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Infrared Receiver

Discussion in 'General Electronics' started by Andrew, Aug 23, 2004.

  1. Andrew

    Andrew Guest

    Hi all

    I've been trying to design an infrared receiver, but have several grey
    areas. My transmitter signal is a simple 10us pulse with a period of
    2ms (500Hz)- it's just a remote 'on' switch. The system needs to work
    in bright ambient conditions over at least 40m from a mobile hand-held
    Tx.

    No off-the-shelf products are available becuase they all have a BPF
    tuned to, say, 36KHz. (Plessey SL486 used to do this exact job). The
    output will connect to a microcontroller so the software can detect
    the signal.

    Using my limited knowledge I've come up with a circuit where the
    signal is processed as follows:

    1) A photodiode connected to a wideband transimpedance amplifier (this
    works well)

    2) A voltage controlled gain amplifier (AD602 or similar)

    3) A Peak Detect that is connected to the VCG amp to detect the peak
    voltage of the incoming signal and adjust the amp's gain to avoid
    saturation if the Tx is too close. And increase it if it's far away.

    4) A comparator to output to TTL

    5) A pulse stretch (555) to interface with microcontroller

    Am I barking up the right tree?

    I'd love to know what it is I really need in this circuit. Do I need
    an AGC to stop my amp saturating? Do you think I should include
    filters into my design? Is a peak detector the best way of controlling
    the gain of the amp in this case?

    Any help at all that anybody can offer will be very much appreciated.

    Andrew
     
  2. Robert Baer

    Robert Baer Guest

    It is my guess that the "bright ambient conditions" (= noise) alone
    will give S/N problems, and if not present, the distance (= diffusion)
    will also give S/N problems.
    Instead of a CW signal that you describe, if you would use a carrier
    (hmm...try 36KHz for grins), then one can have a much better chance at
    success.
     
  3. Andrew

    Andrew Guest

    Robert

    Thank you for your reply.

    Unfortunately I am bound by the 10us pulse because the system I'm
    designing has to be backwards compatible with the Tx.

    The ambient light causes the photodiode to conduct resulting in a DC
    off-set. This DC voltage can prevent the Tx signal from being detected
    and so needs tracking and cancelling. Fluorescent lights are a problem
    because they introduce AC noise. Therefore, I feel that I need active
    filters to prevent this noise from propagating, but I don't know how a
    square wave (10us pulse, period 2ms) works with regular op-amp
    filters.

    Cheers
     
  4. Robert Baer

    Robert Baer Guest

    Two things you could do: 1) use lenses, both ends, to help keep the
    beam on the photodetector; 2) use a long tube, flat black inside, as a
    light shield around the photodetector.
     
  5. There is a reason for using 36/38/40 kHz modulation, to avoid the very
    problems you're dealing with. So, you have a 10uS pulse every 2 mS ?
    That is plenty of time to do some elementary modulation on the TX side
    and use a commercial IR receive module. I used a 68JC908JK3 running 32
    MHz a divide by 15 prescale and a 22V10 for a further divide by 56 to
    make 38095 Hz (for a TV). For what you want,you likely don't even need
    a processor, just a little extra gating, maybe even a 555 (I don't
    like using an electronic hourglass to measure time) to make your 10 uS
    into say, 1mS. Then detect it with a Radio Shack $3 IR module.

    GG
     
  6. Andrew

    Andrew Guest

    Hi Glen

    I'd love to use a mudulated signal, but unfortunately I can't change
    the transmitter at all.

    An interesting point I discovered last week though: I was able to set
    up an off-the-shelf 38KHz IR receiver to pick up my obscure signal
    even though my signal isn't anywhere near that frequency- in fact, it
    doesn't have a carrier at all! I simply powered the device up and
    fired my Tx at it and it worked. How can this be when the IR Rx is
    supposed to filter out all signals other than 38KHz? I need a device
    that will give me more range though, which is why I'm going to plod on
    with my own Rx design.

    Many thanks

    Andrew
     
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