Built one for the hell of it when I first started my build. Used 3 gate valves and an oversized pipe plumbed into the return pipe/pump of my sump. The oversized pipe had 2 valves. Both are kept closed. Opened top valved, inserted some pellets and closed top. Opened bottom valve at bottom of angled oversized pipe. Had another smaller gate valve on the main return pipe (above the bottom gate valve on main return line). Closed this valve. Opened 2nd valve on oversize feeding pipe, then opened the smaller valve on the main return line and off the food would go. Closed the bottom valve on oversized feeding pipe and was good to go.
Initially I tried with just a combination of 2 valves but it didn't work too well. Food would get stuck in the pipe and valves. Using the 3rd valve helped a lot more to flush water into the oversized feeding pipe.