Raised platform iron farm

An iron farm for three villagers and one zombie on a small raised platform. A redstone clock lifts the zombie in and out of the villagers' view, the golems spawn on the platform and wash into a lava blade over the hoppers, and composters turn the poppies into bone meal. Four modules can be joined around one drop shaft. Java only.

Bed orientation decides whether this works. The head of the bed has to face away from the zombie. Head towards the zombie and the villagers sleep through the night without seeing him, and nothing spawns.
Let the villagers sleep in the beds once before you add the zombie, and let them sleep again if it stops later. Breaking the redstone torch drops the zombie out of sight, and a daylight sensor wired to that torch automates it.
Build on flat ground, or raise the farm 5 to 8 blocks and fill the caves below. Golems spawn on any valid ground within range, not only on the platform.
Make the wall around the golem platform two blocks high instead of one. Golems and their drops fall over a single high wall.
Kill every golem within about 16 blocks in every direction before you start. One live golem nearby blocks all new spawns.
One builder reports the villager groups have to be 16 blocks apart when you expand to four modules, not the 10 the video shows, or the extra modules spawn nothing of their own.