Solar Energy Projects for the Evil Genius

Gavin also shared his thoughts about Evil Geniuses, solar power, and more in an interview with EcoGeek.org as part of the EcoGeek of the Week series, which was also presented here on Green Options. The following project is an excerpt from this book:

It’s the same old story—just when you want to talk on your cellphone, the battery goes flat and the conversation is irretrievably lost! Invariably, you haven’t got your phone charger on you, and even if you did have it wouldn’t be an awful lot of help as the chances are there is no power for miles around . . . At the Centre for Alternative Technology, U.K., there is a solar-powered phone (see picture at right); while this is powered by clean green energy, it can’t claim to be very portable!

In this project, we are going to build a circuit that will provide a supply capable of powering either a cellphone or PDA charger. A PDA is about the limit of what you can charge using small cells, a laptop charger is probably a bit ambitious.

One of the problems with trying to build this circuit is that finding a suitable connector for many mobile phones is a real problem. While Nokia makes life easy by providing a simple jack that can be readily obtained from many component suppliers, many other manufacturers rely on proprietary connectors which are nonstandard and awkward to source.

For this reason, we have based this project on hacking a cellphone car charger.

There are two schematics here for projects that tackle the project from slightly different angles. The first method involves creating a solar array that will provide above 12 V—regulating this supply to 12 V, and charging the device via a hacked “car charger” (Figure 1). The other device is suitable for where a USB type charger is available—this is ideal for USB mp3 players, PDAs and mobile phones, most of which now come with a “data” lead. We have an array of solar cells, which charges a couple of batteries when there is spare power; a voltage regulator then turns this into a clean 5 V, which can be used to drive the device (Figure 2). The advantage of this circuit is that even if there is not a lot of sun— or it is night-time, you can pop a couple of freshly charged batteries in (maybe from your solar charger?) and things will start working.

Standard charger wiring diagram (Figure 1)

Car chargers are designed to allow you to plug your phone into your vehicle’s cigarette lighter or accessory socket. They are cheap and readily available; however, they rely on having a car present to allow you to charge your phone!

There are a couple of ways of making this project. You can either build the project as a box into which you plug your car cellphone charger, or, if you are a little more adventurous, you can take apart the cellphone charger and integrate it properly into the box. The plus side of keeping the two pieces separate is that you can use the car cellphone charger as a stand-alone item, or, you can power it from the “solar box.” The plus side of integrating it all together is that it makes for a neat, stand-alone project and the two parts cannot become separated.

USB Charger wiring diagram (Figure 2)

Note: A note on cigarette lighter sockets—the usual wiring scheme is that the casing of one of these sockets is connected to the negative terminal of the battery.

Excerpt from Solar Energy Projects for the Evil Genius: 50
Build-It-Yourself Projects by Gavin D.J. Harper, copyright McGraw-Hill,
2007.