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I am creating a redstone system that has a dropper facing upwards into a hopper that in turn points back down into the dropper. Dropper & Hopper

My goal is to be able to lock the top hopper by powering it with redstone. But however I seem to go about this, it seems to power the dropper, sometimes only when it receives a redstone update.

The rules of quasi-connectivity are that "anything that can power a redstone component above your dropper can power the dropper itself, even if the redstone component isn't there."
This means that any way of directly powering the hopper will indirectly power the dropper, and there is no way to lock the hopper without powering the dropper.

Is there a way to lock the top hopper without powering the dropper?

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It's impossible not to power the dropper, but it's possible not to make it fire. All you need is not to provide a block update, which would make the dropper check for power and activate.

enter image description here

This example setup works - a solid block does not produce updates even when powered, locking a hopper does not produce an update, and a repeater will produce an update 2 blocks ahead, but it updates the hopper, not the dropper.

Of course at this point the dropper is BUD'ed, which means any update, from any source will make it fire. But that's the most you can do in this configuration.

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  • But wouldn't the change from hopper[enabled=true] to hopper[enabled=false] send a block update already? – Fabian Röling May 30 at 12:55
  • @FabianRöling: It's one of the situations where a BUD behavior differs from an observer: Hopper's block state change is observable by an observer, but it does not produce updates detectable by BUD. – SF. May 30 at 21:53
  • Wow, that's weird. But also… how does the observer get its update then? – Fabian Röling May 30 at 22:17
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    @FabianRöling: I don't know the inner workings of the observer but it doesn't use block updates at all - it observes block state changes. The two are usually, but not always connected - e.g. plants advancing a stage of age change their state but don't produce updates. Similarly, redstone dust redirection. Conversely, opening a normal chest produces an update but doesn't change its block state. – SF. May 30 at 22:40
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This is impossible, because of the bug MC-108 (currently closed as "working as intended", but the proper resolution would be "won't fix"). A dropper always activates when the position above it is powered and it gets a block updates (which it always gets from the state of the hopper changing).

You need to work around it, for example by keeping the dropper powered from the last use or by putting the hopper on its side.

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    There are quite a few redstone contraptions that make use of this bug, and would break if it were fixed. In fact, there are so many that Mojang has come out and said that they won't ever fix this specific bug, because it would break so many people's projects. – HiddenWindshield May 26 at 2:29
  • Complex redstone contraptions break in every update, that's not a valid argument. And Panda4994 has shown that nothing becomes impossible to do without QC. On the other side, many things are impossible because of QC, like the question here. Also, comments on SE are not a place for general discussion. – Fabian Röling May 26 at 8:21
  • @FabianRöling If Mojang has told that's why they do it, it's a valid argument. – Mast May 26 at 11:09
  • @FabianRöling there's a difference between "more than zero break per update" and "one of the most-used mechanics will stop working after this update." – TheWanderer May 26 at 12:21
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Power the hopper indirectly with a block. enter image description here

Edit:

I did some more testing and using a redstone block has proven to be much better because the dropper won't activate if a block update occurs. However, the block can only be above the hopper. enter image description here

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  • I edited my answer. – MegaCrafter10 May 25 at 19:48
  • Have you actually tested this answer? Because of MC-108, it's impossible to do what the asker requested. – Fabian Röling May 25 at 19:53
  • Of course, I have. The redstone block method works fine. – MegaCrafter10 May 25 at 20:13

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