Pressure Plate

Pressure Plates were introduced in the third Seecret Friday update. If connected by wire to an object (or placed adjacent to a block that requires electricity), 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 three types of switches currently in the game.

Stone and wooden plates can be recovered with a pickaxe. Both can also be recovered by destroying the block they are on.

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

Crafting
There are currently only two types of pressure plates: wooden and stone. Wooden pressure plates are made from 2 wooden planks in a horizontal line, and stone pressure plates are made with stone (not cobblestone) in the same fashion:

Triggering
Pressure plates may be triggered in the following ways:
 * A player or mob standing on a plate.
 * 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 may trigger a pressure plate adjacent to the track (think about the cart's corners sticking out). However, this is not reliable.
 * Arrows (wood pressure plates only):
 * A plate enabled with this method remains turned on until the arrow is recollected or despawns (this takes exactly one minute as of 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.

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.

Druckplatte