My girls got closed out once since I've had it. We'd gone out to dinner and there they were on their little chicken porch all gathered outside the closed door...lol. Thankfully my run is pretty daggone secure. But I always check on them every evening anyhow to do a head count, so even if one DID get shut out, I'd discover it within an hour or two. Mine is on a timer, so as Coloradomike said, you have to be sure and adjust the time as the days lengthen. Just today I changed mine to open at 6:50 am and close at 7:15 PM.
DH made mine from a drapery motor (can't remember the name of the company off the top of my head...but you can just google drapery motors). We bought our timer at Lowes. That was it. We just used the same door we'd been using before on our pulley system pop door. If you want more info, feel free to PM me. I do have a pic. of our set-up on my BYC page.