Check out the ready made motion detector/photocell units available at
places like HD. They have ones that are sold seperately from the
lights and include a photocell and motion detector all in one unit.
They can be set to give you 2/4/6 hours of illumination after sunset
and during that time you can have the lights on at 1/4 power if you
want. During the time they are at 1/4 power, or after they go out,
they will come on at full power if motion is detected. You can also
set how long you want them to stay on when motion is detected.