> I think perhaps I was unclear.  The condition will be such that there is 
> always a load on the magnet.  All I have to do is counter the magnetic
> field enough to allow the trap door to open.  I don't need to push and hold, 
> just convince the magnet that it should no longer hold onto the core of the
> electromagnet.  And I only need to do that for a few tenths of a second;
> after that, the magnet will have moved away from the EM, and it can be
> powered down.
> 
> Picture a seesaw with a child at one end, up in the air, and a magnet at
> the other, stuck to a metal post in the ground.  Now make that metal
> post an electromagnet, and turn it on.  Make the field generated by the
> EM equal to or greater than the field from the magnet, and what happens?
> The child's end drops to the ground.  Now turn the EM off.  What happens?
> Nothing.  The magnet is too far from the post to be strong enough to pull
> the child back up.  Only instead of a child, I'm dumping a trapdoor with
> pig food behind it.

I knew what you were talking about Mike.  Just didn't know how much 
power was going to be required to reverse the polarity on your magnet. 
For all I know you have a 10 ft^2 door of 1 inch thick steel holding a 
ton of food.  ;-)  In that case, your magnet would have to be pretty 
tough and I would imagine it would require a bit of power to reverse it. 
  But then, I've never built such a circuit before, so I really couldn't 
say.

Best of luck either way.

Do try and keep us informed.  I'm sure some of the guys here would like 
to see pictures of your "project", when complete.

-Shawn
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