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27928Re: [SeattleRobotics] Re: Sensing Lasers..

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  • Jim McBride
    Jun 2, 2006
      I am not sure if there are any advantages other than their size. Take your
      volt meter and connect it to the leads of an LED. You will find that it
      gives off voltage proportional to the amount of light that you shine on it.
      Beam robots use flashing LEDs. The LEDs will flash at a certain threshold
      voltage. As the cap on the robot slowly builds up voltage the LEDs get
      closer to that threshold voltage. The voltage that is generated due to
      light is added to the caps voltage. In this way when the cap has a
      sufficient charge, the LED with the most light on it will hit that
      threshold voltage first which triggers it. The "neuron" engine is tripped
      by the trigger and the caps power is dumped to the opposite motor, spinning
      it. This will make the robot turn in the direction of the light.
      Are you trying to see the direct light of the laser?

      At 07:37 AM 6/2/2006, you wrote:
      >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
      > >
      >Visit the SRS Website at http://www.seattlerobotics.org
      >Yahoo! Groups Links


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