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Detecting IR frequencies

Discussion in 'Electronic Basics' started by [email protected], Sep 14, 2005.

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  1. Guest

    I want to give my bedroom the special treatment for CHRISTmas and
    decorate it up. I have one of those Wild Planet (the company that makes
    all that spy gear for kids) motion detectors. I am wondering if there
    is some way to find out what frequency(without buying an ir detector
    from Radio Shack, the remote for my Altec Lansing iM3 Ipod speakers
    told the Ipod to play, so that whenever somebody walked into the room
    Manheim Steamroller, and other Christmas music, would start playing.
  2. Most remotes are at 38 kHz.... See for an
    interesting read on remotes. There are directions on the site for
    building transmitters. It's not trivial - but it is fun.
  3. Guest

    38 khz? Are you saying these remotes run off AM not FM?
  4. I don't know --- all I know is that is stated frequency of the receivers
    I use and they work.... Check out vishay 1738 - I have made several of
    these and they work with common IR remotes.

  5. Why would you get that from what he said? It could mean a carrier
    frequency for an AM signal, or the center frequency for an FM signal.

    Besides, if it was amplitude modulated, more likely you'd just see
    a modulation of the IR LED, rather than have a subcarrier feeding it.
    But there are very good reasons for not doing it that way, because
    it will mean the circuitry is more likely to be interfered or confused
    with by light levels in the room (or for that matter random light levels
    could cause the unit to change channels). Finally, when you start
    amplitude modulating an LED, you can't set things for optimum performance.

    Which means that it will likely be FM, because it doesn't matter if
    you directly modulate the LED or modulate a carrier that feeds the LED.

    With FM modulation, the feed to the LED is constant, hence it can
    be set for maximum output.

  6. Gareth

    Gareth Guest

    The method of modulation does not depend on the frequency. You are
    probably thinking of commercial radio where FM radio is around 100MHz
    and AM is ~100kHz.

    Most remote controls have very simple modulation. There is a carrier,
    which is probably around 38kHz. This carrier is switched on and off (at
    a frequency much lower than 38kHz) to give a pattern of pulses which
    corresponds to the button you pressed. I'm not sure how you would find
    this pattern unless someone else has done a similar project - try a
    Google search.


  7. Tom LeMense

    Tom LeMense Guest

    I would hazard a guess that the motion detector uses "Passive IR" or "PIR"
    detection, which is essentially a heat sensor that registers the change in
    heat in its field of view and sounds an alarm. This is the principle that
    the vast majority of home security motion detectors, automatic lamp sensors,
    etc. employ.

    If my assumption is correct, there's no emitted IR signal to detect. The
    Wild Planet box is simply a sensor and makes noises when it detects motion.

    I'm basing this on what I found at this link:

  8. Jasen Betts

    Jasen Betts Guest

    the frequency of the IR is the leasst critical thing,
    what you need to track down is the exact IR pulse sequence the remote sends.

    if you don't want to buy an IR detector open up the remote and attach an
    apropriiate device to the LED and record the pulses it gets.

  9. Jasen Betts

    Jasen Betts Guest

    No he's wrong they operate at around 320THz :)

    the IR beam is amplitude modulated (well pulsed would be more acurate)
    at around 38KHZ and the 38KHZ itself is pulsed to send
    a digital signal coressponding to t he keypress etc that the remote
    wants to send.

    there may be remotes that do FSK or PSK on the 38KHZ "carrier" or use some
    other frequency...
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