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Reflective opto switch

Discussion in 'Electronic Design' started by martin.shoebridge, Oct 6, 2005.

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  1. Can someone point me in the right direction.... I want to make this with a
    sensitivity distance of around 300mm, without using any special type of
    reflective tape. I've had a go, but can only get 20 - 30 mm
     
  2. Guest

    You can probably do better if you use a pair of lenses, one to focus
    the light from your source LED on the area where you hope for the
    reflection, and a second one to collect a larger solid angle of the
    reflectd light and focus it on your receiving photo-diode.

    On the face of it, you will want at least one hundred times more signal
    at your receiver than you are getting at the moment, and that may be
    difficult, even with quite big lenses.

    One useful trick is to modulate the light being emitted by you LED -
    something like 10kHz is a popular modulation frequency - and use a
    synchronous (lock-in) detector on the signal coming out of your
    receiver.

    These days, the "synchronous detector" is often the A/D converter you
    get in a PIC microcontroller and similar parts. and you do the
    demodulation in the digital domain inside the microcontroller.
     
  3. It needs to fit in an 18mm dia tube ( like the commercial units- no
    lenses)so lenses are out. I already modulate at 38Khz ( carrier) + 1 khz.
    signal...... Line of sight I get 30 metres...
     
  4. Guest

    Line of sight is pretty much irrelevant - your reflecting surface
    spreads the incident light over a large sold angle, so the proportion
    that gets back to your detector is much lower than your are looking at
    with line of sight.

    That you modulate at 38kHz is a good beginning, but do you do
    synchronous detection at your receiver? You can reject a great deal of
    irrelevant noise if you build your detection circuit correctly (Win
    Hill's "The Art of Electronics" ISBN 0-521-37095-7 discusses this) and
    can get away with relatively high gain amplification at the modulation
    frequency.

    If you have got the signal processing optimised, the next step is to
    get a more intense light source. A laser diode module might do the
    trick, since the tightly collimated beam won't spread out much over
    300mm and should give you a much high intensity of illumination on the
    reflective surface, but when I last looked they started off at about
    $150.
     
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