Using h-bridge with LED fairy lights
An h-bridge provides the means to switch power, but not the control. With typical fairly light string, a simple small signal transistor based h-bridge would work as not alot of current is needed. Something like http://members.tripod.com/robomaniac_2001/id206.htm
Then you need a way to drive the h-bridge. A microcontroller or timer ic circuit like the 555 would work. There will be considerations for the led string resistor that you need to confirm based on the current draw.