user8762: I have an Arduino Uno (a SainSmart model, actually). I intend to power it with three 1.5V batteries. I measured them and they yield 4.67V in total. The Arduino requires 5V.
The question is, if the arduino will work this way.
I'm not powering complicated microcontroller components here. These components are used:

A buzzer
An LED
A unit of 4 seven segment displays
A couple of buttons as input elements

If the volume or brightness of these components is slightly decreased, it doesn't matter at all. But I would like to know if it will work in the first place.
And also...
Where exactly do I plug the power source in? The power jack, the USB jack, pins, somewhere else?

The schematic looks like this, just to illustrate it. I'm not a good drawer, but you'll get the idea...


user23096: You can power the Arduino via the 5V pin - but you need make sure you disconnect it when you are connected via USB - ie. don't power both the 5V pin and via the USB.
Any even better way is to power it from the USB socket - I use those emergency Phone battery packs, but you could cut up a usb cable and use that - this is a much better socket fit to use than the 5v pin and more energy efficient than passing a higher voltage through the 9V supply.

user6740: As per @Majenko's answer, yes, this will work fine.
Some caveats: the Atmel chip at the center of the Arduino has brown-out detection - if it drops below a certain voltage, the chip will shut down. This voltage is selectable: 4.3v, 2.7v, 1.8v, or off. The factory default for the chip is "off", I found a reference that the Uno sets this at 2.7V.
Also, if you are powering anything else with your battery, e.g. a motor, the voltage is will drop - if it drops below the brownout, the arduino will shut down (usually also shutting down the external device, causing the voltage to rise again, causing it to power up again). An LED (with a correct resistor) should be fine.

user26717: For battery-operated Arduino Uno's I recommend thinking bigger (in terms of power, not physical size).  
I power several by using 3x 18650 batteries in series, (providing about 14 Volts, connected to a 4x USB 5 Volt module.  
The Unos are connected to power by a standard USB cable (into the programming port), and then you also have 3 other 5V USB charging or power ports available.
Or you can get a small converter unit that provides just one 5V USB.
Either way, bringing the power to the UNO is as simple as using the programming cable that comes with the board.
Or you can use any other converter and use wires to bring in Vin and GND.  
You don't need to use 3 of the 18650s.  2 will do fine.  Or 4 in series/parallel for longer battery life.  No converter would be needed but again you would need wires to bring the voltage into the Unos.
18650s are very common, and are inexpensive.  They pack a whole lot more mAh than any AA battery you will find.  
One warning about buying 18650s:  There are Chinese companies that advertise 9,800 mA batteries.  Don't believe it.  3,000 is about the max any reputable manufacturer will claim.  

user4143: The Arduino doesn't require 5V.
The Atmel chip on the board requires somewhere between 1.8V and 5.5V, the Arduino just happens to have chosen 5V as a good common widely used voltage to run at.
If you provide a voltage within that range to the +5V pin the board will operate.  There's a couple of caveats though:

Anything below about 4V will mean the 3.3V regulator won't be working right, so the 3.3V pin won't be functioning properly
At lower voltages the Atmel chip can't operate reliably at higher frequencies, which is why 3.3V Arduino boards operate at 8MHz instead of 16Mhz.

So 3 x AA batteries is a perfect power source for running the Arduino, direct into the 5V pin.  Just make sure that you don't power it any other way at the same time or you'll be pumping 5V into your batteries.  You may want to add a Schottky diode in series with your battery + (between battery + and the 5V pin) to stop that ever causing a problem.  Choose one with a very low forward voltage drop though, otherwise you could end up dropping too much of your precious voltage over the diode.
By the way, when you're powering it from USB the 5V could be anywhere between 4.75V and 5.25V.

8762: <p>I have an <strong>Arduino Uno</strong> <em>(a SainSmart model, actually)</em>. I intend to power it with three 1.5V batteries. I measured them and they yield 4.67V in total. The Arduino requires 5V.</p>
<h1>The question is, if the arduino will work this way.</h1>
<p>I'm not powering complicated microcontroller components here. These components are used:</p>
<ul>
<li>A buzzer</li>
<li>An LED</li>
<li>A unit of 4 seven segment displays</li>
<li>A couple of buttons as input elements</li>
</ul>
<p>If the volume or brightness of these components is slightly decreased, it doesn't matter at all. But I would like to know if it will work in the first place.</p>
<h1>And also...</h1>
<p>Where exactly do I plug the power source in? The power jack, the USB jack, pins, somewhere else?</p>
<hr />
<p><strong>The schematic looks like this</strong>, just to illustrate it. I'm not a good drawer, but you'll get the idea...</p>
<p><a href="https://i.stack.imgur.com/6OazX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6OazX.png" alt="schematic" /></a></p>
 23096: <p>You can power the Arduino via the 5V pin - but you need make sure you disconnect it when you are connected via USB - ie. don't power both the 5V pin and via the USB.</p>

<p>Any even better way is to power it from the USB socket - I use those emergency Phone battery packs, but you could cut up a usb cable and use that - this is a much better socket fit to use than the 5v pin and more energy efficient than passing a higher voltage through the 9V supply.</p>
 6740: <p>As per @Majenko's answer, yes, this will work fine.</p>

<p>Some caveats: the Atmel chip at the center of the Arduino has brown-out detection - if it drops below a certain voltage, the chip will shut down. This voltage is selectable: 4.3v, 2.7v, 1.8v, or off. The factory default for the chip is "off", I found a reference that the Uno sets this at 2.7V.</p>

<p>Also, if you are powering anything else with your battery, e.g. a motor, the voltage is will drop - if it drops below the brownout, the arduino will shut down (usually also shutting down the external device, causing the voltage to rise again, causing it to power up again). An LED (with a correct resistor) should be fine.</p>
 26717: <p>For battery-operated Arduino Uno's I recommend thinking bigger (in terms of power, not physical size).  </p>

<p>I power several by using 3x 18650 batteries in series, (providing about 14 Volts, connected to a 4x USB 5 Volt module.  </p>

<p>The Unos are connected to power by a standard USB cable (into the programming port), and then you also have 3 other 5V USB charging or power ports available.</p>

<p>Or you can get a small converter unit that provides just one 5V USB.</p>

<p>Either way, bringing the power to the UNO is as simple as using the programming cable that comes with the board.</p>

<p>Or you can use any other converter and use wires to bring in Vin and GND.  </p>

<p>You don't need to use 3 of the 18650s.  2 will do fine.  Or 4 in series/parallel for longer battery life.  No converter would be needed but again you would need wires to bring the voltage into the Unos.</p>

<p>18650s are very common, and are inexpensive.  They pack a whole lot more mAh than any AA battery you will find.  </p>

<p>One warning about buying 18650s:  There are Chinese companies that advertise 9,800 mA batteries.  Don't believe it.  3,000 is about the max any reputable manufacturer will claim.  </p>
 4143: <p>The Arduino doesn't <em>require</em> 5V.</p>

<p>The Atmel chip on the board <em>requires</em> somewhere between 1.8V and 5.5V, the Arduino just happens to have chosen 5V as a good common widely used voltage to run at.</p>

<p>If you provide a voltage within that range to the +5V pin the board will operate.  There's a couple of caveats though:</p>

<ul>
<li>Anything below about 4V will mean the 3.3V regulator won't be working right, so the 3.3V pin won't be functioning properly</li>
<li>At lower voltages the Atmel chip can't operate reliably at higher frequencies, which is why 3.3V Arduino boards operate at 8MHz instead of 16Mhz.</li>
</ul>

<p>So 3 x AA batteries is a perfect power source for running the Arduino, direct into the 5V pin.  Just make sure that you don't power it any other way at the same time or you'll be pumping 5V into your batteries.  You may want to add a Schottky diode in series with your battery + (between battery + and the 5V pin) to stop that ever causing a problem.  Choose one with a very low forward voltage drop though, otherwise you could end up dropping too much of your precious voltage over the diode.</p>

<p>By the way, when you're powering it from USB the 5V could be anywhere between 4.75V and 5.25V.</p>
