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40d:Pressure plate
Pressure plates work somewhat like levers, but instead of being actively toggled, they're activated by the weight of something on them.
Pressure plates are not configured during use, but upon placement. While placing a pressure plate, you can configure it to trigger when the following things are on it:
- Water/Magma - choose minimum and maximum levels
- Creatures - choose minimum and maximum size; creatures of the size selected will be listed as example. You can pick whether your own citizens will trigger the plate or not. Friendly NPCs (traders ect) count as civilians.
Pressure plates default to toggleable, where they send an On signal when the determined weight is upon them and send an Off signal when it's released. Pressing o will change them to one-use mode, where upon being activated, they do their work then deconstruct themselves, ceasing to operate altogether.
Pressure plates can be linked to all the same objects levers can. The effects are listed in the Lever page.
A toggleable trap may prove useless when there is a large amount of invaders walking back and forth over it, as there will not be enough time for it to activate before it is switched again. Berserk dwarves will set off pressure plates.
Permanent Effect Pressure Plate System
It is possible to hook a toggleable pressure plate up to a device which makes its effect permanent until a lever is flipped. One way to do that is to build a water-sensing pressure plate in a 1x1 room, link that to your trap (a drawbridge, for example), build a hatch above it, link the hatch to the original pressure plate outside, and then put water on the hatch. When an enemy steps on the pressure plate, the hatch opens, the water falls onto the other pressure plate (water-sensing pressure plate), and it raises the drawbridge.
To make it resettable, you can use pumps controlled by a lever or by dwarf-power to move the water back to where it started. You can also build a flood-gate next to the water-sensing pressure plate (linked to a trigger obviously) and have the water flow and spread-out into a room where it can evaporate, however, water in a 1x1 tile will not require a lot of room to evaporate.
You can use this pressure plate system as an inverted trigger system. In otherwords, a lever that controls the permanent-effect pressure plate system can be flipped from off to on and subsequently trigger a trap or mechanism from on to off as a result, and vice versa. It depends on the water level settings used for the water-sensing pressure plate:
- 3 to 7 - Normal, on is on and off is off
- 0 to 2 - Inverted, subsequent state is opposite the main trigger
The water-sensing pressure plate can be set to trigger for water levels 0 to 2 or 3 to 7 because if the room above is filled to the brim with water, then the 1x1 room will fill-up to level 7 quickly and when opening the floodgate or pumping it out will drop to level 2 or below quickly.
- Shortcut b-T-p
- Components used: Mechanism
Common procedures
Closing a floodgate when submerged
For this example, say we want to close a floodgate when a reservoir reaches a depth of 5.
- Build the pressure plate, set to activate under water (w) from 0 (a, s) to 4 (z, x). Make sure it is set to reset (o) (The default is auto-reset and does not need to be changedv0.28.181.40d, you'll know it's set because the screen will read 'Resets').
- Place the floodgate
- Connect a lever to the floodgate, and open it.
- Optionally remove the lever.
- Connect the pressure plate to the floodgate.
- Allow the pressure plate to be submerged.
The floodgate should close a few seconds after the water reaches a depth of 5 or more, and open again when the depth falls to 4 or less.
It's worth noting that the reaction time of the pressure plate-floodgate system is quite large - the reservoir may overflow before the floodgate shuts. In order to prevent this (for example, when building a well filled from a river or brook), build a large run-off pool. This allows the gate enough time to close, and the water will instead continue to spread out instead of overflowing, as it would have in a smaller pool.
A Word of Warning
Depending on your setup, it can be dangerous to rely on pressure plates for automatic drains. Because of the opening/closing delay on Floodgates, a pressure plate can sometimes become 'desynced' with the floodgate it's connected to. Floodgates ignore any ON/OFF message they receive while they're in the process of opening or closing, so if your pressure plate is triggered multiple times over a short period (for example if the water on top of it is "sloshing around"), they can sometimes become 'stuck' in a position where a 7-7 plate is covered with 7 water, but the connected floodgate remains closed. It is strongly recommended that if you're building a pressure plate operated drainage system, you also link an "emergency lever" to the floodgate to avoid flooding your fortress in the event this happens. Also, if you intend to flood a passageway as a trap with a pressure plate, ensure that the plate is one-use only, otherwise goblins will march back and forth across it, opening and closing the floodgates as they do so.
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