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Published on Nov 24, 2011
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The Block Update Detector, or "BUD switch", is a redstone mechanism that exploits bugs and quirks in the game in order to detect changes in nearby blocks. There are many different designs, but they are all based on the same general idea: a redstone circuit is put in a state that should be unstable, but is stable due to some block within not being notified of changes in the circuit that would otherwise affect it. This lazy block then functions as the "sensor". When an adjacent block changes state in some way, it sends a notification to the sensor block and "wakes it up". The circuit then performs some other action and goes back into the sleeping state.
The method was discovered by Cored. It was later popularized by Etho and further developed by many different players.
The BUD switch detects the following events when they happen next to the sensor block:
Placement of blocks Destruction of blocks Re-flowing of water or lava Freezing or melting of ice Gravel or sand landing from a fall Movement of blocks by pistons, and the movement of pistons themselves Growth of plants other than trees, including: wheat mushrooms sugar cane flowers tall grass cactus pumpkins, melons, and their stems Leaf growth and decay Growth and death of grass Dirt becoming tilled soil, soil becoming moist, soil reverting to dirt (but this can be more easily detected with a repeater) Fire igniting or burning out Redstone ore starting to glow, or stopping Changes in redstone state or power level, and activation of redstone devices Setting the delay on a repeater Getting in or out of a bed A furnace starting to smelt, or finishing Opening or closing a door, trapdoor, or Fence Gate Inserting or removing Music Discs from a jukebox Placing an Eye of Ender in an Ender Portal block Rails changing orientation TNT igniting and becoming an entity
There are many BUD switch designs based on pistons. This simple and popular design exploits a bug in which pistons aren't properly notified when powered through the space above them. The piston acts as the sensor.
Here i showcase 4 different piston design, one for each orientation of the piston itself. The floor model buf switch, a ceiling model, and 2 different bud switches where the piston is on its side.
I also show how to convert my favourite bud switch into a clock, and a half tick Pulser.