Instructables

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.
 
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Step 1: Figure out the Circuit

Picture of Figure out the Circuit
C:\Documents and Settings\All\Desktop\breadboard.JPG
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.
rabidy1 year ago
just used these instructions to power an mp3 player i use in my car, very useful.
agis685 years ago
of course u can do it without a dammy battery but its ok....thans
Roger_That5 years ago
I applied this instructable to a FM transmitter I have, and I love it! The only thing I changed, was that I hard wired the "battery", instead of creating a dummy. Thank you so much for writing this!
Maddkatt5 years ago
idk much about writing drivers or any programming for that fact, but you think it would be feasible to control this (or other usb toys like it) with the pc? where to find some kind of gui driver would be nice lol.
yanivn5 years ago
Question: according to this diagram, the Gnd and Vout are connected, so what is the point of the 470 ohm bridging them ? It's like the + battery connects to the 100ohm, no ? I want to get this clear before I hookup something that might fry my pc...
berserk (author)  yanivn5 years ago
Oi, that;s a long time ago! I suspect you found an error in my diagram.

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.
yanivn berserk5 years ago
Wow, thank you so much for that . Since my understanding or electronics is limited (i can follow a sketch and solder basically) I'm hoping you can confirm the sketch I sent (maybe update online). Also, in the worst case scenario, if I short something out, did you say it will not fry my motherboard and all the rest ? is there a fuse or protection somewhere ? Again, thank you for your time and reply.
plan.gif
berserk (author)  yanivn5 years ago
Hi Yanivn,
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?
yanivn berserk5 years ago
I guess so. But to be sure, I bought a voltmeter ... Thank you very much. Y
berserk (author)  yanivn5 years ago
Hey, If I gave someone an excuse to extend their tool collection, I have done my job. :-)
Padlock5 years ago
For those who decide to try this, remember a USB port can supply 500 mA at the most.
munchman5 years ago
I love the battery Idea.
nerdzilla6 years ago
w00t ALLER CANADA!
marquoise7 years ago
Good Instructable, The wooden dowel to emulate a battery is a great idea. I'd recommend taking the time to create some sort of circuit diagram, in case anyone unfamiliar to electronics wants to have a go at this. This would be ideal as a first electronics project, being simple AND very useful.
berserk (author)  marquoise7 years ago
Yes, I was being a bit lazy there. I added a diagram now, though it probably doesn't follow any conventions :-). Has anyone else noticed some of the instructable editing being really slow now? Thanks for the feedback!
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