The Observer Detects Block Changes and Sends a Redstone Signal
The Observer is a redstone block that watches the block directly in front of it and sends out a one-tick redstone pulse whenever that block changes. The change can be anything: a block being placed or broken, water flowing in or out, a crop growing, a door opening, a piston extending, or even a block's rotation changing. When the Observer detects the change, it emits a redstone signal from the back face of the block, which you can use to trigger doors, lights, traps, or other redstone contraptions.
The Observer has a face (the side with the two dark spots that look like eyes) that points toward the block it is watching. The redstone signal comes out the back, opposite the face. This makes it useful for automating things that would otherwise require you to flip a switch or press a button manually.
Key Takeaways
- The Observer emits a redstone pulse when the block directly in front of it changes in any way.
- The redstone signal comes out the back of the Observer block, not the front.
- The pulse lasts only one game tick, so you often need a repeater or other redstone component to extend it if your contraption needs a longer signal.
- Observers are essential for building automatic farms, sorting systems, and other redstone machines that respond to block updates.
How to Place and Orient an Observer
When you place an Observer, the face with the eyes automatically points toward the block you are looking at. If you need to rotate it after placement, you can break it and place it again, or use a piston to push it into the correct orientation. The block directly in front of the face is the one the Observer watches—that is the only block it will monitor for changes.
The redstone signal exits from the back of the block, the side opposite the face. If you want the signal to travel in a specific direction, plan your placement so the back of the Observer points toward your redstone wire, repeater, or other component. You can also use redstone dust, redstone repeaters, or comparators to redirect the signal if needed.
What Counts as a Block Change
An Observer triggers on almost any block update. This includes blocks being placed or broken, water or lava flowing, crops growing one stage, leaves decaying, snow accumulating, ice melting, pistons extending or retracting, doors or trapdoors opening or closing, and redstone components changing state. Even a block rotating (like a log or stairs changing direction) will trigger the Observer.
The Observer does not trigger on things that are not block updates—for example, an item entity moving through the air, a player walking past, or a mob standing on the block. It only cares about the block itself changing.
Using the Observer's One-Tick Pulse
When an Observer detects a change, it sends out a redstone signal for exactly one game tick (one-twentieth of a second). This is very fast and very short. If you connect the signal directly to a door or lamp, the door will open or the lamp will turn on for only that one tick, then turn off again. For most uses, you need to extend the pulse.
The most common way to extend a pulse is to place a redstone repeater set to a delay of 1, 2, 3, or 4 ticks right after the Observer. This stretches the one-tick pulse into a longer signal. You can also use a redstone repeater locked by another signal, or a comparator, depending on what your contraption needs to do. Some builds use multiple repeaters or even a redstone loop to keep a signal active as long as the Observer keeps detecting changes.
Building an Automatic Farm with an Observer
One of the most popular uses for an Observer is automating a crop farm. You place an Observer facing a crop block, and when the crop grows to its final stage, the Observer detects the update and sends a signal to a piston that breaks the crop and collects it. The piston retracts, the crop regrows, and the cycle repeats automatically.
To build a straightforward automatic wheat farm: place an Observer facing a fully grown wheat block, connect the Observer's output to a redstone repeater (set to 1 tick), then connect that repeater to a sticky piston positioned so its rod will push the wheat block when it extends. When the wheat grows, the Observer triggers, the piston extends and breaks the wheat, then retracts. The wheat regrows, and the Observer detects that growth and triggers again. You can expand this design to multiple rows by using redstone dust to connect multiple Observers to the same piston, or by building separate piston-and-Observer pairs for each crop.
Using Observers in Sorting Systems and Redstone Doors
Observers are also useful in item sorting systems. When an item moves into a hopper or container, the block updates, and the Observer detects it. This signal can trigger a redstone door or gate to open, allowing the item to move to the next stage of sorting. Some players use Observers to detect when a storage chest is full (by watching the chest for updates) and then trigger a system to redirect items elsewhere.
For redstone doors and gates, an Observer can detect when a player or mob pushes a button or walks on a pressure plate in front of it, and trigger the door to open. This is less common than using the button or plate directly, but it allows for more complex automation where the door responds to multiple types of block changes at once.
Common Mistakes to Avoid
The most common mistake is forgetting that the Observer's output is on the back, not the front. If you connect redstone dust to the front of the Observer, it will not work. Always connect to the back face, the side opposite the eyes.
Another mistake is expecting the one-tick pulse to last long enough to trigger something. If you connect an Observer directly to a door, the door will flash open for a single tick and close again so fast you might not even see it. Always add a repeater or another component to extend the signal. Finally, remember that the Observer only watches the block directly in front of it—if you want to monitor multiple blocks, you need multiple Observers.
Frequently Asked Questions
Can an Observer detect a player breaking a block?
Yes. When a player breaks a block in front of an Observer, the block disappears and the Observer detects that change, sending out a pulse. This is useful for traps or security systems that respond when someone mines a specific block.
What is the difference between an Observer and a Comparator?
An Observer detects any block change and sends a one-tick pulse. A Comparator measures the signal strength of a block (like how full a container is) and outputs a signal based on that level. They serve different purposes: Observers are for detecting changes, Comparators are for measuring states.
Can I use an Observer to detect when water flows?
Yes. When water flows into or out of a block in front of an Observer, the block updates and the Observer triggers. This is useful for water-based contraptions and automatic systems that respond to fluid movement.
Do Observers work on the Nether and End?
Yes. Observers work the same way in all three dimensions. They detect block changes in the Nether and End just as they do in the Overworld.
How do I rotate an Observer after I place it?
Break the Observer and place it again, making sure to look at the block you want it to watch. Alternatively, use a piston to push the Observer into a new orientation, though this is less practical for most builds.