Do you want the alternation to happen automagically, or by manually operating a switch? If it's automatic, how quickly should the alternation happen? Does the duration of the on/off cycle need to be adjustable? Do you want the lights to "click" on and off, or fade in and out? If they fade, does the speed of the fade need to be adjustable? What kind of bulbs are you using, and what is this going to be used for?