A Sensor is a Hardmode mechanism that emits a signal under specific circumstances. Currently, there are 7 different types of sensors. There are 2 sub-categories of sensors. There are Logic Sensors, and there are Liquid Sensors.
There are 3 different Logic Sensors:
There are 4 different Liquid Sensors:
It has a 5 blocks wide and 10 blocks tall detection zone, although it is actually about 1-2 pixels bigger than that.
When a player moves into the zone, it sends a signal. When a player exits the zone, it also sends a signal.
Note : In multiplayer, if one player moves into the zone, and then another enters the same zone while the first player is still in the zone, the sensor will not send another signal, making it useful for automatic doors in multiplayer (where multiple players may pass the same door). The Player Above sensor will also trigger on the position of dead players until they respawn, which should be considered during Expert Mode planning in particular.