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

G

Guest

Jan 1, 1970
0
How to modulate laser pointer
=====================
I want to modulate a Red and Green laser pointer (laser diode) with audio
sweep signal 1-10KHz square wave with adjustable duty cycle.
For the modulation I'm planning to use a simple transistor circuit where the
base of the transistor will be conneced to a signal generator and the laser
pointer will be connected to the collector of the transistor. The transistor
will be used to switch on/off the laser diode.

How to sweep the modulation signal and rotate the laser pointer
===========================================
Signal generator will sweep the frequency from 1KHz to 10KHz in 1second.
During this 1 second period laser pointer will be physically rotated from 0
degree to 180 degree (From Left to Right). Then signal generator will sweep
the signal from 10KHz down to 1KHz and laser pointer will be rotated from
180 degree to 0 degree (From Right to Left).

Laser receiver sensor
===============
There will be a receiver/sensor on an arbitrary position at a distance of
100meters (or more). When the laser light hits the sensor, I want to detect
the laser signal and demodulate the received signal. The frequency of the
demodulated signal will indicate the angle of the receiver/sensor with
repect to the laser pointer.

Questions
=======
1.) Can you advise a good way of modulating the laser pointer?
2.) What can I use as receiver sensor which can operate even in daylight?
3.) Can you advise a good way of demodulating/decoding the received signal?
4.) Would it make any difference if I use either Red or Green laser
pointers?

Thx.

Rahgu
 
L

Louis Boyd

Jan 1, 1970
0
Rahgu said:
How to modulate laser pointer
=====================
I want to modulate a Red and Green laser pointer (laser diode) with audio
sweep signal 1-10KHz square wave with adjustable duty cycle.
For the modulation I'm planning to use a simple transistor circuit where the
base of the transistor will be conneced to a signal generator and the laser
pointer will be connected to the collector of the transistor. The transistor
will be used to switch on/off the laser diode.

How to sweep the modulation signal and rotate the laser pointer
===========================================
Signal generator will sweep the frequency from 1KHz to 10KHz in 1second.
During this 1 second period laser pointer will be physically rotated from 0
degree to 180 degree (From Left to Right). Then signal generator will sweep
the signal from 10KHz down to 1KHz and laser pointer will be rotated from
180 degree to 0 degree (From Right to Left).

Laser receiver sensor
===============
There will be a receiver/sensor on an arbitrary position at a distance of
100meters (or more). When the laser light hits the sensor, I want to detect
the laser signal and demodulate the received signal. The frequency of the
demodulated signal will indicate the angle of the receiver/sensor with
repect to the laser pointer.

Questions
=======
1.) Can you advise a good way of modulating the laser pointer?
Direct pulse modulation is easiest. A mosfet or bipolar transistor will
do.
2.) What can I use as receiver sensor which can operate even in daylight?
A silicon pin photodiode would be sensible. You don't state the range
you need. A narrow band interference filter at the laser wavelength
in front of the diode will greatly reduce the effect of sunlight.
3.) Can you advise a good way of demodulating/decoding the received signal?

I'd use a carrier in the 100khz to 1 mhz range (fixed frequency) so it
would be easy to easy to filter out low frequency noise. THen you can
audio modulate the signal. Rather than using tone you could send the
angular positon as pulse coded data whhich is likley to yield better
angular resolution vs time than an angule vs frequency scheme. I'm
assuming you have a miroprocessor on both ends
4.) Would it make any difference if I use either Red or Green laser
pointers?

Red will have a little less solar interference, better sensitivity for
silicon detectors, and much lower cost for a given power level laser and
have less absorbtion by the atmosphere. Infrared in the 800 to 950 nm
range would do better still in all respects.
 
S

Sam Goldwasser

Jan 1, 1970
0
Louis Boyd said:
Direct pulse modulation is easiest. A mosfet or bipolar transistor
will do.
A silicon pin photodiode would be sensible. You don't state the range
you need. A narrow band interference filter at the laser wavelength
in front of the diode will greatly reduce the effect of sunlight.


I'd use a carrier in the 100khz to 1 mhz range (fixed frequency) so it
would be easy to easy to filter out low frequency noise. THen you can
audio modulate the signal. Rather than using tone you could send the
angular positon as pulse coded data whhich is likley to yield better
angular resolution vs time than an angule vs frequency scheme. I'm
assuming you have a miroprocessor on both ends


Red will have a little less solar interference, better sensitivity for
silicon detectors, and much lower cost for a given power level laser
and have less absorbtion by the atmosphere. Infrared in the 800 to
950 nm range would do better still in all respects.

In fact, if you get the absolutely dirt cheapest Far East import red
laser pointers, they have no regulator and no filter caps (just a current
limiting resistor) so directly controlling power will be able to modulate
from DC up to many MHz.

Better ones will have some sort of filter capacitors on the power and
modulating at more than a few Hz may not be possible without going inside.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
G

Gordon McComb

Jan 1, 1970
0
Louis said:
A silicon pin photodiode would be sensible. You don't state the range
you need. A narrow band interference filter at the laser wavelength
in front of the diode will greatly reduce the effect of sunlight.

<other good responses snipped>

Curiously, this poster has the same posting headers as "Rico Maxle," who
asked about long distance IR and collimation a few weeks ago. The end
application of this *still* sounds to me suspiciously like an
Opticon-buster.

-- Gordon
 
L

Louis Boyd

Jan 1, 1970
0
Gordon said:
<other good responses snipped>

Curiously, this poster has the same posting headers as "Rico Maxle," who
asked about long distance IR and collimation a few weeks ago. The end
application of this *still* sounds to me suspiciously like an
Opticon-buster.

In a free society each citizen has the right to learn and understand how
technology work and a moral and legal responsabilty to use his knowledge
safely and within the limits of decency and legality. Any technology
can be used for good or evil. Horrid as an "Opticon-buster" (whatever
that is) might be, having knowledge of how it works is not illegal or
immoral. I've been well trained in now to kill people by the US Army
paid for by your tax dollars. Is that knowledge evil? Only if I misuse
it, which I don't.
 
B

Brendan Gillatt

Jan 1, 1970
0
Rahgu said:
How to modulate laser pointer
=====================
I want to modulate a Red and Green laser pointer (laser diode) with audio
sweep signal 1-10KHz square wave with adjustable duty cycle.
For the modulation I'm planning to use a simple transistor circuit where the
base of the transistor will be conneced to a signal generator and the laser
pointer will be connected to the collector of the transistor. The transistor
will be used to switch on/off the laser diode.

How to sweep the modulation signal and rotate the laser pointer
===========================================
Signal generator will sweep the frequency from 1KHz to 10KHz in 1second.
During this 1 second period laser pointer will be physically rotated from 0
degree to 180 degree (From Left to Right). Then signal generator will sweep
the signal from 10KHz down to 1KHz and laser pointer will be rotated from
180 degree to 0 degree (From Right to Left).

Laser receiver sensor
===============
There will be a receiver/sensor on an arbitrary position at a distance of
100meters (or more). When the laser light hits the sensor, I want to detect
the laser signal and demodulate the received signal. The frequency of the
demodulated signal will indicate the angle of the receiver/sensor with
repect to the laser pointer.

Questions
=======
1.) Can you advise a good way of modulating the laser pointer?
2.) What can I use as receiver sensor which can operate even in daylight?
3.) Can you advise a good way of demodulating/decoding the received signal?
4.) Would it make any difference if I use either Red or Green laser
pointers?

Thx.

Rahgu

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!)
 
Curiously, this poster has the same posting headers as "Rico Maxle," who
asked about long distance IR and collimation a few weeks ago. The end
application of this *still* sounds to me suspiciously like an
Opticon-buster.

What's a Opticon-buster ..?
 
In fact, if you get the absolutely dirt cheapest Far East import red
laser pointers, they have no regulator and no filter caps (just a current
limiting resistor) so directly controlling power will be able to modulate
from DC up to many MHz.

How far will a such reach on a clear summerday outdoors ..?
(detected by some photodiode to off/on state)
 
G

Gordon McComb

Jan 1, 1970
0
Louis said:
In a free society each citizen has the right to learn and understand how
technology work and a moral and legal responsabilty to use his knowledge
safely and within the limits of decency and legality. Any technology
can be used for good or evil. Horrid as an "Opticon-buster" (whatever
that is) might be, having knowledge of how it works is not illegal or
immoral. I've been well trained in now to kill people by the US Army
paid for by your tax dollars. Is that knowledge evil? Only if I misuse
it, which I don't.

Sure, it's a free society. I just thought it was interesting the same
guy has (apparently) posted with different names, and never seems to
really indicate what the project is about.

-- Gordon
 
G

Gordon McComb

Jan 1, 1970
0
What's a Opticon-buster ..?

An "Opticon-buster" is a device that emits modulated light for the
purpose of affecting traffic signals. They're illegal except for fire
and police...and dangerous for other drivers, because it causes the
lights to change out of sequence. In any case, a penlight laser does not
have the power output to trigger an Opticon at 100 meters. I guess with
very careful alignment and a very good receiver circuit and optics you
could capture and decode modulated signals at 100 meters, during the
day. Otherwise...

Of course, maybe the OP has something different planned, but it should
would be nice to have some additional information, like "I want to build
a...". Makes for better answers, if nothing else.

-- Gordon
 
An "Opticon-buster" is a device that emits modulated light for the
purpose of affecting traffic signals. They're illegal except for fire

System of traffic bypass controlled by laser is only used in usa ..?
(I think in europe it's through radio)
and police...and dangerous for other drivers, because it causes the
lights to change out of sequence. In any case, a penlight laser does not
have the power output to trigger an Opticon at 100 meters. I guess with
very careful alignment and a very good receiver circuit and optics you
could capture and decode modulated signals at 100 meters, during the
day. Otherwise...

It almost appals me that systems like this won't have even basic crypto.
Even just sending a static code (like remotes) would improve it :)
Of course, maybe the OP has something different planned, but it should
would be nice to have some additional information, like "I want to build
a...". Makes for better answers, if nothing else.

Well people can always abuse technology..
 
G

Gordon McComb

Jan 1, 1970
0
System of traffic bypass controlled by laser is only used in usa ..?
(I think in europe it's through radio)

It's usually a strobe, but the cops can see these -- the strobe flashes
as a warning for people, and the IR content triggers the preemptive
controller. The all-IR models that are for sale in the US (where it's
illegal) are invisible. There are plans all over the Internet on how to
build one.

Frankly, I don't think a laser or other narrow beam would do much good,
and in thinking more about this, it's probably NOT the OP's intention.
There's no way it could be aimed accurately in a moving car.
It almost appals me that systems like this won't have even basic crypto.
Even just sending a static code (like remotes) would improve it :)

The later systems do have a more sophisticated modulation than the
originals. You have to remember this stuff has been around a long time
-- the 1960s for some of them. Kinda like how the music industry never
saw PCs coming, and the ability to record CDs...

-- Gordon
 
G

Gordon McComb

Jan 1, 1970
0
Terry said:
what do you mean, current limiting resistor? thats included in the
battery....

I was surprised they'd only use a resistor, however Sam knows his stuff
so I assume he's run across them.
most of the not-ultra-cheap (ie laser + battery) pointers I have
dismembered have had lasers with monitor photodiodes, and a crude
control loop:
<snip>

This has been my experience as well. I've taken apart maybe 20 of them
over the years, and they all had a control board with a MOB or SOIC chip
on them. I would imagine it's possible that some of the later versions
could have the control circuit on the laser die itself. Some of these
wholesale for sixty cents each, with a couple of holographic filters,
case, and battery, so they must be manufactured for about 5-10 cents.

-- Gordon
 
S

Sam Goldwasser

Jan 1, 1970
0
Gordon McComb said:
I was surprised they'd only use a resistor, however Sam knows his stuff
so I assume he's run across them.

:)

Yep, I've seen lots of them but never actually bought one myself. I can
tell no one is surprised. ;-)

Apparently they were available for awhile at Dollar Stores. These typically
came with multiple pattern heads. They have also been given away as
freebies at conferences and trade shows. Really crappy construction,
just hold together well enough to work for a short time.

See:

http://www.repairfaq.org/sam/slasptr.jpg

I don't see the surface mount 50 ohm resistor so it must be underneath
the PCB.

Interestingly, the laser diodes used in these have a much
wider operating current range (lower slope efficiency) than
the typical high quality devices which almost have to use
some type of optical feedback to deal with a wide
temperature range and differences between devices. The ultra
cheapies can be safely driven like LEDs. Perhaps crappier Far East
manufacturing techniques have an advantage or maybe specifically
designed that way.
<snip>

This has been my experience as well. I've taken apart maybe 20 of them
over the years, and they all had a control board with a MOB or SOIC chip
on them. I would imagine it's possible that some of the later versions
could have the control circuit on the laser die itself. Some of these
wholesale for sixty cents each, with a couple of holographic filters,
case, and battery, so they must be manufactured for about 5-10 cents.

Those are the ones above.

The older red pointers had discrete parts; some newer ones have an IC,
others still use discrete parts. There are several circuits in:

http://www.repairfaq.org/sam/laserdps.htm

I've never seen any with control on the laser diode chip.
For one thing, it's not practical to implement circuitry on the
GaAs-whatever material, so there would have to be a separate
silicon chip. Not worth it for the volumes involved.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
G

Gordon McComb

Jan 1, 1970
0
Sam said:
Yep, I've seen lots of them but never actually bought one myself. I can
tell no one is surprised. ;-)

Apparently they were available for awhile at Dollar Stores. These typically
came with multiple pattern heads. They have also been given away as
freebies at conferences and trade shows. Really crappy construction,
just hold together well enough to work for a short time.

See:

http://www.repairfaq.org/sam/slasptr.jpg

Hmmm. No doubt it's a laser substrate, but I'd question if it's really
lasing. If one doesn't care about actually making the material lase,
these have quite a wide latitude for current input. The beam will still
be "laser-like" to the average consumer.

Have you tested these pointers to see if they are operating in true
lasing range? If they are, they've figured out how to do something
pretty nifty with laser diodes: operate them without a current mirror or
other feedback!

-- Gordon
 
B

Bob May

Jan 1, 1970
0
Actually, Sony had a digital data scheme way back when CDs first got
started. I've seen the specs for audio, digital storage and mixed mode of
digital data and audio on a disk. There were some CD players that didn't
know about data disks but that nonsense was soon only on obselete CD
players. Company I was working for at the time was the first outside of the
major players in the digital data field that made a mixed mode CD. The
process was a bit involved but in the end, we got a glass master disk from a
place in the LA area.
 
S

Sam Goldwasser

Jan 1, 1970
0
Gordon McComb said:
Hmmm. No doubt it's a laser substrate, but I'd question if it's really
lasing. If one doesn't care about actually making the material lase,
these have quite a wide latitude for current input. The beam will still
be "laser-like" to the average consumer.

Have you tested these pointers to see if they are operating in true
lasing range? If they are, they've figured out how to do something
pretty nifty with laser diodes: operate them without a current mirror or
other feedback!

Yep, no doubt it's a laser. There's a distinct threshold, the speckle
is present, they can probably output 10 mW with a very well collimated
beam, and all for something that costs 10 cents to make. These do not
just look like a laser. :)

I should have noted that it's not just that one can't find a regulator,
varying the input voltage does have a major effect on output power.

I think what they figured out is how to make a really mediocre laser
diode with low slope efficiency. And from what I've heard, these
things are not very reliable in the long term, partially due no doubt
to the exposed die with no protection, but possibly also a result of
material impurities.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
G

Guest

Jan 1, 1970
0
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.

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
 
J

John Nagle

Jan 1, 1970
0
Rahgu said:
How to modulate laser pointer
=====================
I want to modulate a Red and Green laser pointer (laser diode) with audio
sweep signal 1-10KHz square wave with adjustable duty cycle.
For the modulation I'm planning to use a simple transistor circuit where the
base of the transistor will be conneced to a signal generator and the laser
pointer will be connected to the collector of the transistor. The transistor
will be used to switch on/off the laser diode.

How to sweep the modulation signal and rotate the laser pointer
===========================================
Signal generator will sweep the frequency from 1KHz to 10KHz in 1second.
During this 1 second period laser pointer will be physically rotated from 0
degree to 180 degree (From Left to Right). Then signal generator will sweep
the signal from 10KHz down to 1KHz and laser pointer will be rotated from
180 degree to 0 degree (From Right to Left).

Laser receiver sensor
===============
There will be a receiver/sensor on an arbitrary position at a distance of
100meters (or more). When the laser light hits the sensor, I want to detect
the laser signal and demodulate the received signal. The frequency of the
demodulated signal will indicate the angle of the receiver/sensor with
repect to the laser pointer.

Questions
=======
1.) Can you advise a good way of modulating the laser pointer?
2.) What can I use as receiver sensor which can operate even in daylight?
3.) Can you advise a good way of demodulating/decoding the received signal?
4.) Would it make any difference if I use either Red or Green laser
pointers?

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
 
J

James Waldby

Jan 1, 1970
0
:
....
....
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.

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: r=180, 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
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) .
....
 
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