This is the most compact (so far) way of doing it:
First, get at least 4 redstone dust and one redstone comparator and a lever or redstone torch. Next, lay down three dust in an L shape. The final redstone dust will extend anywhere except the side with the lever. In the 4th corner of the L shape, put a comparator facing any way. After that, toggle the comparator so the redstone torch is on. Finally, place a lever behind it and power on.