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The VSB automatic door openers seem to be the most popular and from what I have read appear to be very reliable. There is more information on them including reviews on the Wells Poultry site. You do need a vertical operating door with sufficient head height as they aren't designed to operate sideways. I made my own and wouldn't be without it. You might want to consider putting an indicator on your coop somehwere to identify when the door is closed from a distance. An LED light would be good idea as they don't consume much power. This would prevent you having to trail accross your field in the darkness in order to check if the door has closed.

A cycle reflector or hi vis strip on a stick which drops behind something when closed works well (like a railway signal) consumes no power you just need a maglite torch or similar powerful torch light.

HF