I wonder how long the relatively fragile electronics of a scanner will last in the semi-protected environment of a chicken coop. I'd think that chicken manure will contain (precursors of) corrosive chemicals. The mechanism of a scanner is definitely not constructed with this use in mind.

The difficult bit is usually to take all the corner cases into account; e.g. if some straw gets stuck in the door and prevents it from closing. A simple program will power the motor until the closing switch trips, which in the aforementioned case would probably burn out the motor.

Using electronics in an extremely flammable environment like a chicken coop is something else to think about. One spark could be sufficient to burn the whole coop down.

And of course you'd want to construct the door in such a way that it would be impossible for a predator to pry open.

(BTW, cycling computers usually contain magnetic reed switches. Usually, the magnet is clipped to a spoke and the magnetic switch is placed on a leg of the front fork and connected to the cycling computer mount with a wire)

The delicacy of the scanner is an excellent point. And that's also a great point about how the real world setting of a chicken coop poses problems that the engineer might not consider.

I love the simplicity of your solution!! If we were going to build a coop door opener/closer, I think I'd definitely vote for that answer.

Part of the job of an engineer is to think of everything that pertains to solving the problem at hand. Of course we do not always succeed. But we do try and learn from our mistakes (see FMEA).

But the method for solving problems is in principle not difficult. It basically amounts to using logic and common sense, combined with a knowledge of the physical laws of nature;

• Define the problem you are trying to solve. (for the coop it could be "prevent predators from entering the coop")
• Gather possible solutions. This is where you'll need domain-specific knowlegde. In this case you'll need to know about the abilities of chickens and their local predators. (E.g. Chickens might be able to "fly" to an opening high enough so that predators cannot reach it. Surround the face of the coop around that opening with a smooth and slippery material so predators cannot climb in) Do not hesitate to look for solutions elsewhere. Engineers rejoice in re-using solutions. Laziness is a virtue here.
• Grade the solutions. How this is done depends on the situation. Effectiveness, ease of implementation and cost are usually factors in the decision. An important factor is the KISS principle. Simple solutions are generally better than complex ones. IN this case you could translate that into; if possible, go for a solution without moving parts. (E.g. a U-turn maze entrance just big enough for a chicken will keep out larger predators.)
• Implement the chosen solution. When (generally not if ) difficulties arise, you might need to revisit the previous steps. Engineering tends to be an iterative process.