3V (2 AAA batteries) to USB

Hey I was wondering if I can get some help from some of you tech junkies out there. I have one of those new Cortana figure from the new Halo 3 figures by McFarlane toys. So it has a base which has three blue LEDs that is powered by 2 AAA batteries and what I want to do is make this base USB powered. I have a USB cable which I took my old Dell optical mouse, I know that I don’t need the green and white wires and the only two wires I need is the red and black but there’s also a slightly ticker black wire which I’m thinking I don’t need because I’m thinking it’s the shield twisted together. I’m thinking I can just simply connect the two ground wires together same goes with the postive wires. But I know USB gives out 5V while the 2 AAA batteries give out 3V, so here’s what I need some help on. Do I need any sort of regulator or resistors to convert 5V down to 3V, and if so what kind of regulator or resistor will I need and how would it be connected? Or can I simply just go without any regulator or resistor? Any help will be gladly appreciated and thanks in advance.

Does it have to be USB powered? You might be better off getting a AC adaptor that can output 3v instead.

But to answer your question, you probably can do it, I would use a voltage regulator otherwise you’d most likely blow the LED’s, but seeing as the voltage is so close…

…if you know exactly how many LED’s are in the figure (or you can work out how it’s wired up, you could use one of the many LED calculators on the internet and use a resistor.

Or you could be both chinsy and safe and use three diodes in a row. Each drops the voltage about .6v, so you’ve be giving 3.2v to Cortana.

Or if you want to be kinda cool about it, get a potentiometer (5k ohm or 10k ohm I’d say off the cuff. The more turns, the easier and smoother you could adjust things.) and install it in the base somewhere so you could dial the brightness as you like. If you dial it all of the way down, it wont really be any different that just wiring it straight up, but the ability to adjust brightness would be cool.

A voltage divider will do the trick. Pics of the ‘cortana figure’ in action would help.

Two resistors. USB can supply up to 500 mA, we’ll stick to 100 mA max.

For 20 mA, we have 250 ohms total. Use a 100 ohm and 150 ohm resistor. Connect as follows:

USB 5V+ — 100 ohm —\cortant/— 150 ohm — USB GND-

where the + from the cortana gets short cicuited to \cortana/
and - from cortana goes go USB GND-.

If you want it at full brightness, use 82 ohm(or close) instead of 100 and 220 instead of 150.

hey thanks for the help and suggestions everyone. well for those who are curious why i want it to be usb powered is because i figure it’ll probably the most easiest and cheapest way to mod it so i dont have to keep on buying new batteries. sorry i can’t get pics of the base itself but i’ll describe the wiring of the LEDS. the LEDs are not wired in either series or parallel. basically each LED has a red wire (+) and a black wire (-). all three black wires just connect directly to negative side battery compartment. all three red wires connect to a on/off switch then a single wire goes out to the positive side of the battery compartment. so here’s a pic it’s not my pic though but you can get a sense of what it looks like. oh yeah the LEDs are place so they’re almost shining up her butt crack, i’m not lying either.

Believe it or not, that is actually a parallel circuit you got there.

Blue LED’s can be anywhere up to 4.5v voltage drop on each LED (that’s for high intensity), so by using the calculator here and working on that voltage, you will need a 10 ohm resistor.

If you want to be safe for starters (because we’re not sure what LED’s are installed), work on 2.2v for the voltage drop on each LED. That will show you need a 47 ohm resistor.

Exactly, the are wired in parallel. It doesn’t matter since you only have to worry about feeding voltage to the device.

Two AA batteries can run up to 3.5 volts combined.

If you have a multimeter and can measure the current that the figurine uses on two batteries, I can tell you the exact value for a single resistor.

You’ll need to wire it up as:
USB(+ red) --> resistor --> switch on figurine
USB(- black) --> negative wire on figurine

If the thing is using 20 mA total, the resistor needs to be 50 ohms per volt. So to drop from 5 volts to 3.5 volts would require a 75 ohm 1/4watt resistor. You can temporarily try a 22 ohm, 47 ohm, and 100 ohm resistor to see which matches the brightness best.