I don't think there are any exact rules on detection range. Whatever makes sense, but probably about 10-15ish feet to see outline or different height of pressure plate or blood from last victim, etc.
This is hard to say, there aren't really many hard rules about specifically how far away a player notices the traps. I would judge it off of their perception (passive or rolled), the lighting, and if they have darkvision or not.
If their perception beats the traps DC by 5 or more then the player sees the trap as soon as they have line of sight and the trap is in dim light. If they beat the DC, but by less than 5, the player sees the trap as soon as they have line of sight and the trap is in bright Light.
This is based off of how Disadvantage affects passive skill checks and how lighting conditions affect perception checks. I have extended how Disadvantage affects passive checks to also affect a rolled check as well. But this is just too simplify how players check for traps. This way a single roll can be used to determine if the player sees the trap as soon as it enters their torch light or if they have to get closer first.
But all of this is just how I would play it, and isn't strictly RAW.
There aren't any specific rules as far as I know so the DM has to come up with the mechanics.
Some things to keep in mind. If it is dark and the creatures are using darkvision, or if it is dim light for creatures without darkvision, perception checks relying on vision have disadvantage. This means -5 to passive perception and disadvantage on perception rolls. It is much more difficult to spot traps in the dark unless you have devils sight or a light source. Dim light needs darkvision or a light source.
Other than that, I would usually use 10' as a baseline unless the trap is relatively easy to see. A pressure plate might be fairly difficult to notice unless there is something that makes it easy. Usually a pressure plate might be identified by a slight unevenness in the floor or a slightly different color/tint to the plate area. Also, the characters might notice whatever is triggered rather than the trigger itself. For example, if there are holes in the walls - the characters might notice the holes before the pressure plate since they are more obvious. They won't know whether the holes shoot darts, fixed spears that suddenly extend and retract, or whether they could act as conduits for acid or some sort of noxious gas. Noticing the holes might be easy while figuring out the trap and its trigger might be more difficult.
I don’t know about RAW, but it makes sense to me that it would be that they notice it from a minimum of 5 feet away for matching the DC to notice the trap, plus an additional 5 feet of distance for each “point” by witch the noticer exceeded that DC. For example, imagine a hallway with 2 traps:
If the DC to notice a particular trap is 15, and the “noticer” has a Passive Perception (or the result of their check) is 15, then they notice it “just in time” to not step on it from one 5-foot space away because 15-15=0×5=0 feet, and 0+5=5 feet total distance.
If the DC to spot the second trap is only 13, then they would notice that one from 15 feet away because 15-13=2, 2×5=10 feet, and 10+5=15 feet total distance.
If the result of their Wis (Perception) check (or their passive Perception score) was 16 instead, then they noticed the first DC 15 trap from 10 feet away, and the second DC 13 trap from 20 feet away. Make sense? That’s how I treat it anyway.
There are a couple of traps and secret doors in Offical dnd adventures where they actively state someone must take an action perceiving to even find them. COS has a at least one(i dont want to say any more for spoiler reasons. If you want an actual example Ask). This is similar to false appearance where a creature not moving will not be detected.(unless other obvious signs are available )
Still IMO good game play will want to reward People who build towards High PP. I recommend increasing the dc appropriately for relying on PP or PP only gives a clue but not a absolute detection unless they stop to look around.
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Setup:
Party it walking down a sixty-foot corridor at thirty feet there is a pressure plate that triggers a trap.
Question:
At what distance would Passive Perception or a Perception Roll trigger to detect the trap?
I don't think there are any exact rules on detection range. Whatever makes sense, but probably about 10-15ish feet to see outline or different height of pressure plate or blood from last victim, etc.
This is hard to say, there aren't really many hard rules about specifically how far away a player notices the traps. I would judge it off of their perception (passive or rolled), the lighting, and if they have darkvision or not.
If their perception beats the traps DC by 5 or more then the player sees the trap as soon as they have line of sight and the trap is in dim light. If they beat the DC, but by less than 5, the player sees the trap as soon as they have line of sight and the trap is in bright Light.
This is based off of how Disadvantage affects passive skill checks and how lighting conditions affect perception checks. I have extended how Disadvantage affects passive checks to also affect a rolled check as well. But this is just too simplify how players check for traps. This way a single roll can be used to determine if the player sees the trap as soon as it enters their torch light or if they have to get closer first.
But all of this is just how I would play it, and isn't strictly RAW.
There aren't any specific rules as far as I know so the DM has to come up with the mechanics.
Some things to keep in mind. If it is dark and the creatures are using darkvision, or if it is dim light for creatures without darkvision, perception checks relying on vision have disadvantage. This means -5 to passive perception and disadvantage on perception rolls. It is much more difficult to spot traps in the dark unless you have devils sight or a light source. Dim light needs darkvision or a light source.
Other than that, I would usually use 10' as a baseline unless the trap is relatively easy to see. A pressure plate might be fairly difficult to notice unless there is something that makes it easy. Usually a pressure plate might be identified by a slight unevenness in the floor or a slightly different color/tint to the plate area. Also, the characters might notice whatever is triggered rather than the trigger itself. For example, if there are holes in the walls - the characters might notice the holes before the pressure plate since they are more obvious. They won't know whether the holes shoot darts, fixed spears that suddenly extend and retract, or whether they could act as conduits for acid or some sort of noxious gas. Noticing the holes might be easy while figuring out the trap and its trigger might be more difficult.
I don’t know about RAW, but it makes sense to me that it would be that they notice it from a minimum of 5 feet away for matching the DC to notice the trap, plus an additional 5 feet of distance for each “point” by witch the noticer exceeded that DC. For example, imagine a hallway with 2 traps:
If the result of their Wis (Perception) check (or their passive Perception score) was 16 instead, then they noticed the first DC 15 trap from 10 feet away, and the second DC 13 trap from 20 feet away. Make sense? That’s how I treat it anyway.
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I like this logic as it gives some concrete distance I can work off for my players.
Happy to help.
Creating Epic Boons on DDB
DDB Buyers' Guide
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There are a couple of traps and secret doors in Offical dnd adventures where they actively state someone must take an action perceiving to even find them. COS has a at least one(i dont want to say any more for spoiler reasons. If you want an actual example Ask). This is similar to false appearance where a creature not moving will not be detected.(unless other obvious signs are available )
Still IMO good game play will want to reward People who build towards High PP. I recommend increasing the dc appropriately for relying on PP or PP only gives a clue but not a absolute detection unless they stop to look around.