I'm trying to create a skeleton farm that can double as an xp grinder or an automatic mob killer. My plan to do this, is to put a hopper and then a stone block on top where the skeletons will fall, and then swap that stone block with a magma block on the push of a lever, so that the skeletons start to take automatic damage as soon as they land on the block.

However, all of the designs I've found so far are for flush designs, ones that remove a block from a wall/ceiling or floor and then push in a new block. I don't want this, all I need is two pistons that extend and retract at the correct times to push in the magma block and retract the stone block, or vice versa. The problem is, I don't understand how the redstone timings work well enough.

Here's my current set-up:

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The left repeater is at 2 ticks, the right repeater at 4 ticks. This works correctly one way, as the left piston retracts before the right piston extends, but then once I turn the lever off, I want the right piston to retract before the left piston extends, but the left piston always activates first, because the repeater is set to 2 ticks.

How can I fix this so that the pistons activate at the correct times?

  • What's wrong with flush? You say you don't need it, but if it works fine, use it. Aug 9, 2017 at 14:28
  • Because I need the block swap to be near enough instantaneous that the skeletons don't fall into the block below. The flush designs pull the block away and there's a second or so delay before the new one gets pushed in.
    – Lou
    Aug 9, 2017 at 15:05
  • Can't you just do [Piston to the right][stone][magma][air][piston to the left] and push it to one or the other side? Aug 9, 2017 at 16:10
  • It's not that simple, timing wise.
    – Lou
    Aug 9, 2017 at 16:34

2 Answers 2


This seems to work, tested it with skeletons. It uses a button, but works the same

Tutorial on Youtube: Note: I did not make this redstone and i did not make this video.

  • That's an interesting solution! I tested it with skeletons standing on the block, and while it does the job, it only works one way - if you swap it back, the skeleton falls through the block. I guess this isn't a fatal flaw, I could just make sure that all the skeletons are dead before moving the block back, but it would be nice if there were a way to swap the blocks fast enough. Nonetheless, upvoting for a good attempt.
    – Lou
    Aug 9, 2017 at 16:40
  • 3
    Hey @Some Guy, you should outline the basics of the solution directly in your answer in case the youtube video is removed or otherwise doesn't work.
    – Bob2Chiv
    Aug 9, 2017 at 18:05

I've just realised there's a far easier solution to what I'm trying to do... feeling pretty silly now. Slime blocks do it all with one piston.

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