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Thread: PIR Motion Detector interference

  1. Default Have you seen these in your

    Have you seen these in your office?*

    *Intrusion Detection devices.* Download the recording and see if it looks similar.

  2. #12

    Default That is a good find - it

    That is a good find - it could be it. The signals that cmaiolani submitted were stronger, but similar.

  3. #13

    Default Hi, thanks a lot for


    thanks a lot for replay.
    There are these devices but they are not wireless.

  4. #14

    Default Hi, I would agree with one

    I would agree with one of the earlier posts, that the interference is probabaly due to PIR intrusion detection equipment. I have been regularly coming across this type of problem since the mid 1990s and it has always been my biggest interference issue.

    Although most modern PIRs are cabled it is the sensors themselves that cause the problems as they contain what is known as dual-tech components, usually infra-red with a radio back-up. These type of sensors are less prone to 'going off' accidentally. Unfortunately for WLANs a lot of the sensors use 2.4GHz, which can cause havoc accross two-thirds of the ISM band. However there are two other types of dual-tech units that are totally benign to 2.4GHz WLAN operations. These still use microwave radios as the active component but operate at much higher frequencies. The two frequencies used by these sensors are known as K-band units which operate within the 24GHz range and X-band detectors which operate around 10GHz. Neither of these types of PIR sensor have any affect on 802.11a/b/g WLANs whatsoever, being able to coexist with 2.4GHz or 5GHz WLANs without detrimentally affecting their operation.

    Over the years I have managed to persuade a few customers to totally rip out their existing PIRs and install all new non-interferring types. One of them had over fifty of the things installed in a small warehouse in London... his WLAN handheld devices never worked correctly for three years until they were replaced!

    You can glean whether the PIRs being installed have radio components onboard as there is usually a small sticker on the box it comes in, which gives its operating frequency. If its not given on the box it is always stated somewhere on the instructions that are inside. Of course in most cases they are already in situ. If that is the case then all you can do is take a photo and then trawl through PDF catalogues to try and identify the culprit. I've spent many a fun hour doing this!

    Hope this helps.

  5. #15

    Default Hi to all, i have found the

    Hi to all,
    i have found the source of interference...the sensors of presence of the alarm!!
    they work with double tecnology (INFRARED and Microwave).
    But in this case the microwave used are the wifi range.
    Each sensor has a pick in one frequence.
    6 sensors = 6 peaks
    i want to thanks all for the support, in particoular way philbee (your feedback was very precious for the discovery of the problem!) and i would like that you can see a photo of these terrible sensors..

    Last edited by MetaGeek; 10-22-2009 at 03:53 PM. Reason: added images

  6. Default thanks for posting

    thanks for posting this!

    We're glad you found it! * Will you email me a recording? A good clear close recording would be great.* Otherwise, can I use the recordings you posted earlier and post them in our library?

  7. #17

    Default Sure. You can post this


    You can post this recording:

    At the start one sensor is active. Then i turn on the second sensor and you can see a new peack, also when i put the wispy near the sensor.

    Then i turn off the sensor again and the peack goes down.

  8. #18

    Default Hi Gents, Glad to have been

    Hi Gents,

    Glad to have been of assistance cmaiolani, PIRs have been one of our biggest interference problems ever since we first discovered it in a supermarket in Northern Ireland in 1997. As I said in the earlier post I have been working with proprietary 2.4GHz and then 802.11 onwards since then. In fact when we first purchased a Wi-Spy I was suprised that no signature waves for these critters was would have been my number ONE!

    Maybe Trent could enlighten us, are these type of PIRs unknown in the US? If you would like some more traces of PIR interference I'm sure I can oblige.


  9. #19

    Default You are correct, microwave

    You are correct, microwave only detectors used 2.4 - 2.5 Ghz as well as automatic store door openers until WiFi came along.

    When PIR and Doppler Radar are combined the alarm industry calls them Dual-Tec units (regardless of the maker) however DUAL TEC(C) is a trade mark of Honeywell Security and an excellent product.

    To my knowledge all newer Dual-Tec units (1996 and newer) typically used "K" band. See:

    Frequency Chart:

    Microwave Frequency Band:
    Frequency at which the microwave signal is radiated and
    received. Higher frequencies generally penetrate non-metallic objects less than lower frequencies.
    S-band: 2.54 GHz
    X-band: 10.525 GHz
    K-band: 24.125 GHz

    WiFi is an allowable use of the "S" band:

    A significant source of interference for the "S" band was fluorescent lighting rf interference, hence the switch to Dual-Tec.

    Motion Detectors use:

    Digital Fluorescent Light Filters:
    Software algorithms provide infinite rejection of microwave interference due to fluorescent lights.

    Analog Fluorescent Light Filters:
    Hardware circuitry provides rejection of microwave interferences due to fluorescent lights

    This may still be an issue today with WiFi.


  10. Default Thank you both for the very

    Thank you both for the very useful input!

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