I've just built this intervalometer for my Canon 814XL Electronic, allowing me to shoot a single frame every 0.5 second, up to one every 60 seconds. I can also set the pulse width, which means I can shoot at 18fps for between 0.5sec - 5sec, then pause for 0.5sec - 60sec before shooting another burst.
Looking forward to giving it a go with some time lapse
Cheers,
David.