SYSTEM REFERENCE DOCUMENT 3.5
Dungeons are frequently equipped with deadly mechanical (nonmagical) traps. A trap typically is defined by its location and triggering conditions, how hard it is to spot before it goes off, how much damage it deals, and whether or not the heroes receive a saving throw to mitigate its effects. Traps that attack with arrows, sweeping blades, and other types of weaponry make normal attack rolls, with a specific attack bonus dictated by the trap’s design.
Creatures who succeed on a DC 20 Search check detect a simple mechanical trap before it is triggered. (A simple trap is a snare, a trap triggered by a tripwire, or a large trap such as a pit.)
A character with the trap sense class feature who succeeds on a DC 21 (or higher) Search check detects a well-hidden or complex mechanical trap before it is triggered. Complex traps are denoted by their triggering mechanisms and involve pressure plates, mechanisms linked to doors, changes in weight, disturbances in the air, vibrations, and other sorts of unusual triggers.