After losing 3 hens to raccoons/possums on 3 occasions, two which were break in's to the hen house, I decided to design a better coop door.
For a while, I was working on a threaded rod design, but I ran into complications due to wobbling of the rod and that affecting the whole system. Basically, I had to attach the motor rod to the threaded rod. I did this with a weld, but there was enough wobble for it not to work well. I'll probably just use that motor/rod setup for another auto door which uses the winding of twine on the rod. That is my current design, which is similar to my original.
The difference with this design is that the door closes like a guillotine and rests in a frame or sleeve, if you will. That way, there are no door edges or cracks for a predator to work the door open from. It would have to either pull the cord up to open the door, and also manage to get inside, or actually slide the door up with paw pressure and lift from the front of the door. Both ideas I think are too complex for a raccoon/possum.
This door stays with my original intention of economy and practicality, and I think its a lot safer than the top hinged door I had before.
As I write this, I just thought of an alternative design, which would be a bottom hinged door that would open like a draw bridge. The twine would be mounted on the inside top (when closed) corners of the door and then feed through to a single winding spool controlled by a motor and identical timer system I have now. This way, a predator would not be able to pull open the door that is essentially cinched shut by the twine and held fixed by the motor. I suppose keeping twine out of the way of the hens would be an issue, even if its fixed on the corners of the drawbridge.
Anyway, I am gonna roll with the guillotine design for now and see if any complications arise. The door/timer/mechanism is now even more out of the weather, which I like as well. I just took a photo after completion so some wires still need to be tidied up.
If anyone is interested, the motor cost about 12 bucks on ebay, two switches cost about $4 a piece, a double pole double throw relay cost 10 bucks. I got the two timers at Goodwill for less than $3 a piece. The wood and other materials are all reused from other projects.