Grab a Kit
1x USB Charging Circuit
1x 6V Solar Cell
3 x Rechargeable NiMh AA Batteries
1x 3 AA Battery Holder
1x 1N914 Diode
1x Toggle Switch
18 inches Stranded Wire
3 inches Shrink Tubing
Wooden Enclosure Parts
1/8th Inch Plywood
16 x 6-32 Screws
16 x 6-32 Hex Nuts
Double Sided Foam Tape
Hot Glue Gun
Access to a Laser Cutter
Sourcing Your Own Parts
This section will be most useful for those sourcing their own parts, if you are building from a kit, feel free to skip ahead.
USB Charging Circuit
USB charging requires 5V at around 500mA of power to charge most gadgets. In order to meet these criteria in a small package we use a USB Charging Circuit, which boosts low voltage (2V) DC up to 5V DC. Nothing comes free though; we trade our increased volts for fewer amps. To avoid this tradeoff, we could use larger solar panels and larger batteries, but this would result in a larger, heavier, and more expensive charger. Take a look at our Lithium Heavy Duty Solar USB Charger 2.0 if you want something with some extra power!
If you're using an older Apple product, make sure your USB Charging Circuit is compatible. Typically a USB gadget requires just power from a USB port to charge. Most iPhone models, however, require a tweak to USB to work. iPhones will check the USB data ports in order to identify what kind of device it's plugged into and change charging speeds if it thinks the port can support it. Unfortunately if it doesn't see anything on the data tabs it will refuse to charge. This is a problem that plagues many generic USB chargers, especially older ones.
If you don't have an iPhone or iPod, then any old USB Charging Circuit will work. If you do have an iPhone or iPod you may wish to grab one of these charging circuits, as they're the only ones we have found that seem to work.
Our USB Charging Circuit needs a minimum of 2V to operate, so we need to choose power sources with this in mind.
We always advise the use of power storage when doing a solar project. Solar is inconsistent due to the inconsistent nature of the sun. Using batteries helps stabilize the flow of power, and also lets us hold onto that energy for later use.
In this project we use three rechargeable AA batteries. Rechargeable batteries put out 1.2V of power and using three in series gives us 3.6V. Using two batteries will cause our voltage to drop below 2V too quickly, and our USB Charging Circuit won’t be able to operate. Four batteries would require a much larger solar cell to charge.
Batteries require a minimum voltage in order to charge, but raising the voltage will not cause the batteries to charge faster. The general rule is to provide 1.5 times as much voltage as your battery needs (for us 3.6V x 1.5V = 5.4V), but with solar we want some wiggle room. We want our solar panel to meet the minimum voltage, even on days with a few clouds, so a 6V panel does well.
When charging AAs using a wall adaptor, we're able to charge them at high speed due to smart chips that constantly monitor the battery. Since our circuit is "dumb," with nothing to monitor the batteries, we trickle charge the batteries using the 10% rule. AAs can be safely charged as long as they don't have more than 10% of their capacity thrown at them at any one time. This means our 2,000mAh batteries should only get 200mA of current.
Our solar cell is rated for only 80mA, so we're completely safe! Different panels and batteries vary, so make sure yours work well together. One good aspect of the trickle charge method is that it will never over-charge the batteries. Once they're full they just stay topped off.
Above is a circuit diagram for this project. It's designed so that the solar cell is always charging the batteries and the toggle switch turns the USB circuit on/off. A diode has been put in place to prevent power from flowing the wrong direction into the solar cell. This is a very common design for solar chargers.
Our kit comes with a wooden enclosure made from 1/8-inch laser cut Baltic Birch plywood, with holes cut for wires, switches, and ports. Wood works well because it is non-conductive and easy to paint or decorate. We have included the files here for anyone who has access to a laser cutter.
If this isn't an option for you there are more simple ways of making an enclosure. Inexpensive and easy to find, plastic food containers or bins work well. If possible find one with a rubber o-ring, which helps keep moisture out. These makeshift enclosures are also are easy to modify with a box cutter or exacto knife, just watch your fingers!
It s also possible to find "Project Boxes" at many online stores. These also work quite well for enclosures, but tend to be more expensive. However, we advise against metal enclosures. Altoids tins and metal project boxes may look nice, but they are difficult to modify and can easily short out projects if left uninsulated.