Thank you both very much!

BTW, how come the regulator doesn't mind if you put 5v on it's output, when not powering the input?

Is there a way I can run the Uno from a 4.3v supply? I know the AVR can be run from 5v or 3.3v, at 16 or 8 mHz, but I haven't ever heard about a voltage between.

Here is the datasheet. Scroll down to the 5v one (1117 5.0).

Did you read my second to last reply? I said that the NXT can only supply 20 ma the way you are getting it. That is no where near enough!

Yes, a separate supply should solve the problem.

And about it working with USB... USB devices are usually allowed to pull 500 ma (IIRC), and up to 1A if properly configured.

Well, that makes sense then. The NXT sensor port pin 1 can only provide about 20ma, so you are way over-loading it. Hopefully you didn't burn out the transistor.

I agree with Runaway Pancake, but to be more specific, the Arduino 5v regulator can only supply 800ma (according to the datasheet). Many servos can pull 1.5A or more. You should also use a better 5v supply, as well as a fairly nice size capacitor on the power rails of the 5v supply (servos can be extremely noisy).

I'm curious though, how are you getting the 9v supply from the NXT? Did you wire directly into the battery compartment?

Okay thanks, but why doesn't it? The Arduino supports float type variables, so sprintf aught to as well.

How can I format floats using sprintf?

Thanks y'all!

That's more what I was looking for! I'll try it in practice, and see how it goes.

"5 - 9.4" = "n Volts", which is the voltage to be measured. ADC is the measurement point.

Back up a tad further in the first post:

...r2 and r3 can be whatever. I would probably go with 10k for each.

r1 is 10k...

...So assuming all resistors were 10k, how would I calculate the voltage of "n volts"?

Or are you saying I can count them in parallel, and then calculate them to be pulled down to the average of 4.7 and 0 (2.35 volts)?

r2 and r3 are not fixed, and can be any value.

That equation gives the value of r1 and r3 in parallel, which they are not. Doesn't the fact that r3 is pulled down 4.7 volts further than r1 make a difference?

[R1 is not optional!]

Okay, that all makes sense.

I don't mean to be rude in the slightest, but R1 is not optional! I have no choice of removing R1 from the equation, so I need to find an equation that takes all three resistors into effect. I would go without the equation before suggesting people open their NXTs and remove parts from the PCB... it just simply won't happen.

I really appreciate you all trying to help, but if what you all are saying is that there is no equation that any of you can come up with, I will look elsewhere for the answer.

Sorry, I thought I made the setup clear in the first post... The application isn't specific to NXT, it applies to any ADC with a fixed 10k pullup to a mid voltage.

So, I'm still not really sure exactly how to compute it. Do you think you could help me out a bit more?