On the Thursday Trick, Triggers Pressure Plate

Pressure sensors detect and measure pressure and convert that into some kind of output. 

We are not escalating our interactions with the players to engage esoteric knowledge, but rather to allow them to enjoy problem solving. This is the lens through which we examine the pressure plate.  
Pressure Sensor Platonic

There are a few types of pressure sensors: Strain based sensors require a physical material placed under strain such as a coil or strip of wire. There are also Resonant pressure sensors which measure frequency to determine if a machine is experiencing unexpected pressure. Finally there are thermal or ionization sensors measuring pressure via temperature change. There is a heating element balanced delicately, and any movement causes a change in temperature due to airflow.
  • Historically, pressure plates were given a chance to trigger as each character passes. Players poking and prodding ahead would have a chance to trigger the trap with their poles. Mechanically this meant the more likely the trap was to trigger, the easier it was to detect. Pressure plates that trigger 100% of the time are always detectable. This ratio is a good starting baseline.
  • Pressure plates are not necessarily a trap trigger. Cities use pressure plates
    Spring Plate
    Here, the medium is air, 
    the strain element is the left spring
    and the output is the latch.
    (tubes) to map traffic to better design roads. Detecting a pressure plate doesn't give a character any insight into what it does. 
  • The illustrations indicate one case where the pressure plate itself is the trap, and another where the pressure plate is a trigger for the trap. This is not merely a matter of semantics, but describes what the problem the player actually faces. 
  • There are many many ways that people become aware of pressure plates, consider your own awareness when using pressure plates to open handicapped doors. It is difficult to completely integrate pressure plates into surfaces undetectably, though there will always be a dwarf who will claim otherwise. Bipedal creatures are acutely concerned with the stability of the surface they are standing on. We are very aware of instability. 
  • The plate being triggered must be abstracted to a random chance. Because of this abstraction, a failure to trigger could mean "Didn't step on the plate at all," "Plate didn't trigger due to weight distribution," to "The player's patron used mystical force to protect them". Consider this when framing the fictional positioning to the players. 
  • Variations on the pressure plate include many thousands of tiny pressure plate, each step triggering some percentage of them. A pressure plate that disarms a trap or produces a safe space. 
  • The reuseablity of pressure plates makes it difficult to conceal them. Plaster and paint will crack, space that allows the plate to move will crack and grow, plates will lean, et. al.
    Electric Plate
    Here, the medium is air,
    The strain element is the electrical
    resistance in the air, and the output
    is an electric current.
  • Common classic player behaviors to discover these include pouring water, tapping ahead with poles, and being a dwarf. 
  • Pressure plates and their environments are frequently decorated or part of decoration and tile mosaics to hide their presence. In addition, the effects of the pressure plate are usually visible—scorch marks, blood stains, gouging. 

Traditional use

A1: Slave Pits of the Undercity, By David Cook

20. TRICK STAIRS
A staircase goes down about 10’ and ends in a small landing with a door facing the stairs. The steps are made of flagstones set in mortar. The flagstones are loose. 
This staircase will become a slide if the door to room is not opened properly. A thief may not detect the trap, but a dwarf could detect shifting stone if checked for. If the door is opened without first locking a secret catch (detectable only by a thief), the stairs will suddenly slant, creating a steep incline. Unless previous precautions have been taken, characters will slide into room #21, automatically losing surprise and initiative for the first round. Characters roped together, but not fastened to anything, will have even greater difficulties (the characters will be surprised for two segments and may not take any action during that time). After the door is closed and pressure released from the slope, the stairs will return to normal. If the door is opened after locking the catch, nothing will happen and the party will find stairs going down.

C1: The Hidden Shrine of Tamoachan by Harold Johnson & Jeff R. Leason

2. The Hall of Thrashing Canes
The sides of this corridor are carved to resemble walls of bamboo-like logs. The passage slopes down from a single door on its western leg, the lintel of which has been crafted to represent a stylized cavern entrance, to double doors of beaten bronze, worked to resemble a forest of seaweed

There is a pressure plate halfway down the hallway which triggers a trap. Several of the logs will swing out from either wall and buffet the paw towards the double doors. For tournament play, the trap will always work. For campaign adventure, the trap will be triggered on a 1 or 2 in 6. Characters will suffer no damage if the trap swings out behind them, stunning for 1-4 rounds. If a character is walking up the slope towards the trap, he or she will receive 1-6 points of damage from the trap, and will be stunned for 1-4 rounds. Once triggered, the stone logs will not swing back and will effectively block the passage, since they bar the way from ceiling to floor with only a 6” gap between the logs.

T1-4 Temple of Elemental Evil by Gary Gygax

103. PILLARED HALL
One hundred pounds of pressure on the floor at a point five feet in from the entry triggers a delayed-action mechanism. The pressure point limit is shown by X marks on the map. One round after this is triggered, a heavy barred grate drops and seals the doorway. Two harpies roost atop the two central eastern pillars, having special niches cut into the eastern faces 34 feet above the floor. When the grate crashes down, the harpies start singing. After one round, they cease their song and fly around, attacking any creature that makes a hostile gesture. If sounds of fighting occur here, the occupants of area 104 arrive 1 round later. If there is no such turmoil, these creatures arrive five full rounds after the grate's noisy descent.

Pressure Plate design

Early pressure plate design should trend toward obviousness and small hit point tax. Escalating this should be both pressure plates that trigger nothing or are broken, or plates that work to allow the players to advance. (e.g. a plate that requires 800 pounds for a door to open). Consider the construction of the plate, specifically the medium in relation to what the players do to examine it. The pressure plate is not a critical feature. The simplest option when dealing with it is "I ignore it". This is a feature.  Pressure plates (and pits) are most effective when they are placed in areas where characters will be fleeing: tombs, in front of fear runes, outside of a room filled with 12 beholders.



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