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Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


Don't remember the name but they were in the City of Industry.  Enter on the
left (as you look at the front of the building) and there were two stations
for looking at the video tapes that the customers brought in.

--
Why do penguins walk so  far to get to their nesting grounds?



Re: Modulation/demodulation of Red & Green laser pointer and laser sensor



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It could have been them. When I worked for them, they were located
in Anaheim. But I seem to recall hearing they were once located
elsewhere and C of Industry sounds familiar. I know someone I
could ask.

But even so, it was fun playing with those 'toys'.

Brian
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Seismic FAQ: http://www.skywise711.com/SeismicFAQ/SeismicFAQ.html
We've slightly trimmed the long signature. Click to see the full one.
Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


They started the tape and immediately heard the noise of digital data which
confused them and the volume was loud.  The software engineer was somewhere
else but I was there  and told them to just turn down the volume.  Later on,
in the audio section, the engineer had come back in (they heard the noise
all over the front part ot the factory) and he told them to turn up the
volume.  The guys were amazed that they heard the sounds of the audio
section so clearly (no background noise as the audio was all digitally made,
probably another first).  We got the glass master about 4pm. that day.

--
Why do penguins walk so  far to get to their nesting grounds?



Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


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Red laser pointers can be fairly simply modulated.
Many green laser pointers have the laser not running all the time, at
maybe 1Khz.

Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


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Have a look at:
http://sci-toys.com/scitoys/scitoys/light/light.html#laser_communicator
It pretty much tells you step by step on how to do it - have fun (I did!)

Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


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so the end of the beam (100m out from the source) will be traveling at 314m/s
(over 1000Km/h) and to detect a 1KHz signal and have any chance of
determining the frequency you'll probably need atleast 3 cycles, so 3ms,

which means the overlap of your sensor and the end of the laser beam will need
to be .003*314 metres wide, (a little under one metre)

you'd probably do better to sweep from 20Khz to 10Khz or to modulate with
digital position data or send a radio pulse at the start of each sweep.
or possibly sweep at different rates...

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picking "gel" films that block the unwanted frequencies may help.
using a prism or difraction grating may work too.

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start with an oscilloscope and work up from there.

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mostly the way it looks.
The sun outputs more green than red light so red may see less interferance,
but sunsets and sunrises are often red and close to the horison.

--

Bye.
   Jasen

Re: Modulation/demodulation of Red & Green laser pointer and laser sensor



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Dear Jasen,

Thank you for your informative reply.
As you have pointed out, the scan speed is a critical issue and I'll try
much higher frequencies than 1-10KHz.

As I have mentioned in my original post, my aim is measure/determine the
relative angle between "transmitter" and the "receiver" (nothing to do with
traffic lights etc as some others suggesting) .

Another critical problem is to accurate pointing the transmitter laser with
the receiver. The receiver must be on the same horizontal plane generated by
the scanned laser beam. So I'm thinking to generate vertical line and scan
this vertical laser line from left to right and from right to left instead
of scanning a point shaped laser beam. Vertical line elimantes the problem
of accurate positioning the receiver and transmitter on the same horizontal
plane.

Images on the following link (http://www.solarstop.net/mrshims/l58.asp )
explains the concept of vertical line. Problems with the line generations
are (A) finding low cost components to generate line (B) when I have
vertical line, the effective power/light intensity reaches to the receiver
will me much less compared to point laser.
Do you think it can still be detectible?
Which receiver/sensor component would you recommend?

Regards,

Rahgu



Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


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Rather than modulating with a swept frequency, just use a constant
high frequency, for easier detection; and use the direction and time
between hits to determine the angle.  For example, if the sender
does r degrees per second and the time between illuminations
from the right and then the left [OP says sending head rotates 180
left, 180 right, repeatedly] is t, the receiver is r*t/2 degrees
clockwise from the counterclockwise stop.  Example:  r18%0, t=.3
gives 27 degrees.  For extra credit, do least squares analysis on
successive hits.  If you don't want to use two sensors (to enable
whether hit is left or right) alternate pairs of 1-second sweeps
with pairs of 2-second sweeps, or some such arranglement.
 
-jiw

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Re: Modulation/demodulation of Red & Green laser pointer and laser sensor



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Thank you Jiw.
Using direction and time is very good idea!.

If we assume that the receiver is mobile (ie. can move) then am I right that
sender and receiver must be in sync so that the receiver can start a
timer/counter when sender start scanning and stop the timer/counter when it
receive the laser hit?

I have one more issue; the photodiodes and phototransistors that I have seen
so far have upto 120 degree view angles. I'm thinking to position  multiple
sensors at different orientations/directions, to make the receiver omni
directional ie. can receive the laser signal from any direction. Is there
any better, more efficient method to receive laser pulse from (almost) any
direction ?

Regards

Rahgu



Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


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     Actually, the swept frequency idea is a good one.  You just need
to modulate at a high enough frequency that you see a reasonable
number of cycles when the beam goes by.  You can modulate a laser diode
at a few megahertz easily, although that may require something
more than a modified laser pointer.

     The main problem with the swept frequency idea is that you'll
have to build some custom electronics for both the transmitter
and receiver.  The parts count and cost won't be that high,
but you need to know what you're doing.  "The Art of Electronics",
by Horowitz and Hill, may be helpful, even though it's dated.
Expect to spend considerable time with a solderless breadboard and scope.
(You must have a scope to do this, or you'll never figure out
what's going on.)  But you'll learn something.

                John Nagle

Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


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He was talking about having the laser scan back and forth in a sweeping
motion, not in a circle.  You could even do it with a full 360 deg sweep.
If you do that, then the receiver doesn't need to be directional, and
doesn't need to be in sync with the transmitter.  It simply looks at timing
of the pulses created when the beam hits the receiver.  It will create an
uneven duty cycle between each pulse.  So, you receive a pulse, and start a
counter to see how long it takes before you receive the next pulse, then do
that again to see how long it takes to receive the third pulse.  You then
look at the ratio between those two times to determine where in the sweep
pattern the receiver is located.

If the spacing is even, the receiver is in the middle of the sweep.  If
it's uneven, it's to one side or another relative to the difference in
timing.

Now, without more help, this doesn't tell you which side of the sweep you
are on.  But I assume you are doing this to allow the receiver to locate
itself.  So you are going to need at least two transmitters.  If you use
three then I suspect it could always solve for the correct location using
all the data.  And actually, if two transmitters are pointed so the
mid-line of the sweep doesn't cross, I think you can do it with only two
transmitters and always solve for the correct location.

Or, if you are modulating the laser, and could modulate it with one
frequency when scanning in one direction, and modulate it with another
frequency when scanning in the opposite direction, then the receiver could
tell which direction it was sweeping and correctly solve where in the sweep
pattern it was located.

Another possible option is to use multiple receivers side by side and see
which one gets hit first to understand which direction the beams are
sweeping.

I have no idea however if any of this will produce the accuracy you need.

--
Curt Welch                                            http://CurtWelch.Com /
snipped-for-privacy@kcwc.com                                        http://NewsReader.Com /

Re: Modulation/demodulation of Red & Green laser pointer and laser sensor




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some sort of cylindrical lens ?

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I don't know. What do they use to detect police lidar speed traps?

Bye.
   Jasen

Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


Another way to this is to make an "angle of arrival sensor" Depending
on the angle the incident light makes with light makes with the
aperature the position of the focused spot will change. You can detect
this by placing a camera at the focal plane of the lens. There are
other ways to do this with wavefront sensing etc as well.

john muth


Re: Modulation/demodulation of Red & Green laser pointer and laser sensor



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Hi John,

Thank you for  the new ideas.
I want to be able to measure relative angle between transmitter (sender) and
receiver where they may be located 100 meters (or more) apart from each
other and receiver is mobile (moving). I think determination of the position
of the focus sounds a bit difficult (I may be totaly wrong if there is a
simple method for this).

I do not have any clue about your other suggested method of "wavefront
sensing" . Can you give us a bit info abut this method and the other methods
that you've summarized as "etc" ?

Once again, thank you for your contribution.

Regards,

Rahgu



Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


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    That's a nice approach to indoor navigation.  I gather that the
idea is that you have several of these beacons rotating in a room,
and the detector on the robot gets a bearing from each of them.
That should work.  Another approach is to have a rotating
beacon on the robot which hits retroreflectors on the walls.

    First, modulating the laser with a few KHz should work.  You can
buy small laser modules with modulation inputs.  See

    http://www.lasermate.com/GRImodule.htm

But modifying laser pointers should work into the KHz range.

On the receive side, check out the detectors at the above site.
Also consider putting a narrowband interference filter (from
Edmund Scientific) on the receiver to reject light other than
the laser wavelength.

    You can use any wavelength you like, including IR lasers.

                John Nagle

Re: Modulation/demodulation of Red & Green laser pointer and laser sensor



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5.) Is there any advantage or disadvantages using IR instead of Red or Green
laser?

  Thx.
  Rahgu
 



Re: Modulation/demodulation of Red & Green laser pointer and laser sensor


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Advantages for infrared:
Silicon detectors are most sensitive around 800-900 nm
Sunlight is weaker.
If you're working long distance (>100m) atmospheric scatter will be less.
powerful lsaers (> 50mw) are more readily available in the IR,
particularly between 800 and 840 nm.
Humans cant see it.


Disadvantage of infrared.
Alignment and focusing can't be done visually.  you need do do that with
an image intensifer or a video camera.
Humans can't see it.


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