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27927Re: Sensing Lasers..

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  • Mustafa
    Jun 2, 2006
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      Thanks alan, but i had already changed the lens to spread out the
      laser. The phototransistor and photodiode i used are clear plastic
      types and so i do not expect them to have an ir filter.

      Also i am having different problems while using phototransistor and
      photodiode. The phototransistor is able to sense unmodulated laser
      quite well, however when i observed the output from the
      phototransistor (for this the output was observed directly from the
      collector terminal, i.e. before any bandpass filter stage) with
      modulated laser on a CRO, i found narrow and small amplitude (around
      1.2V) spikes at the same 10 KHz frequency instead of a square wave
      with 5V amplitude.

      On the other hand the photodiode shows very little response towards
      laser (unmodulated) and that too when the laser is extremely close to
      the diode.

      The application actually requires sensing the laser from about 1 m and
      that is the reason behind using a laser instead of ir led.

      Btw Jim could you please give more information on how leds can be used
      as light sensors.. do they offer any advantage over photodiodes that
      are supposed to do the job?

      --- In SeattleRobotics@yahoogroups.com, Alan King <alan@...> wrote:
      > Mustafa wrote:
      > >Hi everyone..
      > >
      > >I am making a circuit for sensing laser modulated at a frequency of
      > >10kHz. I tried using phototransistors and photodiodes to sense the
      > >laser, but both are unable to give sufficient response for laser. the
      > >photodiode circuit works fine with ired, but does not show any change
      > >in output with laser.. Does anyone know what can be used to solve the
      > >problem? The laser used is a small ordinary one available from
      > >road-side vendors.
      > >
      > >
      > >
      > Look at the beam angle and sensitivity. Lasers put out very little
      > light except down the beam, unless your spot is right on the detector
      > you won't see much of anything. Get one that has changable lenses and
      > spread it out, or if you need long distances get the spot on the
      > detector. If you've already got the spot on the detector, you may be
      > just swamping out the detector, with so much light coming in it may
      > not be turning off at all. If it's working with the relatively low
      > amount from an IR LED it may be way too sensitive for a direct laser
      > beam.. Also, working with IR LED? Cheap red laser pointer? If it has
      > a dark IR filter you may not be getting much of anything to the sensor
      > at all if you're using the same sensor. Of course if you've tried
      > several then most likely at least one or two were clear enough.
      > Alan
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