NPR: DIY Solar Backpack

Diversions

by Dick Demenus

With the help of some DIY-expert friends, reporter Jon Kalish was able to get this solar backpack constructed for $40 -- far less than the $200 or so that similar backpacks cost in the store.

Courtesy of Dick Demenus

It sounds great, doesn't it? Powering your portable devices while you're on the move. But like most things, building your own solar backpack involves some compromises.

The solar cell we chose to power our apparatus cost less than $30 and can deliver a maximum of 100 milliamperes (mA) of current at 6 volts. That's when it's in full sunlight and aimed directly at the sun. (To put that into perspective: A computer's USB port provides up to 500 mA.)

The time it takes to charge an iPod in an electrical outlet varies depending on the model, but four hours is typical for an iPod Classic. To charge an iPod using a solar backpack it would take more than a day at full sunlight.

Of course, you can always add another solar panel or two and linearly increase the current. Most commercial chargers use a different strategy that you can also adopt: They include rechargeable batteries in the charger. This allows them to charge slowly over time and then quickly charge your device off the batteries.

Materials You'll Need

One used backpack
A solar panel
A voltage regulator
A capacitor
A USB extension cable with the male plug taken off
Wire
Solder
Duct tape
Double stick tape (the foam type is best)
Two sheets of laminating film

Tools You'll Need

A soldering iron
Wire stripper
Scissors
A razor blade
Hot glue
A stapler

How To Do It

Expose the solder pads on the solar cell (left). Carefully scrape off a small (1/8-inch diameter) patch of the insulating film on the edge contact strips.

Tin the pads. This is essentially priming the surface. Apply heat and solder quickly (be careful because sustained heat can kill electronic devices) until the solder "wets." When this happens, the ball of molten solder flattens out into a shiny film.

Begin to wire it up. Again, it is best to tin the leads before soldering. Since this is a very simple circuit, we used a minimalist approach: Solder everything together and then smush it onto a piece of double-stick tape to hold it all together and keep it from shorting.
[Wiring Diagram]

Solder two flexible wires (each about 3 inches long) to the ground and input terminals of the voltage regulator (left). Then solder two more wires (each about 5 inches long) to the ground and output terminals of the voltage regulators. These 5-inch wires will go through a small hole you poke into the backpack with an Exacto knife.

Weatherproof it. Lay down one sheet of laminating film and arrange your solar cell and circuit on it. Now apply the other sheet and trim the edges of the film, being careful to leave a 1-inch border of just laminate around the perimeter of the entire circuit.

Stabilize it. Cover the perimeter of the laminate with duct tape or colored, 1.5-inch cloth furniture repair tape. The laminating film is too fragile for attachment so we need a strong, flexible border that won't tear easily.

Poke the hole through the backpack with an Exacto knife and run the wires coming out of the circuit through the hole.

Attach the solar panel circuit to the backpack (left). This turns out to be the trickiest task. Good luck sewing it; and keep some band aids handy. We scrapped that approach. Glue is too messy. Tape won't hold. Nuts and bolts...inelegant. Our solution: staples! If you have access to a saddle stapler, all the better.

Wire the inside. Now it's time to connect the USB cable and the capacitor (it's needed to make the voltage regulator happy) to the circuit. Solder the 5-inch wires coming through the hole in your backpack to the positive and negative terminals of the capacitor (left) and the USB extension cable. We put the capacitor on the inside because it's fat. We put the other parts outside just to show off.

Secure the connection. Hot glue is a big disappointment as an adhesive. Besides the burns that keep on giving, it fails so often when you least expect it. One place it shines, though, is in attaching to porous surfaces like fabric. It is also the poor man's molded plastic; squirt it on generously, imbedding the capacitor, wire and cable. We added an optional strain relief with a nut and bolt.

You're done. Like cooking, this is but one recipe. Experiment! That's the beauty of doing it yourself

Dick Demenus is the co-founder of Tekserve, a Mac repair store in Manhattan.

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