I do love your photography projects 
I built something similar, but instead of a relay, I used an IR LED to control my Nikon DSLR. Actually, right now the project is still on my workbench waiting to be assembled, but the code is pretty much done. I'm using a rotary encoder to set the interval and an 8x2 LCD for a display.
I'm not sure how much I'll use it though. 1440 exposures per minute of finished video seems like a lot of wear and tear on the mechanics of a very expensive camera body.
I get about 700 exposures from a battery charge so I'm not sure how far this project will get me without an external power source for the camera too.