You're halfway through listening to "Layla" when it happens: Your MP3 player's battery dies. Normally you'd have to wait until you were at your computer to finish rocking out, but there is an easy and eco-friendly way to do it on the go. First, slip a piezoelectric transducer -- a device that generates a tiny charge when touched -- into your shoe. A connected module collects the voltage created every time you take a step and continuously powers up a rechargeable AA battery. (It takes a lot of walking to get a full charge, but it's perfect for reviving or topping off a gadget.) Once the battery is charged, put it into a DIY five-volt converter, and plug in your dead MP3 player. Now you can listen to the guitar solo while you walk some more juice into another battery.
Time: 6 hours
(1) Energy Harvesting Module (Mouser #585-EH300; $49.83)
(1) 1.25mm 2-position receptacle (Mouser #798-DF13-2S-1.25C; $0.29)
(1) 1.25mm 4-position receptacle (Mouser #798-DF13-4S-1.25C; $0.28)
(6) 1.25mm female contacts (Mouser #798-DF132630SCF; $0.78)
(1) Piezoelectric buzzer element (DigiKey #102-1170-ND; $1.62)
(1) 5V DC-DC step up (SparkFun Electronics #PRT-08248; $10.95)
(1) USB type-A receptacle (DigiKey #ED90065-ND; $1.34)
(1) AA battery holder (Mouser #12BH311-R; $0.69)
(1) AA NiCd 900mAh battery (DigiKey #SY116-ND; $2.33)
(1) USB enclosure (Mouser #789-P3A-210603; $2.10)
(1) Project box (Mouser #546-1551HTBU; $1.91)
(1) Plastic pocket clip (Mouser #546-1599CLIPGY; $1.00)
1. Download the wiring diagrams. Build the AA battery charger [A]. Connect the piezoelectric device [B] to the energy-harvesting-module input.
2. Mount the battery holder on the project box. Connect the battery holder's terminals to the energy-harvesting-module output, and install the module in the box.
3. Test the battery charger by tapping on the piezoelectric device and measuring the voltage output. Then measure the energy-harvesting module's voltage output.
4. Adapt the converter [C] for USB output. Snip off its plastic connector. Solder the wires from the circuit board to the USB receptacle's pins. Install the converter inside the USB enclosure. Test the converter with a fully charged AA battery.
5. Put a rechargeable nickel-cadmium battery [D] into the charger, and get a move on.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.