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Motion detectors

  1. Jun 3, 2009 #1
    hi all

    im new in this forum and i have a quick question:
    i need to know which motion detectors can i use to trigger inputs
    i.e , if a motion detector detects another motion detector , then it will trigger the input, otherwise, nothing will happen if the motion detector dosent detect the another motion detector.

    thx alot :)
  2. jcsd
  3. Jun 3, 2009 #2


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    Staff: Mentor

    Welcome to the PF. There are many types of motion detectors, and your requirement sounds a bit unique. Can you tell us what the application is so that we can understand your requirements better?

    The simple motion detectors use a multi-segment IR lens and an IR detector behind the lens. They are called PIR (passive IR) motion detectors. As a source of IR (like a human body) moves in front of the PIR device, the multiple images of the moving IR emitting object are projected back towards the IR sensor. Because there are multiple "blobs" of the image formed by the multi-facet lens, the motion causes the output of the IR sensor to vary. This variation is detected, and trips the motion sensor.

    So are you trying to detect motion of humans and animals, or motion of robots, each with a motion sensor on it?
    Last edited: Jun 3, 2009
  4. Jun 3, 2009 #3


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    Gold Member

    This does sound a bit peculiar. If it's just a matter of one detector being triggered by another one, then I assume that you have control of the design of both. Wouldn't it be easier to just directly couple the two with an optical or radio link?
  5. Jun 3, 2009 #4


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    Science Advisor

    Motion detectors are just passive devices and they can't trigger each other unless you connect them electrically somehow.
  6. Jun 6, 2009 #5
    thx all for the reply

    what im trying to do exactly is: I need to install a motion sensor in my car, and the other motion sensor will be placed in the gate opener circuit in a place that will keep the circuit open, so when the car reaches home or near the gate, the motion sensor that placed on the circuit will close it in order to complete the circuit to open the gate without pressing a button from the remote control :)

    thanks a bunch
  7. Jun 6, 2009 #6


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    Science Advisor

    It sounds more like you need an RFID stystem.

    These are used in some big cities (like San Francisco) to automatically collect revenue from vehicles going over a bridge or a toll road.

    Cats and dogs have a RFID chip placed under their skin so they can be identified

    You don't want every person out walking their dog to open your gate do you? You want only your car to trigger it. Motion detectors are pretty crude devices and they can't discriminate between targets. They just receive infra red from anywhere and trigger if it changes suddenly.

    I don't know if you can buy these RFID devices as a pair. I tried to get some for a running club to automatically log the times of marathon runners, but it got too expensive.
    Try Google for RFID?
  8. Jun 7, 2009 #7
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