Hi, I am using an Arduino Uno with 2 micro servo motors and 2 PIR sensors for a robot that I am currently building. My question is will I need two seperate power supplies for this robot? Like maybe a 9V battery for the Arduino and PIR sensors, and a 4.8V rechargeable battery pack for the servos..? Or could I simply just use one power supply for this robot? And if so, what would be an appropriate single power supply? Also if you could give me a quick explanation on why so, that would be very much appreciated. 
Hi SMM2,
Yes I use a single supply for all my bots (See www.melsaunders.co.uk). But never use the arduino 5v line to drive other devices, it has limitted output and power. Make sure you connect all the grounds together.
For some time I have beed using 2x 3.7v lithium cells, suppling the 7.4v to the motors and the arduino Vin pin, are you using PIR's or U/S modules, not sure about PIR's but the U/S usually needs 5v, but little current so a small 5v reg should be OK.
Hope it helps, Regards.
Mel.