While I was reading through this thread, Dub's solution got me thinking. While the 150 lbs actuating magnet might not be the most cost effective, an would certainly only be realistic with a pretty expensive shield, it does seem like the sort of solution that would result in a shield that would go nowhere when you didn't want it to and yet still move freely when you did.
So, in the spirit of true over-engineering, I started wondering if you could control the state of the actuating magnet with something like a solenoid to push or pull the lever.
The problem of proximity sensing could be solved by running a wire through glove that completes a circuit when it comes in contact with the shield. When the glove comes in contact with the shield the mechanism trips and flips the lever on the magswitch/actuating magnet. By only flipping the lever once per contact you could avoid the magnet returning to a default "on" and reattaching to the shield each time you tried to get it off. This also avoids making you walk around with a very powerful unshielded (no pun intended) magnet on your back.
The power consumption of the system would be very low, as really the only power expenditure is the device that flips the lever. The battery could be fairly small, yet still last for a while. And if worst comes to worst and the system dies, just do it by hand.
The problem with this system is that something like a solenoid would reset itself back to a default position. This means that even if you could get the shield off of your back before the magswitch turned back on, you would still be walking around with a very large, very powerful magnet on your back. As this could potentially pose a problem certain instances (whenever you are next to metal), this is a problem that would need to be resolved.
This is where I need some input for y'all. I know that there must be much easier and simpler ways of flipping a lever than a solenoid, but I'm drawing a blank on this one. Any thoughts on devices that fit the bill?
If you got something that had no default state you probably wouldn't even need a micro controller, and this solution seems to me like one of the best ways to replicate Cap's movie magic in real life.
