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Problem with IR detection.

Discussion in 'Electronic Design' started by pimpom, Apr 8, 2010.

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  1. pimpom

    pimpom Guest

    I'm trying to devise a presence detector using an IR beam and a
    standard remote control IR receiver module. I couldn't find
    datasheets for the receiver modules I can find locally and
    proceeded on the assumption that they'd be similar to the ones
    for which I have datasheets, like the Vishay, Sharp and Panasonic
    types.

    To minimize interference from unwanted radiation sources, the
    known types all set limitations on burst length, gap time and
    maximum number of bursts per second, and I made a note of those.
    But I cannot get the system working properly. I get detection
    briefly when the beam is unblocked and I get the expected
    waveforms at the receiver output, but the signal then dies down
    quickly. I've tried varying the burst length from 300-800usec
    with corresponding gap times.

    I'm not fully conversant with RC codes, but since I don't need to
    send data, I assumed that I wouldn't need those codes. Obviously
    I'm missing something, probably to do with using regular bursts.
    Can anyone point me in the right direction towards solving the
    problem?
     
  2. pimpom

    pimpom Guest

    I know about the carrier frequencies. My receiver module is a
    38kHz type and I have the Tx carrier at that frequency with
    pulses of approximately 40% duty cycle. To comply with the noise
    suppression requirement from the datasheets, the pulses are
    switched on and off in bursts. I started with a burst length of
    about 500 usec (about 18 cycles) and about the same gap time.
    When that didn't work, I varied the burst and gap lengths from
    300 to 800 usecs. No difference. The pulses appear at the
    receiver output for a brief moment when the beam is unblocked and
    then disappear. The receiver works with my TV remote control.
     
  3. pimpom

    pimpom Guest

    I'm not using a readymade transmitter. I made my own using a
    38kHz carrier, switched on and off in bursts. I started out with
    about 500usec on, 450usec off. When that didn't work (the
    received waveform appears briefly and then disappears), I varied
    the on-off periods between 300 and 800 usec. No difference. The
    receiver works with my TV remote control.
     
  4. pimpom

    pimpom Guest

    My receiver circuit already complies with the suggestions in your
    last para.

    I tried the freq adjustment but it's no good. My Tek 2236 scope
    with a built-in frequency meter is temporarily out of commission
    and I haven't yet found time to dig out my old Hameg counter -
    the gated type. The scale on the basic 15MHz single-trace scope
    I'm using now seems to be quite accurate (as visual reading of
    scope traces go) when I checked it against mains waveform.
    According to that scope, the adjustment range is about 32-42kHz.

    Most IR receivers seem to be quite tolerant of reasonable amounts
    of frequency mismatches. Datasheets for 38kHz types indicate
    about 50% efficiency at 36 and 40kHz, and I've been testing my
    circuit at Tx-Rx ranges from a few inches to 3 ft.
     
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