Hey guys
So I just bought an Arduino Uno R3 for my science fair at my school. I'm a 7th grader. So I am making a voice controlled car with 4 "3.6-4.5v gear motors", L293D Motor driver, a sonar sensor,HC-05 Bluetooth module and a Tower Pro 9g servo. I have read from the Instructables that a 9v battery will fry the 5v voltage regulator in no time. So I want to use AA batteries as it is available in a cheap price and abundance. So can I use 5 AA batteries to power all the electronic parts with proper power and not harming the board? I am really concerned with this matter. Please help me. Should I use 5 AA batteries to power the WHOLE project?
"Instructables"? Wash your mouth out with soap. 
Maybe wash your mouse out after such profanity! 
Do not power the Arduino for an actual project via "Vin" or the "Barrel jack". The on-board regulator is not capable of supplying devices beyond the Arduino itself.
A four alkaline "AA" battery for the servos. Three "AA"s for the 4.5 V motor but I don't know whether the L293 is suitable to do that. You may get away with a diode in series with the 6 V supply to power the Arduino.
A 9v battery will not fry a voltage regulator. However if you try to draw too much current through the regulator it will overheat and shut down. The bigger the voltage drop the less current that may be drawn.
You should not draw current through the Arduino and out via the 5v pin the Vin pin for motors or servos or anything else that draws a significant current.
You can power an Uno from 3 x AA Alkaline cells (4.5v) or 4 x AA NiMh cells (4.8v) connected to the 5v pin. The same battery could probably also power your servos and motors. However if you find that you need the full 6v for the servos or motors you should power them from a separate battery with the battery GND connected to the Arduino GND.
There is another reason why it would be a good idea to have a separate battery for the servos and motors - if the separate battery is depleted by the motors it won't affect the working of the Arduino. If you only use one battery the motors may pull the voltage too low for the Arduino to function (maybe even for a millisecond) before the batteries are so depleted as to to be useless for the motor.
...R
KM_Ahnaf_Zamil:
I have read from the Instructables that a 9v battery will fry the 5v voltage regulator in no time.
Ah well, that's Instructables for you......
As stated before, "Instructables" doesn't often have accurate information, but sometimes I find good ideas there.
Here is the Arduino reference.
Look at the Arduino Uno Power Circuit:
The big box on the left is the 2.1mm Power Jack. Power from there goes through a blocking diode that prevents power applied from the Vin pin from flowing out of the 2.1mm power jack. The only difference from the 2.1mm Power Jack and Vin is the blocking diode. Meaning that you can power the Arduino within the specifications of the regulator. Just google the regulator part number and you should be able to find the specifications on the datasheet.
You can not fry the regulator if you stay within the regulator specs. The datasheet says the max input voltage for this regulator is 20V, but I would be conservative and not exceed 15V.
You can power something from the Vin pin as long as the current rating for the blocking diode is not exceeded. I don't know the specs of the diode are, but I would say safely 800ma, since that is the spec for the regulator output.
Using the 5V pin to power external devices is tricky. Remember that the regulator is limited to 800ma, and the Arduino itself needs a good chunk of that available current. If your downstream device is a relay, for example, that draws 750mA, you could easily exceed the capacity of the regulator.
In general,
Power the Uno from the 2.1mm Power Jack, or the Vin pin up to 20V. If you apply 5V to the 5V pin, it must be 5V. (The Arduino will work with 4.5V as stated above). Do not ever exceed 5V on the 5V pin or you will likely brick your Uno.
Power external devices separately. They should share a common ground with the Arduino.