Automatic door

[Altairsky]

I'm using run chicken. It's a simple door, very basic, which means the less complicated it is, the less parts there are to malfunction.
I changed the batteries twice and so far the door never failed once.
I change the batteries every 6 months, to be on the safe side.

 

[LuciferRoo]

Last night I got home late and somehow failed to close the little door the chickens use to get into their pen for the night when I tucked in all the birds. Luckily the girls were safe next morning, but as I'm getting to a forgetful age (74) it's probably time to add a failsafe door to the entry.
I'd like a door that that can be time programed to open and close, and that can be installed on the outside face of the existing pen frame, so I can continue to close the old hatch (located on the inside of frame) when I put the birds in for the night, but also have the security of the auto door closing on it's own a little later. Will any of the doors work this way (specifically the mounting option)?

I did that a few times and the girls were fine in the morning, but I also left one of the girls out one time and found her half eaten in the morning.
I had almost 20 chickens at the time and it was difficult to check who's in the coop, as the coop only has a chicken door and a ventilation spacing under the roofline.
I would peak through the ventilation space and check.
After that incident, I began to check all their outdoor hiding spaces before closing the door.
I did find a hen a couple of times trying to camp out.