user639: The Xbee requires a 3.3V power input and the Arduino Pro Mini (5V version) only provides a regulated 5V.
Question: What will be the simplest way to provide a 3.3V rail to the XBee from the Arduino Pro Mini? 
If we use a voltage divider consisting of 2 resistors, the ratio should be 66/100, but how do we choose the resistor values?
If we choose a 3.3V voltage regulator, will the STMicroelectronics L78L33ACZ be suitable?

Between the voltage divider and voltage regulator, which is the preferred method? Is there a better alternative?

user1387: You can use a voltage regulator to get the supply voltage down to 3.3v.
For the Serial Lines, the best way forward is to use a good level convertor like https://www.sparkfun.com/products/11978 from sparkfun.
If you don't want to buy it, you can also make it using a couple of Mosfet's and resistors. Here's the schematic from Sparkfun.

user84: The simplest way, as you ask, is probably a resitor-based voltage divider; its main drawback is the waste of energy in the resistors.

To calculate the "best" resistor values, you'll need to know the amount of current needed by the XBee module.
So you have:

Vin = 5V
Vout = 3.3V
Iout = to be determined (check XBee spec sheet for that)

Then you have to resolve the following equations to calculate the resistor values you shall use:
R1 = R2 x (Vin - Vout) / Vout
Iout = (Vin - Vout) / R1 - Vout / R2

These equations simply come from applying Ohm's law and Kirchhoff's circuit laws.
This site calculates the ideal resistor values for you.
For Iout = 100mA, it gives the following values:

R1 = 17 Ohm
R2 = 33 Ohm

Another simple way is to use a 3.3V zener diode if you have one on hand. You can google for good circuit examples.

user58: This is part of a Fritzing circuit I used to control the x'mas tree lights (the relays and sensors have been snipped out) with an XBee. This circuit uses a 3.3v LDO you may have to change the capacitor values depending on your regulator specs. I chose to use a separate 5V regulator (not shown) but it will work off the Pro Mini's +5V power rail.
There are two FETs that do the actual level shifting, the bonus transistor allows you to reset the Arduino and thus gives you the ability to program it over the air via the XBee (beyond the scope of the question, but if you want to go wireless, go wireless all the way - do the Google for wireless programming over XBee if needed). Note the 0.1uF across the XBee power pins, I recommend this capacitor be placed as close to the XBee as possible to minimize noise (I solder it across the back of the XBee interface board).


639: <p>The Xbee requires a 3.3V power input and the Arduino Pro Mini (5V version) only provides a regulated 5V.</p>

<p><strong>Question:</strong> What will be the simplest way to provide a 3.3V rail to the XBee from the Arduino Pro Mini? </p>

<p>If we use a voltage divider consisting of 2 resistors, the ratio should be 66/100, but how do we choose the resistor values?</p>

<p>If we choose a 3.3V voltage regulator, will the <a href="http://www.st.com/web/en/resource/technical/document/datasheet/CD00000446.pdf" rel="nofollow noreferrer">STMicroelectronics L78L33ACZ</a> be suitable?</p>

<p><img src="https://i.stack.imgur.com/3nr5L.png" alt="enter image description here"></p>

<p>Between the voltage divider and voltage regulator, which is the preferred method? Is there a better alternative?</p>
 1387: <p>You can use a voltage regulator to get the supply voltage down to 3.3v.</p>

<p>For the Serial Lines, the best way forward is to use a good level convertor like <a href="https://www.sparkfun.com/products/11978" rel="nofollow">https://www.sparkfun.com/products/11978</a> from sparkfun.</p>

<p>If you don't want to buy it, you can also make it using a couple of Mosfet's and resistors. Here's the <a href="http://dlnmh9ip6v2uc.cloudfront.net/datasheets/BreakoutBoards/Logic_Level_Converter.pdf" rel="nofollow">schematic</a> from Sparkfun.</p>
 84: <p><strong>The simplest way</strong>, as you ask, is probably a resitor-based <a href="http://en.wikipedia.org/wiki/Voltage_divider" rel="nofollow noreferrer">voltage divider</a>; its main drawback is the waste of energy in the resistors.</p>

<p><img src="https://i.stack.imgur.com/gWyVW.png" alt="enter image description here"></p>

<p>To calculate the "best" resistor values, you'll need to know the amount of <strong>current</strong> needed by the XBee module.</p>

<p>So you have:</p>

<ul>
<li><code>Vin</code> = <strong>5V</strong></li>
<li><code>Vout</code> = <strong>3.3V</strong></li>
<li><code>Iout</code> = to be determined (check XBee spec sheet for that)</li>
</ul>

<p>Then you have to resolve the following equations to calculate the resistor values you shall use:</p>

<pre><code>R1 = R2 x (Vin - Vout) / Vout
Iout = (Vin - Vout) / R1 - Vout / R2
</code></pre>

<p>These equations simply come from applying Ohm's law and Kirchhoff's circuit laws.</p>

<p>This <a href="http://www.power-io.com/library/appnotes/voltage-divider.htm" rel="nofollow noreferrer">site calculates the ideal resistor values</a> for you.</p>

<p>For <code>Iout</code> = <strong>100mA</strong>, it gives the following values:</p>

<ul>
<li><code>R1 = 17 Ohm</code></li>
<li><code>R2 = 33 Ohm</code></li>
</ul>

<p>Another <strong>simple way</strong> is to use a <strong>3.3V zener diode</strong> if you have one on hand. You can google for good circuit examples.</p>
 58: <p>This is part of a Fritzing circuit I used to control the x'mas tree lights (the relays and sensors have been snipped out) with an XBee. This circuit uses a 3.3v LDO you may have to change the capacitor values depending on your regulator specs. I chose to use a separate 5V regulator (not shown) but it will work off the Pro Mini's +5V power rail.</p>

<p>There are two FETs that do the actual level shifting, the bonus transistor allows you to reset the Arduino and thus gives you the ability to program it over the air via the XBee (beyond the scope of the question, but if you want to go wireless, go wireless all the way - do the Google for wireless programming over XBee if needed). Note the 0.1uF across the XBee power pins, I recommend this capacitor be placed as close to the XBee as possible to minimize noise (I solder it across the back of the XBee interface board).</p>

<p><img src="https://i.stack.imgur.com/PzjWI.jpg" alt="enter image description here"></p>
