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MAKE contributing writer Alastair Bland first introduced us to this simple, effective circuit back in 2009. It takes higher-voltage alternating current (AC) from a retro-style bike headlight generator, converts it to direct current (DC), and steps it down to a safe voltage for charging your cell phone or other mobile device.

You may have one of these old bike generators lying around.  If not, they can still be bought new or used online and in some retail bike shops. Technically, this device is an alternator, a very simple and reliable generator that turns rotational momentum into AC electricity.

The phone charger circuit consists of a rectifier, a capacitor, and a voltage regulator.  The rectifier contains four one-way electrical gates called diodes which, working together, convert the back-and-forth wiggling of the charges in AC to a series of DC charge pulses.

The large capacitor connected between the DC terminals of the rectifier smooths out these pulses, charging up when there’s more energy in the system and discharging when there’s less.

Finally, the voltage regulator holds the incoming DC power down at a steady 5V, which is what most cell phones and other mobile devices are designed to accept.  Without the regulator in place, the phone charger might deliver more than 5V to your device, which could damage it.