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laser activated switch

Discussion in 'Electronic Basics' started by, Jul 10, 2003.

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  1. I'm looking for a circuit for a switch that can be activated by a standard laser pointer and nothing else. I know I could probably buy some very expensive filters to only let laser light pass through, but I'd prefer something that uses a phototransistor or photodiode that can adjust sensitivity and tune to the proper frequency and or wavelength. Anyone have any ideas? Thanks.
  2. Well, the wavelength thing sort of makes this tough. But, having faced
    similar challenges before, I can tell you that using a number of gel type
    filters from the Rosco sample book is a good place to start. Keep in mind that
    it will be nowhere near as good as a nice narrow band optical filter, it can
    still get a lot of the ambient light out of the way.
    Start by looking in the sample book at the spectral response curves and pick
    about three filters that pass your longer wave red well while blocking most
    everything else.
    The #19 (Fire) filter is a good, sharp cutoff type to start playing with.
    Overlap that on a #93 (Blue-green) and it will cut out a huge portion of the
    visible spectrum while letting roughly 50% of the laser pointer light through.
    Overlapped once again, this time with a #84 (Zephyr blue) and it will cut nearly
    everything out but allow red laser light to get through pretty well.
    Of course, this is no substitute for a real laser filter, which can
    sometimes be found for a buck or so at good surplus outlets, but if you spend a
    little time experimenting with the gel filter books and their passbands, you can
    get a pretty fair filter together.
    You know, modulating a laser pointer is really very simple and would greatly
    enhance the sensitivity of your circuit. Check my web page and you can find a
    section about laser pointers there which might be helpful.
    Finally, if you go just for light and no modulation, almost any cheap
    phototransistor can be used as your sensor and then any low current relay (or
    even a solid state relay) driven from it. But I would really rather use a
    modulated pointer and a "tone decoder" or PLL to do that.


    Chip Shults
    My robotics, space and CGI web page -
  3. Joe

    Joe Guest

    Sir Charles,
    I checked your webpage (address below?), and could not find a reference to
    laser pointers. I am interested in learning more about this. Would you mind
    posting a link to the page you are referring to?
    Thank you,
  4. Bob Masta

    Bob Masta Guest

    In addition to Chip's comments about filters, I wonder if maybe
    you can do something with the output of your phototransistor
    to detect laser pointer dynamics. I'm assuming here that the
    pointer is hand-held, so that the spot will not be steady on the
    sensor. (In fact, you may have a hard time hitting it at all!)
    So you might hi-pass or differentiate the output to discriminate
    against headlights, flashlights, daylight, etc.

    Just a thought...

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Shareware from Interstellar Research
  5. C.M. Hobbs

    C.M. Hobbs Guest

    i seem to remember a couple of kits that were a game of "laser darts".
    basically, a laser sensetive bullseye (using a normal laser pointer, you
    could play a game like darts). it seems to me that this would be easily
    modified for your purpose, if you can find one.

    i'll do some more research, i believe that they were fairly inexpensive.

  6. The easiest way would be to amplitude modulate the laser light (change
    its intensity a couple of 1000 times a second). The modulation could
    then be detected in the receiver with a bandpass filter.
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