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A couple wires, a diode, a 5v to 9v usb adapter, and you are in business. The reason I did this was to make a fish feeder work for at least a week. I was going away and didn't want anyone to be burden with fish feeding.

Supplies:
Usb cable for NXT brick (included in Mindstorm kit)
5v to 9v usb adapter
A 2 amp diode ( thank you Lan01)

Tools:
soldering iron and solder
wire (small size from radio shack)
Phillips screw driver
wire cutters and needle nose pliers

Step 1: Opening NXT

Take out batteries and the four small screws in each corner (Not now but in a future step you will need to pay attention to the little black button in the low middle of the picture). Turn over brick and pull on the white face plate. Might be a little tough but comes straight out.

The board is attached to the screen by 3 items. 1.) Two screws one on each side of the screen. 2.) The 10 solder spots. If you just pull up from that spot it should come right out but be careful because the side with the speaker has (3.) 2 wires that do not separate. So fold the screen over on its face. You can put something soft down so as if not to scratch it (as you can see in the third picture I did not. Nothing was scratched but better safe than sorry).

Step 2: Inside Case

Now its times to take off the two dark gray ends. The battery pack and the main board is attached by two points of solder but only on one side. In order to pull the pieces off you must pry open the board and battery pack at the opposite end they are attached. The two gray ends are held there by two small plastic pegs. If the gap is big enough you can slide them right out. Remember the little black button in the battery case. If you separate the board from the battery pack this will most likely fall out. This is fine it is really no big deal. Just make sure not to lose it. To put it back in later I just used needle nosed pliers.

Next take two wires cut them decently long and have the diode ready. If you can see from the picture I wedged a battery in between the board and battery pack to give me room to solder.
The bottom of the usb port has 4 major solder points. The two closest to the larger side is the power.
In the picture the wire with the knot is the + (do not put a knot in the wire but a black mark because you will be routing these two wires into a small hole).

After you solder the two points run the wires into one of the screw holes(same screw holes that hold the face plate to the battery pack) to the side of the board that has the screen on it .

Step 3: Final Stretch

Alright the wires should be sticking out of the screw hole closest to the usb port. Now stretch out the wires to go to the two battery ports on the edge of the board. The terminal on the left is + and the terminal on the right is -. Now keep in mind you have to solder the diode (Thank you LasVegas) to the positive wire. The diode has a indicator to show which way you want the power to go. You want the indicator pointing towards the battery terminal. After soldering I electrical taped where the wire met the diode. I next laid them out and cut to where the end of the diode met the battery terminal. The solder on the battery points are thick but if you are patient and let the solder melt. The diode will have no problem plying itself.

Step 4: Finishing Touches

The face plate needs a slight modification since one of the screw holes have wires in it. I just trimmed the one peg a little shorter with wire cutters and it fits together fine. Just make sure you cut the correct peg. In the picture it's the closest peg on the right that I cut.

Putting it back together is no big deal. Just put the correct dark gray pieces on their corresponding sides. The side with the soldered wire on the bottom might be a little tricky but be patient and if you made you solder and wire too exposed to fit the piece. Just take a deep breath, realize you are almost there, and redo it. After the sides it is time to put black the little black button. Now fold the screen back over and push it into it's 10 slots. Screw down the two screws that hold it to the board. Throw the face plate back on and screw it back together.

Step 5: The Moment of Truth

Take the 5v to 9v usb converter (http://www.seeingwithsound.com/usb_powersupply.htm) and plug it into your computer's usb port. Take the usb wire and plug one end into the converter and the other end into the NXT brick. With no batteries in it hit the start up button. Congrads!!!! You have a NXT brick that runs on batteries and usb power. If it did not work start trouble shooting. Open it up again and make sure the solders are good and tight. If that seems all good and well make sure the diode is correctly pointed towards the battery terminal.


sorry for the poor quality video
HEY YOU SHOULD ENTER THIS IN A CONTEST ITS GREAT!
1. NXT motors can draw up to 2A each when stalled. Make sure your diodes can handle it. 2. Can you still use the USB port for downloading programs?
USB only supplies 500mA at 5V. This means the 9V adapter probably supplies about 250mA (a quarter amp). The diodes have nothing to do with it. I wouldn't suggest trying to drive very much directly from the USB supply. Only 1 diode on the positive line is needed to prevent draining the batteries through the supply. The second diode on the ground line is redundant.
Everything works fine though. I've left on the brick and the lego fish feeder and works very well for the duration of the time I have left it on
As long as your motor doesn't stall, you may only be pulling under a quarter amp. One plus to this design is that you're using a third party 5v-9v adapter. The adapter is likely already current limiting, protecting the USB port. This might cause your feeder to run a little slow, but the computer's port is protected.
Big Question I'm trying to figure out how to do a similar idea. I want to be able to program and reprogram to fix bugs without constantly wearing down my limited supply of batteries. so i wanted a transformer to basically do the same job as the batteries did, I had a talk with my friend and he said a 9v 350mA would probably be fine, but the rechargeable battery from lego is 7.4v 1400mA so I'm kinda confused, help or suggestions?
the rechargable battery is only 7.4v but the nxt runs better of alkaline batterys as they supply 9v. the only reason that the rechargeables supply 7.4v is because each cell can only supply 1.2v vers 1.5 for the alkaline batterys
well thats a plus. I have a crappy video I'm putting up that shows it in action. Might be a better gauge for how it is working. Thank you though for your input. Greatly appreciated
speaking of the motors -- they are described as servos, but are are able to make full rotations like if i wanted to use them as wheels am i wrong ??? since the wiki says servos can only make small rotations and their primary use is stuff like robot arms... with only small turns here and there
The motors are basicly regular dc motors with a rotation sensor attached. You can program it to do unlimited full rotation, or to rotate by parts of a degree. Wiki is wrong in the fact that it can <strong>only</strong> do small turns. I know this because I won and use an NXT with its motors.'<em><strong></strong></em><br/>
i think you misunderstood. I was talking about the nxt wiki, saying that the motors are servos. I know they can do full turns, i have an nxt too. just sayin that the motors are not servos, while the nxt wiki describes them as such...
Yes i put in the right diode i was just unaware because i did the project atleast a month ago and wasn't sure. Thank you though I'll redo the instructions. The data has no problem going from the computer to the brick.
Couldn't you just attach the wires to the two termanals outside the nxt casing (where the batteries usually touch) rather than opening the expensive brick?
I've been looking to replace the battery with another energy source. Using the usb is ok, but I don't feel like soldering and opening up the nxt. Can we basically use an AC adapter to power it, i used my multimeter to test the current. It shows up around 125 mA. So can I just buy a 9V and 125 mA AC adapter to do it?
Why not make a bunch of potato batteries?If I remember right from my 2nd grade science project forever ago, 2 potato halves with a zinc plated door hinge with a thick piece of copper wire made close to 1.5 volts or 1 volt. They stay at the same voltage for weeks. I only tested it for 2 weeks. that would work too. amps were a problem though... And then there was the one minute-20 volt battery. That was fun..... Jk about the potatoes. It would work but, yours seems more effective and cheaper than that potato battery stuff.
due to the high price of the nxt, i would rather not open it up there is a 50$ rechargeable battery for it, or you could just wire it up with rechargeable batteries -- this does involve modding it, but the circuit board is untouched
You should post some Lego Mindstorm models i am going to soon when i get my set for Christmas
check out nxtlog at <a rel="nofollow" href="http://mindstorms.lego.com/NXTLOG/default.aspx">http://mindstorms.lego.com/NXTLOG/default.aspx</a><br/>
i finally got a Lego mindstorm & so far Ive built a cool car & a machine gun
I don't see the connection between this instructable and it's purpose, fish feeding. Do you have a fish feeder that runs off a NXT brick? I take a simpler approach to feeding fish when I'm on vacation. I purchase a 10-day food block and drop it into the tank - works every time and not affected by power outages, etc. But your approach looks like more fun.
Yes I do have a lego fish feeder or else the modification would be worthless. And I have done the brick food into the tank before but I was feeling saucy. So one night I threw this all together, hence the bad pictures and not so good directions.
I think i've seen your fish feeder on either NXTLog or in a SERVO Mag... Do you know which?
u have seen the exact one i made?
Pretty Sure

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