Dad built one when I was a kid. I think he got plans from the county extension agent. It is very similar to this one http://www.agriculture.gov.sk.ca/S-691
Electric motor turned a set of paddles inside a rabbit wire drum. You used a handle to turn the drum in the opposite direction. Whole works was inside a screen wire covered cabinet with a box at the bottom to catch the peas. You couldn't do very many at a time or the paddles would jam against the drum. Also the rabbit wire drum let a lot of chaff through so your peas were pretty dirty. We would pour them down a rigged up screen wire shoot with a squirell cage fan blowing thru it and that would blow most of the chaff away. Also it didn't seem to work very good with peas fresh from the field we would spread them out on newspaper on the concrete floor of the barn for a couple days before shelling. We used it a little but it was probably faster to shell by hand. I know I spent a lot of time during the heat of the day in late July and August watching Andy Griffith reruns and shelling purple hulls. I think Mom would freeze 12-15 bushels a summer.