hi! i am a beginner of arduino project, in my project, most of my component is using 12V, but UNO board is using 9V battery. and i have read the information about UNO in http://arduino.cc/en/Main/ArduinoBoardUno the Input Voltage (recommended) is 7-12V. so can i replace the 9 voltage battery with a 12V power source? is it going to affect my reading of sensors, such as pH sensor, temperature sensor, etc?
I went through the same sequence of powering the UNO as you are doing. First the USB connection, then the 9 volt battery, then 12 volts to the Vin pin. Works fine all the different ways of powering the board. Be sure to use the Vin pin!!!!!
Two options You can also use two different power source, good thing about micro-controllers and small devices like transistor is that they can drive high voltage devices with a very low voltage ( powered via arduino). What components are you using, the sensors you mentioned don't require that high a voltage.
I would be careful about the quality of the 12V source. If it's regulated that's cool, but some of these things put out way more than the nominal voltage under low load, so be sure you're not going to zap your Arduino with 20V when you thought it was never over 12.