is the middle terminal. Make sure both wires make good contact with the terminal when you tape them up.

4. Connect the second project wire to the output terminal of the LM7805 regulator. The output terminal is on the right.

5. Test the output. Insert a 9V battery into the holder and use the multimeter to test the output. It should read a steady 5V. Remove the battery after you’ve confirmed that the output works as expected.

6. Connect the ground wire from the regulator to the USB connector ground. This is pin 4 on the connector, the leftmost terminal when looking at the connector from the back, which connects to the leftmost terminal when looking into the plug, with the USB symbol face-up. The pins on the connector are small and delicate, so take care when making your connection with the electrical tape. 7. Connect the positive wire from the regulator to the USB connector. The positive wire connects to the rightmost terminal, pin 1, again looking at the connector from the back.

8. Wrap everything in electrical tape. Use electrical tape to create a finished presentation, wrapping the wires and the back of the USB connector. Do not, however, cover the IC portion of the voltage regulator. The regulator sheds its extra voltage as heat, so you should leave the flat box exposed to open air.

9. Try it out. Attach the 9V battery, plug in your favorite gadget, and watch in wonder as it recharges itself from the battery. Afterwards, detach the gadget and remove the battery.

You’re now ready to put your new, ready-to-travel USB power source and a 9V battery into your Altoids tin. If you want to get fancy, you can screw the voltage regulator to the tin itself to take advantage of the flat metal surface, which will work as a heat sink. Spare power is ready and waiting. And remember to always remove the battery when not in use, or you’ll drain it.

 

Erica Sadun has written, co-written, and contributed to over two dozen books about technology, particularly in the areas of programming, digital video, and digital photography.

Emergency USB Cable Charger On my last annual trek to the lovely Outer Banks in North Carolina, I managed to leave my cellphone charger at home. It was a 20-hour car ride, and turning back was not an option. Worse, my Danger Sidekick has an internal battery that’s good for only a couple of days, and I was going for a full week.

Lucky for me, our rest stop just south of Norfolk, Virg., was next to a RadioShack. I didn’t feel like paying for a charger that I would use for only a few days, so I decided to make my own simple USB cellphone charger cable for just a few dollars’ worth of mostly reusable parts. I shopped for the ingredients I needed, warded off the overly helpful employee with a counter-curse, paid for the bits of my potion, and was ready to go. I put it together in a few minutes.

This charger works for any phone that takes 5V to charge, which is common. Your phone’s wall charger will be labeled with the voltage it supplies.

 

MATERIALS

USB cable with USB-A plug Either A-to-A or A-to-B

Replacement Adaptaplug socket RadioShack Size H Adaptaplug RadioShack

Electrical tape

Nylon wire ties

Multimeter

1. Cut your USB cable, and separate and strip the wires on the male “A” plug end.

2. Plug the cable into your computer, being careful not to let the wires short. Use a multimeter to test pairs of wires and identify which are the +5V and which are the ground. (On my cable, red was +5V and black was the ground, according to the standard.) After you’ve identified the wires, unplug the cable. 3. Connect the replacement Adaptaplug socket to the USB cable, joining the TIP (the wire to the plug’s tip, with the printing on it) to the USB +5V and the base to the USB ground. Use a wire tie to prevent the wires from pulling apart and insulate any bare wire with electrical tape.

4. Plug the “rather expensive” ($4.99) “H” Adaptaplug into the socket, with TIP connected to the + side. 5. Plug the USB cable into your computer and check the voltage at the new plug with a multimeter. If you don’t get a clean, steady 5V, you may not want to risk your expensive cellphone by plugging this in.

Fortunately, mine was dead-on at 5V, so I went ahead and plugged in my Sidekick. I was rewarded with the battery-charging indicator. Before I crashed into bed, I made sure Energy Saver mode on my laptop was turned off, in order to keep the USB bus powered overnight. The charger worked perfectly for my entire trip and now lives tucked away in my computer bag just in case.