Redstone basics in Minecraft: dust, repeater, comparator, observer

A power source sends a signal of strength 15 into Redstone Dust, and the signal loses 1 per block, so a line goes dead after 15 blocks. Repeaters boost and delay it, comparators compare it and read containers, observers create pulses. The values below come from the Java Edition 26.3 game code. On Bedrock Edition the same parts exist, but some contraptions behave differently: in Java, pistons, dispensers and droppers can also be switched on by power aimed at the block above them, and Bedrock has no such rule.

Every circuit starts with a source. Some stay on: a Lever (Stick on Cobblestone), a Redstone Torch (Redstone Dust on a Stick) or a Block of Redstone. Others give a short signal: a Stone Button, a pressure plate, a Target hit by an arrow.

Place a Lever, then a line of Redstone Dust and a Redstone Lamp at the end. Flip the Lever: the lamp turns on as long as the signal reaches it. The dust right next to the Lever has strength 15, the next one 14, and so on. The 16th block of dust gets nothing.

A Redstone Torch also inverts: placed on the side of a block, it turns off when that block is powered. That is how you build a NOT gate.

A Redstone Repeater (2 Redstone Torches, 1 Redstone Dust, 3 Stone) only passes the signal in the direction of its arrow, and always sends it out at strength 15. Put one in the line every 15 blocks or less.

A fourth click brings it back to 0.1 s. When another repeater or a comparator powers it from the side, the repeater locks: its output stays as it is until the side signal stops.

A Redstone Comparator (3 Redstone Torches, 1 Nether Quartz, 3 Stone) takes a signal from behind and another from the sides. Use it to switch modes; the small torch at the front shows which one is on:

• Compare (front torch off): outputs the rear signal, unless a side signal is stronger. Then it outputs nothing.
• Subtract (front torch on): outputs the rear signal minus the strongest side signal.

Placed behind a container, it reads how full it is. One item gives strength 1. Every further level needs another 1/14 of the container:

The table counts items that stack to 64. An item that stacks to 16 counts 4 times as much, a tool or a potion fills a whole slot on its own.

An Observer (6 Cobblestone, 2 Redstone Dust, 1 Nether Quartz) watches the block its face points at. When that block changes (a crop grows, a door opens, a block is placed or broken), it sends a pulse of 0.1 s out of its back. Place it so the face looks at what you want to watch and the back touches your circuit.

Two observers facing each other trigger each other forever: the simplest fast clock. A Redstone Torch cannot do that job: if it switches more than 8 times in 3 seconds, it burns out and stays off for 8 seconds.

This build uses what you learned: the comparator reads the hopper, and the 15 blocks of dust drop the signal so that only a full hopper lights the lamp.

1. Place a Hopper on the ground, so it does not empty into anything, with a chest or a farm output above it.
2. Place a Redstone Comparator against the Hopper, facing away from it, in compare mode.
3. Lay exactly 15 Redstone Dust in a line from the comparator.
4. Put a Redstone Lamp at the end of the line.
5. Fill the Hopper: with 319 items or fewer the signal is 14 or less and dies before the lamp. At 320 items it reaches 15 and the lamp lights up.

Swap the lamp for a Note Block or a Piston and you have a sound alarm or an overflow switch. Once this works, you have every part you need for doors, item sorters and farms.