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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[1]. It was later popularized by Etho and further developed by many different players.


What it detects

The BUD switch detects the following events when they happen next to the sensor block:


Types of BUD Switch

Water Based

This BUD switch takes advantage of the fact that water flows toward the nearest hole, but it doesn't notice new holes until it is updated for some other reason. The water source block acts as the sensor. Though charming, this design is dated and is more complicated than newer designs.

File:Water bud switch complete.png

Water+boat BUD switch, complete

File:Water bud switch lower layer.png

Water+boat BUD switch, hidden details

Piston Based

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.

File:Basic piston bud switch.png

A simple piston-based BUD switch. The piston sticky.

This design can be extended with multiple sensor blocks. All of the pistons in this design are sensors.

Piston bud array

A piston-based "BUD array", with multiple sensor blocks. All the pistons are sticky.

The next design works because pistons can't be pushed by other pistons while extended. And when a piston retracts, it doesn't notify any other pistons that were trying to push it. This design has the advantage of a low profile and also that the quirk it exploits is less likely to be fixed than an outright bug.

File:Stuck piston bud switch.png

A "stuck piston" BUD switch. The piston in the back is sticky.

Redstone Wire Based

Bugs prevent the power level from correctly propagating through redstone dust when it is receiving power from multiple sources. This can be exploited to make the large BUD switch below. 12 of the redstone dust pieces on the bottom are sensors.

File:Redstone dust bud array.png

A 12-long BUD array made with redstone torches and dust

Powered Rail Based

Similar to pistons, powered rails can receive power through the space above them, but don't update themselves when powered in this way. Combined with a detector rail and minecart, this can be exploited to make a BUD switch.

References

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