Hi,
I would like to read a wide voltage range with my arduino. 6-45 volts, I need 0.1 volt accurncy, but the analog in only read 0 - 5 volts. How do I do this in the best way (using the least amount of components?)
Do I have to setup a number of voltage dividers? Or is there a better way of doing this? :-?
Do I have to setup a number of voltage dividers?
Basically yes.
You can have them switch if you want so you can alter the sensitivity from software but I would arrange a series resistance and some rail catching diodes in case of accidents to prevent more than 5V getting on the analogue inputs.
With a single 1/10 voltage divider you would get about 0.05V per step (50/1024) from the Arduino analog to digital converter. This should be sufficient towards a voltage reading with 1 decimal precision as per your requirement.
Thanks that will do the trick, of course, just a 1kohm resistor and a 9kohm resistor will work perfect. 
I would get 1% resistors or better (0.5%). In a voltage divider like that, 1% resistors give you about 1.8% worst-case accuracy (one is at max, the other is at min) and 1.8% of 45V is 0.81V. That's divided down, but still near the range of accuracy you're looking at.
Ahh, thanks for the input, I'm using 5% resistors now. Then I'll be changing to something with a better accuracy. 
Since you're feeding this voltage into an AD converter and microcontroller (Arduino), you can measure the exact resistance with a multimeter and calculate accordingly. If you mass produce it's a differenet ballgame (per unit calibration will be needed), but for a one-off it doesn't really matter whether resistors are 0.5% or 5%.
Once you have it built you can measure what you're reading and look at the difference the Arduino is reading and set up either a lookup table or a formula for scaling the results. Then you can use 5 percent resistors and save money.