Redstone · Mechanic · Java & Bedrock

How to Use an Observer

An observer is a block-change detector. It stares at whatever is in front of its face, and the instant that block changes it fires a quick pulse out the back. That single trick runs most auto-farms and flying machines.

watched block

WATCHING…CHANGE → PULSE!

An observer stares at the block in front of it. The moment that block changes, it fires a quick pulse out the back. That is what makes crops auto-harvest the instant they grow.

What it detects

Any change to the block it faces, never items or mobs.

Crops & saplings growing

Pistons extending or pulling

Redstone power changing

Water or lava flowing in

Levers, doors, trapdoors

Even another observer

Front watches, back fires

The big staring face is the detector, pointed at the block you watch. The back with the dot is the output. Get them backwards and nothing fires, so always aim the face at the block and pull redstone from behind.

Quick answers

What does an observer do?

It watches the single block directly in front of its face. The moment that block changes in any way, it sends a short redstone pulse, about one redstone tick, out of its back.

Which side of an observer outputs the signal?

The back, the face with the small dot. The big staring face is the side that watches. Place the face toward the block you want to monitor and take the pulse from the opposite side.

What can an observer detect?

Block changes only: crops and saplings growing, pistons moving, redstone dust or levers or doors changing state, water and lava starting to flow, leaves decaying, and more. It does not detect items lying around or mobs walking past.

How do you make an observer clock?

Point two observers at each other, or an observer at its own output. Each one keeps detecting the other's change, which makes a very fast 2-tick clock. Use it sparingly because fast observer clocks cause lag.

Why is my observer not detecting anything?

Check the facing. The staring face has to point at the block you are watching, not away from it. Also remember it only fires on a real block-state change, so items or mobs passing by will not trigger it.

Do observers work the same on Bedrock?

Yes. The watching face and the pulse output behave the same on Java and Bedrock, so the same auto-farm and flying-machine designs work on both.

Database →

Observer & redstone

Look up the observer and every redstone component.

Guide →

Use a repeater

Delay or extend the observer’s short pulse.

One pulse, one face, no exceptions

It watches the block in front of the face, and nothing else

The observer has a small face on one side and that is the only block it reads. Put it against a crop and it sees the growth stage change; put it against air and it sees nothing forever. This is what makes observers so precise compared to a redstone torch clock: the pulse happens because something actually changed, not because time passed.

The pulse is short and comes out the back

Output is one redstone tick on the opposite side, which is fast enough that a repeater is often needed to make a piston or a dropper act on it reliably. If a mechanism works when you press a button but does nothing when the observer fires, the pulse length is the first thing to lengthen.

Two facing each other never stop

Point one observer at the back of another and they will trigger each other forever, which is the smallest clock in the game and also the easiest way to lag a farm. Break one face to stop it, and if you plan to switch a farm off, put the toggle between the observer and its target rather than trying to mute the clock itself.