Powering a device via USB

November 30, 2006 12:27 PM

Instead of powering a device with 2 AA batteries, I'm interested in creating something that would allow it to draw power via USB. Is this possible?

My friend and I are looking into the possibility of powering a device by a USB cable that would normally use 2 AA batteries. He knows more about electronics than I do but we both want to be damn sure that we don't fry the device or us, so we are asking around.

I imagine something like this would be possible but we both aren't exactly sure how. A shove in the right direction would be appreciated.

posted by Diskeater to technology (17 comments total)

Maybe you could adapt one of the batteries they sell at www.usbcell.com? (These are batteries in various sizes that twist apart to reveal USB plugs, through which they recharge.)
posted by spaceman_spiff at 12:34 PM on November 30, 2006

What device is it? Are the 2 AA batteries in series? You may have to regulate the 5V USB down to 1.5 or 3V.

http://en.wikipedia.org/wiki/USB#Power_supply

It should work though. You certainly won't fry yourself. If you match the voltage with your device, you won't fry it either.
posted by bloggboy at 12:39 PM on November 30, 2006

Yes, this would be easy USB pin 1 is + five volts, and USB pin 4 is ground (Pins 2 and 3 are data). Just ignore pins 2 and 3, and you'll be good to go. A single AA battery is 1.5 volts. So the USB power would be equivilant to a little more then three batteires.

here is a pinout, with diagrams of the various connectors.
posted by delmoi at 12:40 PM on November 30, 2006

It should be pretty easy to find a 5v to 3.3 converter, which would be pretty close to the 3v two AAs would produce. Check Digi-key or mouser.
posted by frieze at 12:47 PM on November 30, 2006

As others have said, USB power is 5V, and 2 x AA in series will be 3V. You can use a regulator, something like an LM317 or LM338 adjustable regulator will do the trick if you can't find a fixed reg at the voltage you need.

You could probably even get away with a simple voltage divider circuit (just two resistors). The power from two batteries is pretty variable, so the thing you are powering won't be very fussy about the exact voltage it needs.
posted by markr at 1:00 PM on November 30, 2006

The issue here is current.

In USB 1.1, a device is always permitted to draw 100 milliamps. But if the hosting device says so, it can draw 500 milliamps. Determining this requires a protocol handshake.

If you draw 500 milliamps automatically, then if it's connected to a device which would tell you "no" if you asked, then you are at risk of causing damage to the hosting device.

And drawing more than 500 milliamps is always a big no-no.
posted by Steven C. Den Beste at 1:40 PM on November 30, 2006

One caution, the usb port will only provide 500ma max, which should be enough to power your device, but it is theoretically possible to damage your usb port by trying to draw too much current. It's a good idea therefore to measure the amount of current draw by the circuit when connected to the batteries. If it's drawing more or around 500ma, using a current limiting resistor in series will protect you.
posted by puppy kuddles at 1:42 PM on November 30, 2006

Thanks for all the great answers so far. Keep em coming!

The idea here is to essentially create a 'wired' Wii remote control. Between myself and my roommate playing Zelda for the past week and half, we are already on our third set of batteries. Since the Wii has USB ports already, my friend and I figured that we could rig up a 'battery pack' for a remote to draw power thru the USB ports if you don't really need the remotes to be wireless. Say...for playing Zelda or games on the VC.

It looks like the batteries are in series, which brings me to my next question. Should we pump 3v thru one of the battery contacts or should we try and pump 1.5v thru both of the battery contacts?
posted by Diskeater at 1:50 PM on November 30, 2006

Okay, sounds like you guys are new at this. A couple suggestions:

1. Google up a diagram of the USB pin outs. Know your polarities. If you fry it, it will be because you put the + where the - should be.

2. Google up info on basic soldering and attempt a similiar project with much lower stakes than this one first. Buy a soldering iron beginners kit from radio shack. You can also get a book or begining electronics from the library.

3. First build a cable from a pack of batteries to the USB. You do not want to mess with real power coming from the outlet (thats where your pc gets power you kn0w)

4. Test this cable attempting to power the USB slot. The USB excepts 5volts. Not 3v. If the remote can be powered by USB (if this is speculation - then stop right now) you need 5V.

5. Assuming your test rig works, pushing out 5volts then attempt doing it live with the computer.

Honestly, if youre asking such basic questions I suggest you not attempt this. You may end up with a bricked PC and a bricked controller. Chargable batteries are safer and smarter and will cost as much as your soldering iron.

Or you can save your pennies and buy this when it comes out.

posted by damn dirty ape at 2:49 PM on November 30, 2006

Considering a Wiimote costs like, 40 bucks, a soldering iron much more, and your computer power supply at least 40 (not to mention your motherboard!), I wouldn't attempt this. A set of rechargeable batteries that you juice up every night would be much cheaper.
posted by muddgirl at 3:04 PM on November 30, 2006

And I forgot about the multimeter, to make sure the current draw from the Wiimote is never higher than 100 milliamps or so.
posted by muddgirl at 3:10 PM on November 30, 2006

I can appreciate the desire to want to DIY, but I think you're making a mountain out of a molehill. A few sets of high-capacity rechargable AAs and a good fast charger will be a better solution. Plus, although you're using the Wii a lot right now, how long are you really going to be using it this heavily? At least the charger will let you use the batteries for other things later.
posted by Kadin2048 at 3:11 PM on November 30, 2006

I recommend a USB battery charger. I have one of these that I used to use at work when I had a cordless mouse there. Worked well for that application.
posted by krisjohn at 3:16 PM on November 30, 2006

I appreciate the concern from everyone. Yes, I do realize how basic these questions seem. In reality, we have a very good idea how we are going to accomplish this but due to the fact that wiimotes are hard to come by, we want to be *really* sure.

It's not simply a matter of not wanting to buy batteries. This is a fun project that would be phenomenal if we can pull it off.

We aren't going to solder anything to the wiimote itself. We are definitely going to test everything out to ensure that we don't destroy the wiimote or anything else. We also have miltimeters, etc already, so the only cost will be the cost of components.

Also, the Wii itself has two USB ports that may or may not be functional right now but we plan on using those to power our wired wiimote if we can.
posted by Diskeater at 3:21 PM on November 30, 2006

This might have product potential down the line. Apparently the Wiimote is designed to eventually be plugged into a recharging cradle through the port on the bottom that the nunchuck usually plugs into. If you could develop a USB-to-Wiimote-plug power cable (would need to be pass-through for the nunchuck) I bet you could sell it.
posted by Rock Steady at 4:02 PM on November 30, 2006

You could reverse-engineer this.
posted by DrJohnEvans at 4:55 PM on November 30, 2006

The Wii USB ports may permit you to draw 500 mA or may limit you to 100 mA. The only official way to know is to do the protocol handshake.

I am afraid I agree with most other posters here: you're trying for the wrong solution. You should be looking into rechargeable batteries.
posted by Steven C. Den Beste at 10:04 PM on November 30, 2006