warning: it is very important to thoroughly test the the power converter, the charging cable, and Apple charging adapter before attempting to power or charge your devices or you may damage them.  Also, while I have tested these with several devices (an Apple iPod Touch, an Apple Nano, and an HTC Incredible 2) to the best of my ability, I can not take responsibility for any damage this may cause to your device.

After finishing and using the Solar USB Power Supply and Charger, I had some ideas to make it better.  An improved version of the project is shown in the first photo.

There are two improvements:

1. Rather than require a special cable for charging, a switch has been added to the circuit that, when thrown, will connect the data lines as shown in the circuit schematic (2nd image). 
2. The LED has been removed from circuit because it is very difficult to see the illuminated LED in bright sunshine.