Exactly what psycho described it saves on equipment you can have 2 flower rooms running with 1 ballast the flipflop flips the HPS Bulbs over using relay or contactors with a timer. You need a special type relay that can handle at least the inrush or inductive load at startup for 1000 watt HPS lamps. I know I could build this but i dont know what type of relay the retailers use. And I dont want to spend 400 dollars when this could be built for 150 to 200.................