This trigger just regularly springs the trap. Note that the trap may be set to use a timed trigger after the initial trigger is sprung.
Timed triggers are great obvious traps. This can be gouts of flame or swinging blades. They are buzzsaws, vines, and swinging logs. It is spinning pillars, anti-gravity platforms travelling up and down, and sections of floor that disappear or collapse after you step on them. They are rarely a danger themselves, but can make other situations more interesting.
They are best used in situations where other objectives need to be met, rescuing hostages, defeating a bad guy, stealing an item, made all the more complicated by the regular changes in environment.
S2 White Plume Mountain by Gary Gygax
7. The door opens onto a stone platform in a large natural cave. The ceiling averages 50´ above the level of the platform while the floor of the cave 50´ below is a deep pool of boiling mud. Points A and B mark the locations of geysers. Geyser A spouts once every five minutes. Geyser B spouts once every three minutes. Opposite the entrance platform is another stone platform, approximately 90´ away. Between them a series of wooden disks is suspended from the ceiling by massive steel changes. The disks are about four feet in diameter, and three feet apart. Each disk is attached to it’s chain by a giant staple fixed in its center. The disks swing freely and will tilt when weight is placed upon them. The disks and chains, as well as the walls of the cavern , are covered with a wet, slippery algal scum that lives on the water and nutrients spewed up from the geysers. This coating gives off a feeble phosphorescent glow.
When the geysers erupt, they reach nearly to the roof of the cavern, and creatures holding onto the disks or chains may be washed off to fall into the mud below (an almost instant death). Characters with 18 Strength, or better, have a 65% chance of holding onto a disk that is adjacent to an erupting geyser. For each point of strength less than 18, there is a 10% lesser chance of hanging onto the disks (i.e. 16 strength equals 45% chance.) However, for each disk the characters is located farther from the geyser, there is a cumulative chance 5% greater of holding on i.e. one farther away (from the adjacent disk) equals +5%, two away equals +10%, et. Damage varies as the distance from the geyser. Adjacent disk: 5-50 points; one away: 4-40, and so on: 3-30, 2-20, 1-10, 1-6 and 1-4 for anyone in the cavern. Characters who make their saving throw versus breath weapon will take only one-half damage.
Timed trigger design
Early timed triggers can be as simple as a swinging blade, something obviously and trivially avoided, at least until you get hit with a Fear effect or henchmen fail their morale rolls and run into it while fleeing.
Moderate timed triggers are an effective way to challenge mid-level parties with beginning challenges. Take an encounter for characters of 1st-3rd level and add gouts of fire that criss-cross the battlefield and it becomes an appropriate challenge for higher level characters. Another type of timed trigger is one where a normal activity is given a hard time limit, you must defeat the monster/pick the lock/disarm the trap before the room crushes you, the ceiling falls, the guards arrive, or your get cut apart my laser beams.
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