Observers Explained (And What to Build With Them)
Observers watch a block and fire a pulse when it changes. That one behaviour powers most automatic farms — here's how to aim and use them.
An observer does exactly one thing: it stares at the block in front of it and sends a short redstone pulse out of its back whenever that block changes. That is the whole component, and it is behind almost every automatic farm worth building.
Getting the direction right
Observers confuse people because they output from the opposite side to the one that does the watching.
- The face — the side with the small dot or "eye" — is what watches.
- The back — the side with the redstone dot — is where the pulse comes out.
When you place an observer it faces the direction you are looking, so it is easy to place them backwards. If your contraption does nothing, check this first.
What counts as a change
The observer fires on a block state change, not just a block appearing or disappearing. That includes:
- A crop growing to its next stage.
- Sugar cane or bamboo growing another block.
- A piston extending or retracting.
- A door, trapdoor or fence gate opening.
- A dispenser firing.
- Water or lava spreading into the watched block.
It does not fire for entities. A mob walking past, an item lying on the ground or a player standing there will not set it off — that is what pressure plates and comparators are for.
The pulse is very short
An observer emits a 1-redstone-tick pulse. That is short enough to fire a piston or dispenser cleanly, but too short for some components to react to reliably.
If something downstream is not responding, put a repeater between the observer and the component to stretch the pulse. This is the second most common observer problem after facing them backwards.
What to build with them
Automatic sugar cane and bamboo farms. Point an observer at the second block of a cane plant. When it grows, the observer fires a piston that breaks it, and the drops fall onto a hopper line. This is the whole design behind the sugar cane farm and the bamboo farm.
Automatic crop farms. An observer aimed at a fully grown crop triggers harvesting only when it is actually ready, which is far more efficient than a timer that fires whether the wheat is grown or not. See the automatic crop farm.
Cactus and melon farms. Same principle — watch for growth, break, collect. Cactus and melon and pumpkin farms both use it.
Flying machines. Two observers pointed at each other's pistons create a self-sustaining loop that drags a slime-block platform across the world. This is how large tunnel-boring and terraforming machines work.
Instant piston doors. Observers fire the moment a block changes, so they make very snappy hidden entrances. Pairs well with the piston door guide.
Item elevators and update detectors. Anything where you need to know "did that thing just change?" without polling on a clock.
Observer versus clock
A lot of farms can be built either with an observer that reacts to growth, or a redstone clock that fires on a timer.
Observers are better when the event is unpredictable — crops grow at random intervals, so reacting beats guessing. Clocks are better when you want a steady rhythm regardless of what is happening, like a periodic water flush or a dispenser that fires every few seconds.
Most good farm designs use both: an observer to detect, a clock to sweep.
Crafting
Six cobblestone, two redstone dust and one quartz. The quartz means you need a trip to the Nether first — if you have not been, the nether portal guide covers getting there safely.