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.
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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
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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?
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In sci.electronics.design Rahgu Seghar wrote:

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

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!)
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On 2006-05-22, <Rahgu Seghar> <Rahgu> wrote:

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...

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

start with an oscilloscope and work up from there.

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.
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Bye.
Jasen
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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
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"Rahgu Seghar" wrote: ...

...
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: r0, 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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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
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Rahgu Seghar wrote:

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
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<Rahgu Seghar> wrote:

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.
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<Rahgu Seghar> wrote:

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 /
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On 2006-05-23, <Rahgu Seghar> <Rahgu> wrote:

some sort of cylindrical lens ?

I don't know. What do they use to detect police lidar speed traps?
Bye. Jasen
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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
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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
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Rahgu Seghar wrote:

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
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5.) Is there any advantage or disadvantages using IR instead of Red or Green laser?
Thx. Rahgu
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Rahgu Seghar wrote:

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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