My Pi-Controlled LED Strip Porch Light Project

I bought these online for about $25.

5 meter, 18 watt LED strip and 30 watt 12 volt power supply

I plan to stick the LED strip under the outer edge of the soffit. These things are surprisingly bright. And the length is just right to span my deck, the front door and the steps up on either side. I also ordered (but haven't gotten yet) a new PIR motion detector that is small enough to fit inside a 1" PVC pipe ($1!). I need to recess the PIR to limit its angle of view (otherwise it would be constantly tripped by blowing plants or varmints). The detector will be pointed so it covers someone standing in by the front door. In simplified terms my Pi program will work like this:

Only if it's dark:
  Test the PIR every 3 seconds
  If the PIR has detected motion:
    Turn on the LEDs for at least 30 seconds, continue if there is more motion
         
There is a handy soffit vent to bring out the PIR sensor and 12v for the LEDs. This vent is only about 12' from my Pi. I plan to run 5 22 gauge wires, 3 for the PIR (5v, ground and 3.3v signal) + 2 12v for the LEDs.

As an over-ride to my program sketched above, I have already programmed a web interface that will turn the lights on/off from my iPhone.