Can I use a PIR sensor to turn on an arduino? I'd like to use a PIR sensor (HC-SR501) to control the power to an Arduino Uno. Note that I'm not talking about using a PIR sensor as input to the Arduino and sensing the output pin from the PIR, I'm talking about using the output pin from the PIR to turn the Arduino itself on and off.

Hooking it up directly doesn't seem to work as I'm guessing the PIR doesn't handle that much current output. Would I use a transistor of some kind to do this?

My reason for wanting to do this is simple, power preservation. The PIR uses so little current when it doesn't sense anything that I thought it would be nice for something like a mailbox sensor that doesn't need to run that often. Thanks.
Yes you can.

It all depends on how you are turning the power on.  You sensor needs to flip a 'switch' rather than actually power the Arduino directly.  (I'm sure you know that but just in case).

So you need a normally open relay in between you power source and you Arduino.  Normally open because you don't want it powered unless you 'turn it on'.  Your relay might be able to be switched by your PIR sensor's data output, but if you need more current then you are going to have to use a transistor and draw more power from the power source.

You have the PIR sensor, so now find the relay capable of carrying the maximum current you need for the Arduino.