Hi,
I know this may be a really basic/ stupid question but..
I'm using an arduino UNO board for my high altitude balloon project. Connected to the board will be a NTX2 radio transmitter and a GPS receiver. The UNO board says it operates at 5v and input between 7-12v. So what voltage should I supply to the board?
Thanks!
The lower the voltage the better (down to 7V) as that puts the least stress (i.e., heat) on the Arduino's voltage regulator. But for your project the current rating of the power supply is more important, as you have to make sure you can supply enough current for all of the things you are connecting (radio transmitter, GPS receiver, etc.) If you have a 7V 50mA power supply and a 12V 1A supply, I'd definitely take the 12V supply! See if you can research what the current consumption is of your transmitter and GPS receiver.
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Either a 5V regulated voltage to the +5V pin. Or a voltage anywhere between 7.5V and 12V to the Vin or power jack.
I need to use batteries for the project (lithium ion). So how many would I need to use? I can see that the are generaly 1.5v each, so i can add the voltages. But what is the current rating?
It depends on the particular cells you are using (though my guess is lithium ion batteries will be able to do just fine in your application). Do your batteries have any data available?
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I recommend a few 9v batteries in parallel to increase your mah or better yet 6 AA's or a rechargeable lithium ion matching your reqs. Check out this link for more info:
http://www.zbattery.com/seriesparallel-pf.html
For current draw just hook up a multimeter in series to your +ve when everything is running and you can see how much juice it uses at peak. I don't know what the inner resistance of the batteries you will be using is but you can at least see if it works for your needs by running this simple test.
As a rough example from one of my projects; I was able to successfully draw ~500ma from two 6v lamp batteries in series (i.e. a 12 volt battery). Your mileage may vary.
I recommend a few 9v batteries in parallel to increase your mah
Never put batteries in parallel, especially primary cells because they charge each other.
And when one goes faulty the other one starts a fire.
I'd go with Li Ion batteries (1.5V? I thought they were more like 3.7 when low and 4.2 when charged) and a low cost high efficiency switching regulator.
Get a pack that has some built in circuitry, like this 6-cell pack which has 3 pairs in parallel of 2 batteries in series.
http://www.tenergybattery.com/components/com_virtuemart/shop_image/product/1a766e33b0da6739e33c8d72a2ecda26.jpg
http://www.tenergybattery.com/ more options & chargers available also
is it possible to power arduino uno with one step-up circuit like this:
if I understood well, in order to use these kind of circuit with arduino, I must connect it directly to the 5V pin (and ground), bypassing the regulator (the step-up provides regulated power, right?)
how long (more or less) does an arduino uno (let's say with a blinking led) lasts with such kind of AA battery (or two AA batteries) + step-up circuit?
Yes. and Yes.
Check price for same at pololu.com also.
Duration will depend on how much else you have connected.
Generally, 30mA or so for the Arduino.
With NTX2 radio transmitter and a GPS receiver? Not enough info provided.
non-rechargeabe AA battery is good for 2200-2500mA or so.
Individual Li Ion, like 18650, good for 2200mA at 3.7V (check the tenergy website to make sure).
Do some ground testing first; batteries become less efficient at temperature extremes.
CrossRoads:
Yes. and Yes.
Good to know! 
CrossRoads:
Check price for same at pololu.com also.
Same price But thanks for the link to pololu.com (I'm a newbie, I NEED these links)
CrossRoads:
Duration will depend on how much else you have connected.
Generally, 30mA or so for the Arduino.
With NTX2 radio transmitter and a GPS receiver? Not enough info provided.non-rechargeabe AA battery is good for 2200-2500mA or so.
Individual Li Ion, like 18650, good for 2200mA at 3.7V (check the tenergy website to make sure).
Do some ground testing first; batteries become less efficient at temperature extremes.
I realize that I am not able to compute the duration of a battery, given its mah and current consumed by the load, when the battery is behind a step-up circuit. Is it the same? I mean, 2200mAh of a 1.2V battery stepped up to 5V will supply 2200mAh to a 30mA circuit? and thus last for 73.3 hours?
Thanks a lot!
Well 2200mAH x 1.2V = 2640mAVH, or 2640mWattHours. Divide by 5Vx30mA, 150mW, yields 17.6Hours
There's a switching efficiency factor in there too, so probably somewhat less. 90% maybe? so 15.8Hours.
Are you thinking NiCad, is that where the 1.2V is coming from?
CrossRoads:
Well 2200mAH x 1.2V = 2640mAVH, or 2640mWattHours. Divide by 5Vx30mA, 150mW, yields 17.6Hours
There's a switching efficiency factor in there too, so probably somewhat less. 90% maybe? so 15.8Hours.
Ok, it's clear... now I know.
CrossRoads:
Are you thinking NiCad, is that where the 1.2V is coming from?
Well, I'm a really newbie, so I was only thinking about "normal" batteries (those that can be bought in a supermarket). I wanted only to get rid of that big and heavy 6xAA (NiCd) pack that came with my first Arduino pack. And, moreover, I was wondering how to power my future projects, I think your advices about having a look at Tenergy website are very useful, I will study their products.
.. you may consider a MAX1724EZK50 - step-up converter, runs from Ubatt> 0.8V, max Iout=150mA (it depends on the Ubatt and L).
So when using one single 3.7V cell (ion, lipo):