Here is a set of free plans and instructions: http://www.woodworkingcorner.com/feeder.php
I used these plans as more of a guide and changed some of the dimensions to suit my needs, mainly making the entire feeder wider to accommodate more chickens at the same time.
The idea is that the chicken(s) must step on the treadle which then opens the lid exposing the feed. Wild birds aren't heavy enough to open it. With the linkage at its current setting, it takes about 2-1/2 lbs. to open. That is a very large russet potato (from last years garden) on the treadle, it probably weighs about 1-1/2 lbs which isn't enough to open it, but with two of them (about 3 lbs.) it opens.
I built it out of some left over half inch plywood and cedar fence boards. I love using the 1x6" dog-eared cedar fence boards cause they are super light and naturally weather resistant, and they are pretty cheap at $1.50 or so each at Home Depot. Since I usually only need a handful at a time I can sort through the stack and pick out some nice solid heart wood with sound knots. I will probably paint the plywood and stain the cedar.
Here is a view with the roof lid off. I made the feed compartment angled so that it would push out toward the front. I will not finish the inside at all so that nothing contaminates the feed. If you are interested on the exact measurement or a plan, email me and I will send you a measured drawing.