Observer in Minecraft: crafting and mechanics

Crafting recipe

You craft an Observer at a crafting table:

• Row 1: Cobblestone, Cobblestone, Cobblestone
• Row 2: Redstone Dust, Redstone Dust, Nether Quartz
• Row 3: Cobblestone, Cobblestone, Cobblestone

Materials: 6 Cobblestone, 2 Redstone Dust and 1 Nether Quartz. The only ingredient that requires a trip to the Nether is Nether Quartz, and a handful of Nether Quartz Ore blocks is enough to stock up for a long time.

How the Observer works

The Observer watches the block directly in front of its "face" (the side with a texture that resembles eyes). When that block changes state, for example when it is placed, removed, grows, changes its growth stage or changes its properties, the Observer sends a redstone pulse of strength 15 from its back.

What the Observer detects

• Any block being placed or broken
• Plant growth (sugar cane, bamboo, cactus, wheat, pumpkin/melon)
• Changes in the level of water and lava
• Doors and trapdoors opening and closing
• Changes in redstone state (lever, button, piston)
• Changes in a block's state (for example a cauldron filling with rain)

The pulse lasts exactly 1 redstone tick. That is a very short signal, enough to activate a piston or a dropper, but too short for some circuits. If you need a longer signal, use a flip-flop.

Placement direction

You can place the Observer facing any of 6 directions. The face (the side with the "eyes") is the detecting side. The back (the red dot) is the side that emits the redstone pulse. The direction depends on the player's position when placing the block.

The Observer in automatic farms

Sugar cane farm

Aim an Observer's face at the sugar cane. When the cane grows from 2 to 3 blocks, the Observer detects the new block and activates a piston that cuts the cane. A hopper underneath collects the harvest. The farm is fully automatic and needs no redstone clock: the Observer reacts exactly when it is needed.

Bamboo farm

The principle is identical. Bamboo grows faster than sugar cane, so an Observer-based bamboo farm yields a lot of bamboo, which you can use as fuel and as scaffolding material.

Pumpkin and melon farm

The Observer detects a fruit appearing on the block next to the stem. The pulse activates a piston that pushes the pumpkin or melon onto a hopper. One stem keeps producing fruit indefinitely, and the Observer detects each new fruit separately.

Flying machines

Observer + Sticky Piston + Slime Block = a self-propelled machine. The Observer detects the movement caused by the piston and sends a pulse, and the piston moves the structure again. The cycle repeats and the machine "flies" in one direction. This is the basis of bridge builders and tunnel borers in Minecraft.

Observer vs BUD switch

Before the Observer was added (1.11), players used the BUD switch (Block Update Detector), a piston-based contraption that relies on quasi-connectivity (QC) in Java Edition. A BUD detects block updates in a similar way to the Observer, but there are important differences:

The Observer is simpler, smaller and works on both editions. A BUD switch still has uses in niche builds where detecting a wider range of updates matters, but for most uses the Observer is the better choice.

Comparison with other detectors

• Pressure plate: detects entities (players, mobs, items). The Observer detects block changes, not entities.
• Tripwire: detects entities passing through the string. It has a different scope than the Observer.
• Daylight Detector: detects the level of sunlight. The Observer does not react to light.
• Redstone Comparator: reads the state of a container (the number of items). The Observer detects a change in a block, but it does not "see" its contents.

The Observer is the only block that reacts to general changes in block state. That makes it irreplaceable in farm automation.