How to Use an Observer in Minecraft

Guide Java Bedrock

Quick answer: An observer watches the block directly in front of its face. When that block changes - grows, is placed, broken or changes state - the red dot on the back fires a 1-redstone-tick pulse at full power 15. That single pulse powers farms, traps, flying machines and the fastest compact clocks in the game.

What you need

Observer (6 cobblestone + 2 redstone dust + 1 nether quartz)
Pistons and slime blocks for flying machines
Something to watch: crops, a furnace, a door

Step by step

Craft it. An observer takes 6 cobblestone, 2 redstone dust and 1 nether quartz, so it needs one Nether trip.
Place it facing the right way. The face with the two "eyes" watches the block in front of it; the red dot on the opposite side is the output. The face points away from you when you place it, so stand on the output side.
Wire the output. The dot side emits a pulse of strength 15 lasting 1 redstone tick (2 game ticks) - enough to fire a piston, dispenser or note block, or to feed dust, a repeater or a comparator.
Detect crop growth. Point the face at the top of sugar cane, bamboo, kelp or a cactus: when it grows one block, the observer fires once - the trigger behind most automatic farms.
Make a fast clock. Two observers facing each other detect each other pulsing and loop forever - the most compact fast clock in the game. Break and replace one to stop it.
Build a flying machine. An observer moved by a piston emits a pulse as it arrives, so two observers plus two slime blocks and pistons push each other along in a line. That is the engine of every flying machine.

Java vs Bedrock

Java and Bedrock observers detect different changes. Java watches for block-state "shape updates" (very predictable); Bedrock acts as a general block-update detector and fires on some changes Java ignores - and vice versa. Farm designs are rarely cross-edition compatible, so build one made for your edition.

Versions: The observer debuted in Pocket Edition 0.15.0 (June 2016) as the first official BUD block, then came to Java in 1.11, the Exploration Update (November 2016). Its pulse has been 1 redstone tick at strength 15 ever since.

Frequently asked questions

Why does my observer fire randomly?

It detects more than growth: block placement, breaking, water flowing in, even a chest being opened nearby on Bedrock. Check what block its face actually points at - one block off and it watches the wrong thing.

Observer vs comparator for farms - which one?

Use an observer to catch a change the moment it happens (cane grew, melon spawned). Use a comparator to measure a level (how full a chest is). Most automatic farms use observers as the trigger and comparators in the storage system.

How do I stop an observer clock?

Two facing observers pulse each other forever. Power one of their outputs into a piston that pushes a solid block between them, or just break one observer and place it back when you want the clock to run again.