It has a light sensor, so it assumes that once its dark enough, the chickens will have come inside of their own accord. Based on chickens I've met, this is probably accurate.

When I did it, I just closed the door at ~nautical~ civil twilight. The chickens come home to roost before that, and the predators (raccoons) generally were later than that.

My design was Raspberry PI -> sensor/power board -> battery powered drill. And an extra long usb cable to get the wifi dongle to the other side of the roost, because when the chickens were in the coop, they absorbed enough wifi signal to make it drop off the network.

Edit -- changed which twilight. Also, code here: https://github.com/wiredfool/coop-door

Unfortunately, schematics and physical door design are lost to several moves.

TIL chickens are wi-fi jammers

All sacks of meat, humans included, are to some degree. We're made of a lot of water.