This is a fully automatic mob head farm that I call "the guillotine" for its ability to maximize the amount of mob heads collected from each thunderstorm. It works by spawning as many creepers as possible before the first lightning strike, and charging as many as possible from each lightning strike. Charged creepers are then stored in a collection chamber and then used to generate heads one by one. The system constantly tops up the amount of charged creepers while the thunderstorm is ongoing.

During a thunder storm, the farm tries to maintain a balance of three creepers (charged and pre-charged), for every mob used to generate heads. When the storm is over and the sky is clear, there can be up to 30 charged creepers in the collection chamber. The farm will continue to use them to generate heads until the charged creeper chamber is empty, then the farm shuts down and awaits the next rain and thunderstorm. When it starts raining again, it spawns creepers and keeps them in the charging chamber to await the next lightning strike.

The creepers are ignited by a skeleton clad in blast protection IV diamond armour and healed by harming potions from a witch.

For now the rain detection relies on snow golems (this will be replaced with a more reliable system after the 1.19 release), which needs to be topped up with snow and carved pumpkins. A total of 13 extra snow golems can be spawned after one is used to detect rain.

Update: Changed the rain detector to a more reliable one. This design is inspired by one made by ilmango. It's a different design (couldn't get his one to work) but it uses the same base mechanics.

I've also recalibrated some timing circuits to improve the consistency of creeper charging and use of each charged ceeper, and also improved the flow of mobs from the spawners.