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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.
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.
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Signing UpStep 1Figure 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.
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.
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Obviously it would not make sense to have the resistor parallel with that direct connection. If you have this set up on a breadboard, I would suggest trying to hook it up as described in the picture minus the direct connection.
One other option would be to download a spec sheet for an LM 317, mine came with the circuit and explanation.
Sorry to pass the buck. I will try to have a look at the real thing tomorrow at work. All I can tell you right now is it's still up and running fine.
As for frying your PC... better to be safe than sorry, but the USB protocol is supposed to be pretty self regulating as far as I know.
I think that circuit is right. Have a look at the spec sheet for the LM317 too, e.g. here.
As for the safety of a USB port, I really in the end don't know enough about them to be sure. However, wikipedia about USB non std devices looks reassuring. If the power to a port is limited to 500mA, that should take care of a short-circuit, should it not?