How do I make a monster spawner?

Solution 1:

Some of the statues that can be found in the new underground houses function as monster spawners. So far I can confirm that the bat, slime, and jellyfish statues are spawners.

The spawners must be powered by a wire connected to some sort of trigger (a switch, lever, pressure plate, or maybe a timer). In order to place wires you need a wrench and to remove them you need wire cutters. The wrench, wire cutters, wire, and triggers can be purchased from the mechanic NPC whom you have to rescue from the dungeon. Small amounts of wire and pressure plates can also be found in various traps underground.