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I have made a 50x50 mob trap as specified in this tutorial. Every time I enter the area, mobs spawn inside of the trap, but after they die, no more appear. What am I doing wrong?

The floor of the trap is at y = 67 and the collection point is at the side of one of the edges of the trap at y = 54. Is my collection point affecting the mobs spawning?

Mob Trap

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There are some weird glitches with mob spawn behaviour; sometimes it will just stop working. Try leaving the region (a new region starts every 512 blocks, starting at 0) and then come back. –  Kevin Y Jul 22 '11 at 7:59
    
If I go to the nether, and then come back again, then mobs spawn in the trap. But after those ones die, no more do. –  DanielGibbs Jul 22 '11 at 9:32
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If I remember correctly, there's a cap on the number of mobs allowed at any one time. If there are caves under your trap or the surrounding area, and they fill with mobs, eventually you'll hit the cap and no more will spawn in your trap. The solution is to make sure everywhere within 72 (I think) squares of your collection tile (above and below ground) that is not part of your trap is well lit. You can skip lighting the area within 24 tiles of your collection point as per Abyss' answer though. –  Fambida Jul 22 '11 at 9:36
    
youtube.com/watch?v=f8YJfAwYOz0 This is an EthosLab for making a mob trap.. It also explains most of the mob spawning logic in a fairly simple manner.. I would suggest watching it even if you never actually build this system. –  James Jul 22 '11 at 21:41
    
When i built mine, I dug right down and build it under ground and cleared in each direction, once I knew there were no caves I went along and filled it back in so that it was the perfect shape and there could be nothing, it took a long time but was worth it, I did another one on tekkit when the EE was still in it and just used a catalyst which was much more fun –  Marriott81 Apr 29 at 9:48

3 Answers 3

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Depending on whether you consider it "cheating" or not, you should find out if there are other caves nearby that are consuming your mob cap.

If you press F3 in game to get the debug menu and look at the value for E on the second row. This will tell you the number of entities (including mobs) that are visible/in-game. If the second number stays high when all the mobs in your trap have died, then it is likely they are spawning elsewhere.

Spawn region is roughly 272 x 272 around the player, with a 24 block sphere (yes they apparently do exist in MC) around the player excluded from possible spawning. This means if there are any dark spots in that region, mobs will amass there and your mob trap will die out.

If you want to take the "cheating" a little further, you can use something like minutor to show you the caves in your map and head out to light them all up. You'll also need to light up the ground in that 272 x 272 region as well.

This is why building a high rate mob trap "legit" is a pain in the ass! I'm amassing TNT to destroy the entire 272 x 272 area down to bedrock, that should stop them spawning :)

NB: I use the word "cheating" lightly, it's your game, play it how you want.

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+1 for cheating being in the eye of the beholder. ;-) –  Ben Blank Jul 22 '11 at 19:34
    
Thanks, I'm working on clearing out a large enough area. Unfortunately I don't have much gunpowder to make TNT, hopefully the trap helps with that. –  DanielGibbs Jul 24 '11 at 7:46

I know for a fact that mobs don't spawn within a few blocks of you (I think it's 24, or something), nor do they spawn too far away. Maybe it's something to do with that?

The idea is to use water to bring their drops to you, so you don't have to move/stop them spawning, to get the loot.

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Yea, I have a water tunnel to the collection point which is at the side and below the trap. I don't think this is the problem however, as I moved the collection point to y = 6 and nothing changed. –  DanielGibbs Jul 22 '11 at 9:31

Mobs naturally despawn after a while, so the mobs in the caves in the active chunks aren't really the problem.

Even after you carefully find and light all the caves in all the active chunks around where you're standing when you use the mob farm, there is a 20m "border zone" around the outside of the active chunks in which mobs can end up (either they wander there from active chunks or a group of mobs spawn on the edge of an active chunk and some of the group thus spawns in the inactive area.) These mobs that are not in active chunks are immune to despawn, and so eventually reach mob cap and hurt your output.

Details: https://www.youtube.com/watch?v=4MBEdG9Qj-o& (I suggest skipping to 8 or 11 minutes if you've ever felt "tl;dw" about videos)

If you light only the caves just in and outside the periphery of this border zone (20 meters) you'll get constant drops because the remaining mobs will all be either in your mob farm or subject to despawning.

Or, you can just switch to peaceful and then back to hard, which ought to get rid of the mobs in the 'border zone' also.

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