Motion Detection

This page is about mechanical and electronic motion detectors. For motion detection performed by people, animals, or other organisms, see Motion Perception.

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Basic Information

Motion Detection refers to technology and processes that can detect or identify movement or moving objects.

Applications for motion detection technology:

Simple Mechanical Methods

Things a spy might improvise in the field:

  • Tripwire - may be observed later to tell if an intruder has been here
  • Tripwire with alarm - a tripwire that sounds an alarm when tripped. Could be as simple as soda cans (with some rocks inside) tied to string.
  • A hair taped to the inside of a window sill - if the window is opened, the hair breaks free.
  • Very full glass of water - put on top of an object, if the object (or glass) is moved it'll spill.
  • Windchime or noisemaker on the back of a door - if the door is opened quickly, it'll make noise.
  • Fragile ground cover - includes the well-known pit trap, but also strewn twigs make sound when broken or flour that leaves tracks.
  • Rigged doorknob - string attached to the doorknob sets off some mechanism when pulled or slackened; the classic example is a gun pointed at the door.
  • Paired magnets - magnet attached to alarmed object, piece of iron within attraction range. Moving the magnet away from the iron will set off the alarm.

Intermediate Methods

  • The "nightingale floor" - a Japanese favourite that consists of an expanse of deliberately squeaky floorboards.
  • Pressure plate - apply or remove pressure, and it activates some mechanism. Common in bear traps and the like, but more cinematic versions are popular.
  • Attractive nuisances - something valuable-appearing that will likely be stolen by an intruder, may be connected to some sort of trigger.
  • Doorframe button - opening the door releases a switch built into the frame.
  • Alarmed lock or handle - mechanism built into the door activates if lock is opened or handle is pulled
  • Fake out - a detector that seems to have been tripped. Intruders "resetting" it shows that someone has been here.

Electronic Methods

  • Passive Electronic Scans
  • Active Electronic Scans
    • Ultrasound
    • Microwave
    • Laser trigger
    • Dual Technology Motion Detector - uses a Passive Infrared Sensor, which triggers an active sensor. If both score a hit, then it triggers the alarm function. Intended to prevent false alarms.
    • "Pet Immune" motion detector - A modification to other systems so it can't be triggered by motion of objects that weigh less than 80 lbs.

Cinematic Methods:

Things only likely to show up in fiction for various reasons:

  • Anemometer - movement near it produces a small wind. Must be sensitive enough to detect this wind, but not sensitive enough to be set off by regular air movement.
  • Baryometer - room is kept at pressure difference to outside, opening door equalizes pressure. Requires either airtight rooms or active pumping.
  • Ward Magic - a spell of some kind detects when someone has entered. Uses magic.

See Also:


Game and Story Use

  • These may be hazards, traps, and alarms that the PCs have to overcome.
  • Alternately, the PCs may employ such devices for their own protection.
  • Each method has it's own advantages and drawbacks. Some are more easily defeated or triggered than others. Some only record what happened, others scare intruders off, warn authorities, or even take aggressive action.
  • The pet-immune sensors are a liability in a setting that has magic, demihumans or robots. Or, indeed, various species of animals such as monkeys and raccoons.
  • Likewise, passive infrared sensors are likely to be ineffective against anything close to ambient: undead, constructs, some robots…
  • Motion detection, especially for security purposes, is often (when possible) paired with some kind of bypass method to prevent action from being taken every time the sensor goes off: punch in the code to tell the alarms you're allowed in, get buzzed through at the front desk, pull this lever to disarm the trap for fifteen minutes, etc.
    • This gets more important the more force the alarm itself can bring: a sensor that wakes up the receptionist is less likely to have a bypass than one summoning an armed response, and both are less likely than one attached to a sentry gun.
    • This also makes a handy explanation for skills like Disable Device, especially partial successes: you didn't completely disarm the trap, you just figured out how the dungeon inhabitants avoid setting it off every time they pass through.
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