Powering the $100 Laptop

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To customize the generator for children with varying strengths, or so that users can decide how hard they want to work, the design includes a computer chip that continuously adapts to how much resistance users feel. This electronic "variable motor loading" is like changing gears on a bicycle to go up a hill, Bulthaup says. "Each person pedals at the same speed, but a stronger person can push harder with each stroke. Our device automatically adjusts the loading to reach that optimum comfort/power point."

The device meets other key criteria, too, including durability and ease of use. If the string breaks, for instance, it can be easily replaced with a shoe string, or a similar object. And the generators should cost less than $10 apiece, Bulthaup says.

In an e-mail, Nicholas Negroponte, chairman of the One Laptop per Child project, says the device is the best-performing of the many they've looked at so far, and that they intend to use the design with their laptops -- if it continues to perform well in tests and another, better design does not appear. Other human-power options may also be used, however, depending on the situation, says Mark Foster, the project's vice president of engineering and chief architect.

The $100 laptop developers are also working with several firms on an ambitious, related project: developing a long-lasting battery system to be paired with the generator (or to charge off AC power). This battery system will include "custom chemistry, unique electronics, and complex charge and discharge monitoring algorithms to deliver 2,000 battery cycles -- four times more than normal PCs," Foster says. A long charging session in the morning, for instance, would allow kids to use the laptop throughout the day, with the batteries storing enough energy for eight hours of work -- with enough left over for the computer to serve as a wireless mesh network router for another 16 hours.

The $100 laptop, which the developers expect to start shipping to interested countries next year, will actually cost $135 to manufacture at first, before it drops to a projected $100 by 2008.

The project is making steady progress, moving forward on its integrated circuit, software, and industrial designs, Foster says. Once everything is ready, the group plans to conduct extensive testing: they've set aside 500 laptops to be tested until they're destroyed -- to make sure they're rugged enough for rough environments.