A spawner is a switched item that allows players to spawn specific mobs by activating a switch or touchplate. This item is in the Engineering section of your inventory. It is the usual 1x1 block size, has no collision, and is self-protecting.
How to Obtain
Spawners drop as rare loot from mechanical chests. You can also trade for this item on a market world, or buy it in the Weapons And Exoskeletons section of the crowns store.
How to Use
To set up a spawner, first place a switch or touchplate, then place the spawner and choose which mob you want it to spawn. You can choose from a variety of non-hostile mobs (rat, bunny, skunk, crow, bat, racer) or one of two hostile mobs (terrapus, automata). When the switch is activated, one mob will spawn. If the switch is activated again, the first mob will disappear and a new one will spawn.
The racer is a specialized, non-hostile terrapus that will travel in a straight line without making the random stops and turn-arounds that regular terrapi make.
Spawners have placement limitations on them to prevent trolling. You cannot place a spawner anywhere in a world that is less than 75% explored (even private worlds); and you can't place one within 25 blocks of any world teleporter (i.e., a spawn teleporter). You can overcome this second constraint on a private world that you own by temporarily mining the world teleporter, placing the spawner, and then putting back the world teleporter.
In addition, spawners do not function in market worlds.
Due to their self-protecting nature, spawners are often used to claim space or to prevent others from claiming space. And since spawners that you own display a protector icon on your personal mini-map, they can also serve as a private, albeit inelegant, way to mark locations.
Spawners were introduced in June 2013, and were able to be placed and activated anywhere. Their parameters were changed in November 2014 as an anti-trolling measure.