For quick schematic sketches, I suggest the free ExpressSCH software. Its meant to be used with the free PCB layout software which is meant to be used to order quick turn PCBs.

The catch is that the format only works with their service, it is only really suited for the hobbyist. I use Eagle serious stuff but I still use ExpressSCH for quick sketches.

Back on topic, I suspect they use a CdS cell for the light sensor. The circuit shown by AFLM will work buy the LED will come on gradually as the ambient light dims. I would use a comparator with hysteresis so that it snaps on at some point. Actually, I'd use a little microcontroller that sleeps for most of the time and just wakes up to check the light level once in a while.

But that's just me.