This farm works under 16w43a. It checks the growth of the central melon/pumpkin using an Observer. Items are collected by the hopper minecart below (items on the ground are from a test at random tick rate = 3000, in normal functioning the minecart should keep up fine). Shown is the maximum size of the farm -- 14 blocks to either side of the test circuit. Scaling up further will require a repeat of the text circuit or a repeater.