Pressure Plate

A Pressure Plate is a type of switch that can be placed onto a surface of a block. If connected by wire to an object (or placed adjacent to a block that can be powered), they will provide power to that object or block when stepped on by either a player or a mob (as well as other items; see below), and will stop when stepped off. Pressure plates are one of the four types of switches currently in the game.

Stone plates can be recovered with a pickaxe.

History
Pressure plates were introduced in the third Seecret Friday update. Since Beta 1.6, wooden plates do not require a pickaxe to be successfully extracted; before they used to. Both types of plates can also be recovered by destroying the block they are on.

Since Beta 1.3, pressure plates only require two wooden planks/stone to craft. (Previous versions required three).

Since the 1.6 update, pressure plates may no longer be placed on leaf blocks. Pressure plates placed on leaf blocks prior to this update will drop when stepped on and may be recollected.

Since the 1.8 update, pressure plates are placeable on fences, thus representing tables, though they are not functional as pressure plates in this configuration.

Since the 1.9 Pre-release 2 update, pressure plates now function when placed on top of fences; be aware of this when running redstone wires near your tables.

Crafting
There are currently only two types of pressure plates: wooden and stone. Wooden pressure plates are made from 2 wooden planks placed side by side, and stone pressure plates are made with stone (not cobblestone) in the same fashion. Pressure plates are most commonly used to open doors by being placed in front of them.

Triggering
Pressure plates may be triggered in the following ways:
 * A player or mob standing on a plate.
 * A player flying in close proximity on top of a plate. -this may be a bug.
 * A dropped item (the kind you can pick up by walking onto) on a wooden plate (stone plates are not affected). Sand or gravel falling onto a pressure plate will convert into a dropped block, but it will not necessarily land on the pressure plate unless there are walls around it.
 * Minecarts:
 * Any type of minecart will trigger a wooden plate.
 * A minecart with a mob or player riding it will trigger a stone plate (however, storage minecarts and powered minecarts do not trigger stone plates).
 * Pressure plates do not act as rails, so the cart will stop on the plate unless it has very high momentum.
 * A minecart on a curved track triggers a pressure plate adjacent to the track (think about the cart's corners sticking out) if there are two or more successive curves. (Two curves are needed to get the cart on a diagonal path and thus the edge sticking out over the track. If a single curve is laid down the cart just changes by a 90 degree angle.)
 * Fishing rods
 * A fishing rod can activate a wooden pressure plate (seen in this video)
 * Arrows (wood pressure plates only):
 * A plate enabled with this method remains turned on until the arrow is recollected or despawns (this took exactly one minute in Alpha 1.2.4)
 * For arrows to trigger a wooden pressure plate there needs to be a secondary trigger which can be any particle effect:
 * 1) Block destruction debris
 * 2) Torch smoke
 * 3) Furnace flames
 * 4) Dispenser smoke
 * 5) Fire smoke
 * 6) Lava embers and more...
 * There is a mode to allow pressure plates to be activated only by humans, and not mobs. However, no block in the game currently uses this feature, although some mods have implemented it.

Pressure Plates are often used in traps. For example, if you set a pressure plate on top of a TNT block, the TNT block will explode under the player's or mob's feet.

Pressure Plates can also be used to make sure that you do not forget to close the door to your home. Just place a pressure plate just inside of the door.

Note Blocks
With the release of note blocks people have been making songs, using the pressure plates to play a note. They remove the pressing sound by modifying the Plates' sound file in the Minecraft folder. They replace the press with silence, via the aid of sound editing software such as Audacity, then set the file as read only, thus preventing Minecraft from re-downloading the sound file. This enables a screen recording of the created song without having the pressure plate sound messing up their recording. The sound file is .minecraft/resources/newsound/random/click.ogg. Note: this is also the sound for the in-game menu buttons.

Trivia

 * Pressure plates do not make any sounds in SMP when triggered.
 * In the game code, there is an unused option for pressure plates, so only players can trigger the pressure plate, not mobs.