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1st - place down a block.
2nd - place a hopper on the side of the block
3rd - break the existing block
4th - shift-click a hopper into the 1st hopper
5th - place down your comparator
6th - place down your command block (or piston, door, lamp, ect.)
Lastly - put a item into one of your hoppers

- Your comparator should be going back and forth through being powered and not powered

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