Introduction: Replace the Battery on a Desktop Gadget With USB Power
I have a set of noise cancelling headphones at work. The active part does a nice job on the building ventilation and computer fans, and the passive part on my co-worker's phone conversations. I didn't like the idea of wasting batteries, though, and the system didn't seem to like even NiMH rechargeables. So, I decided to run them off the USB port.
The USB port can supply 5V, but I needed the ~1.5V that a single AAA will supply.
I have used this setup for about a year now, and it works fine. If I had to re-do it, I would make sure that the wire lenghts for the original gadget wire and the power/USB wire match so you can tape the boxes together. Same goes for the direction in which the wires exit the box.
Step 1: Figure Out the Circuit
I used an LM317 voltage regulator and two resistors (100ohm and 470ohm) to get the right voltage. I used the USB cable from an old PDA syncing station for the input. The breadboard shows the layout, I don't have a wiring diagram.
If you have not worked with a breadboard before, you stick the parts into the holes, and the holes are connected to each other in rows perpendicular to the long axis of the board.
Please see the image for further instructions.
Measure your USB cable to find the ground/"-" and the positive.
I took the picture from several different angles, but could not find one that really made things clear. I hope the image notes do.
Step 2: The "Battery"
Turns out a large hardwood dowel is just the right size to make up for a AAA battery, once you add a wood screw at each end as the battery terminals. I pre-drilled and screwed in a short screw all the way on the "-" side, holding down that wire. I then screwed in a longer screw on the "+" side and cut it off at about the right lenght. Final adjustments to length can be made by turning the cut-off screw in or out.
I only needed one battery for this project, but if you needed several, you could make dummy cells from a screw or nail that goes right through a dowel...
Step 3: Make It Permanent
You don't really need this last step, but I wanted to enclose the project and to have my breadboard back. I soldered the same circuit onto a piece of project board with the same layout as the breadboard. You could probably just solder things straight together if you don't have any project board around.
I used an old chewing gum tin lined with electrical tape where it matters and some rubber grommets from an old hard drive to protect the wires from sharp edges on the metal. .