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need help with infrared leds

Discussion in 'Electronic Basics' started by vredsdfr, Feb 13, 2007.

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  1. vredsdfr

    vredsdfr Guest

    I am playing with some infrared leds and phototransistors. My project
    is a pinewood derby timer. It's computer based and I use a usb data
    acquisition box so the circuitry is pretty simple. I am a bit confused
    about the emitter and detectors I bought. I got the radio shack 276-142
    pair. Now... with my tv remote I can aim it over my shoulder, without
    looking, bounce the beam off the wall and it will work great. With
    these leds I have to push the current to 100mA+ and even then they
    have to be very close (not more than an inch) and almost perfectly
    aligned. Does anyone know of a better emitter and detector to use?
  2. The problem is not with your emitter detector pair, but with
    the signal passing between them. The remote control
    communicates with the television by producing short bursts
    of 40 kilohertz pulses to form a binary coded message. The
    receiver can ignore all the steady light it receives and all
    the light flickering at other frequencies from fluorescent
    lights and TV screens, because the detector signal is passed
    through a narrow band 40 kHz filter before being amplified.
    Then the amplified 40 kHz carrier is demodulated
    (rectified and averaged over several cycles) to reproduce
    the pulses that make up the binary coded message. The
    message may also have error correction bits added to it, so
    that even if it is a little corrupted, the message is pretty
    likely to be decoded.

    If your emitter is putting out a steady blast of light as
    its signal, that blast must overwhelm all other light picked
    up by the detector for the signal to be recovered. You can
    help this by adding a narrow pass optical filter over the
    detector (if one isn't already molded over it) but it will
    not help nearly so much as modulating the emitter signal
    with a carrier frequency.
  3. vredsdfr

    vredsdfr Guest

    It is.
    The blast of ir light is diluted by the other light sources? It seems
    to me that the other light sources would be additive and cause the
    transistor to saturate too quickly.

    It seems odd to me that I can saturate the transistor easier with a
    pulsed signal rather than a steady one. Obviously I am wrong but I
    don't understand it.
  4. The receiver transistor will not practically saturate as it
    receives the light signal, except for very short range
    applications (slotted interrupters). IR receivers deal with
    tiny signals that get amplifier by later gain stages.
    Give up saturating the receiver, and think about incremental
    signals. How do you distinguish between lots of ambient
    light, including variations at twice line frequency and TV
    scan rates, but find the small additional signal from the
    LED. This is a lot like building an AM band radio that
    finds a tiny carrier signal swamped in wide band noise
    hundreds or thousands of times larger except that the energy
    is in the form of IR instead of low frequency EM waves.
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