Eight 19x19 spawn floors, no walls for spiders to climb, and endermen can't teleport to safety.
Oh, and only 1 water source block flushes the whole thing!
In this thread I will offer an overview of the farm and its features along with a world download.
The farm uses a single water source block to flush all 8 floors. Flushing is controlled by 2 redstone clocks and a piston (dispensers are unreliable), and carefully designed water channels that split and direct the flow.
Mobs are flushed to a 5x5 drop with overhanging water, so spiders are pushed out over the center of the drop and can't grab the sides. There are no walls around the drop area for spiders to grab onto or climb back up (no drop chute). The landing pad is 11x11, which allows enough room for mobs to drift in the air and still hit the platform.
The drop is 40 blocks, which is enough kill everything except a zombie or skeleton with feather falling boots or endermen who teleport down. Water and double carpet prevent endermen from teleporting anywhere other than the landing pad, which is made out of magma blocks, so mobs who survive the fall die anyway.
The kill/collection system platform consists of the magma block landing pad, the minecart track under that, the hopper minecart unloading and switching station below that, with the item sorter and AFK area making up the bottom layer.
It all fits in the 13x13 area under the landing pad. A dual hopper minecart system collects the drops and feeds them to an item sorter. Two hopper minecarts are needed because one hopper minecart cannot collect drops fast enough.
The farm produces up to 14 different kinds of drops.
The sorted drops are:
1. Gunpowder (lots)
2. Redstone (a little)
3. Glowstone Dust (a little)
4. Sugar (a little)
5. Glass Bottles (a little)
6. Sticks (a little)
7. Spider Eyes (a little)
8. String (lots)
9. Bones (lots)
10. Arrows (lots)
11. Rotten Flesh (lots)
12. Ender Pearls (not a lot, but more than a little)
And the occasional chicken jockey will give you raw chicken and feathers (collected in an unsorted items chest).
The world download (click picture to download):
For those who want a bit more information, keep reading...
The dual hopper minecart collection system uses an RS NOR Latch to toggle between two hopper minecarts. When one comes in to unload, the latch toggles and sends another cart out to continue collecting. When the first cart finishes unloading the latch holds it there until the other cart returns, then the latch toggles back and the first cart goes out again.
The darkroom spawner has three types of floors. The bottom floor has six layers, the middle floors have five, and the top floor has six layers. You can add more middle floors to increase the spawn area, as long as all floors remain within 128 blocks of the player (hostile mobs more than 128 blocks away will instantly despawn).
Here is a picture showing the layers of the bottom, middle and top floors.
Note that some layers have glass blocks in the water channels.
I have built this in my Mushroom Peninsula SSP world and it has been working very well for me. I built my farm with the top of the spawner all the way up at the build limit over a deep ocean biome with the bottom at Y30. That puts me 128 blocks above the ocean floor when I am on the item sorter level of the farm, so I didn't have to do any spawnblocking area prep.
Building the farm lower down and spawnblocking the surrounding area should yield even better results, but that means you will not only have to block the surrounding area from spawns (8 chunks in every direction), but also enderman teleportation (32 blocks out from the spawner horizontally, but any distance down).
I have not done any timed tests to see what the farm's output is, but I can tell you it outputs quite a lot of items, and one of those items you get a lot of is gunpowder. If you were thinking about building a creeper farm just for gunpowder, you might want to consider building this instead. You'll not only get lots of gunpowder, you'll also get lots of bones for farming, taming wolves and crafting bone blocks, lots of arrows, which could be useful now that mending and infinity are mutually exclusive, lots of rotten flesh for trading with clerics or feeding dogs, lots of string for trading or crafting wool, and a decent amount of ender pearls (along with small amounts of the other drops).
If you skip the collection system and item sorter the farm is not too resource heavy. You'll need 4 sticky pistons and 1 regular piston, 4 hoppers, 4 redstone dust, 4 comparators, 2 redstone blocks, 1 redstone torch and 1 repeater for the timer-flusher system. The spawner itself just needs a lot of building blocks and 8 glass blocks for each floor (except the bottom floor which only needs 4). The glass blocks prevent mobs from spawning in the water channels, but you should be able to use slabs or stairs instead (I haven't tested that). You will also need a bunch of carpet to cover the roof and other areas.
An alternate collection system I considered was water flushing the drops into hoppers, which was much lighter on resources, but keeping the timing synchronized with the mob spawner was challenging.
While I do consider this farm to be my own design, I must credit Redstone Jazz for his ideas on single source flushing systems, and Snocrash for information about trapping endermen in the farm. If anyone builds this in their world, post pics and let me know how it works for you.
A few more pictures and some additional info about the farm I built in my SSP world.
Flushing system and timer (on top of the darkroom spawner):
This area is 2.5 blocks tall so it's easy to fly into, but too short for enderman teleporting (it's also carpeted to prevent spawning).
The switch on the redstone lamp will lock the farm in whatever mode it is currently in. Locking it while the water is on (flushing) will leave the spawner floors wet so no mobs can spawn, effectively turning off the farm.
The timer circuit uses 2 Etho hopper-piston clocks. The outer clock controls spawning. The inner clock controls flushing. The flushing clock uses 1 sticky piston and 1 regular piston, so it locks at the end of its cycle, and remains locked until the spawn clock unlocks it for the next cycle (the redstone torch pulses at the end of the spawn cycle and triggers the flushing clock).
I placed glowstone on top of the redstone torch here (and under the ones in the item sorter) to reduce lag caused by lighting updates that happen when they activate or deactivate. When you illuminate the area around a redstone torch brighter than the light they emit, the game doesn't process lighting updates. I also placed droppers on top of the hoppers here and in the item sorter, which is supposed to reduce hopper lag (even more than furnaces).
The item sorter and loot collection level:
12 items are sorted, and anything not in the item filter system is placed in the final unsorted item chest.
One thing I forgot to mention before are the 2 redstone lamps in the ceiling here. They turn off when the hopper minecart on their side is unloading. As you can see, the lamp on the right of this screenshot is off, so the cart on the right station is currently unloading. Why is it important to know that? It isn't, I just like indicator lights. You can also climb up the ladder on the left to the unloading level and watch the hopper minecarts.
The landing pad, where the tragically short lives of the mobs that spawn above come to an abrupt end:
Assuming you are using elytra to go to the farm, you could fly up and land on top of the containment wall around the landing pad. The wall is here to keep the occasional zombie, skeleton or enderman that survives from wandering away while the magma blocks finish them off, but it's also a good place to see the farm in action.
When you fly up to land here, don't miss and fall inside, or the magma blocks might finish you off as well. If that happens, don't fret about it, a hopper minecart will be along shortly to collect YOUR drops, and you'll find them in the unsorted item chest.
It's raining mobs!
Here is what we want to see. Lots of mobs, which means lots of loot!
If you are going to hang out here and watch the mobfall you might want to raise your shield, because those darn aimbot skeletons will shoot at you on their way down and knock you off the wall. See that skele to the right of those creepers? Yea, he's looking at you (well, actually me). This is definitely not the place you want to AFK at this farm.
Hmm... MobFall, that would be a good name for this farm. What do you think of that?
looks good and efficient. i like this design though that i built. as it handles both spiders and slime additionally. much less complicated. though may by more time consuming when you factor caves and such. building in the sky has definite advantages.
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Yes i know, my spelling sucks.
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