The block update detector or bud switch is a redstone mechanism that uses quirks in the game in order to detect changes in nearby blocks.
Minecraft hidden door bud switch.
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Finally mumbo jumbo returns with a range of hidden entrances you can make.
Next replace the removed blocks with ones that can hold a painting but can also be walked through.
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Some work with a piston door while others are entirely new contraptions.
When the furnace is smelting it causes a block update which triggers the block update detector.
To create the door start by digging a hole in a wall.
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As such players are likely to check for secret passages even if they are not trying to invade or thieve.
Another variation on this idea is docm77 s hidden switch which cleverly uses a series of armor stands to create a chain reaction that opens an iron door.
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It uses a bud switch to set it of then it replaces the block you destroyed and opens the door.
The detector used is the classic t bud or toggle bud which can be found on the wiki.