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

Discussion in 'Hobby Electronics' started by Guest, May 22, 2006.

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  1. Guest

    Guest Guest

    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.

    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


  2. Louis Boyd

    Louis Boyd Guest

    Direct pulse modulation is easiest. A mosfet or bipolar transistor will
    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.
  3. 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:
    Repair | Main Table of Contents:
    +Lasers | Sam's Laser FAQ:
    | Mirror Sites:

    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.
  4. <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

    -- Gordon
  5. Louis Boyd

    Louis Boyd Guest

    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.
  6. Have a look at:
    It pretty much tells you step by step on how to do it - have fun (I did!)
  7. Guest

    Curiously, this poster has the same posting headers as "Rico Maxle," who
    What's a Opticon-buster ..?
  8. Guest

    In fact, if you get the absolutely dirt cheapest Far East import red
    How far will a such reach on a clear summerday outdoors ..?
    (detected by some photodiode to off/on state)
  9. 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
  10. 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
  11. Guest

    An "Opticon-buster" is a device that emits modulated light for the
    System of traffic bypass controlled by laser is only used in usa ..?
    (I think in europe it's through radio)
    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 :)
    Well people can always abuse technology..
  12. 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.
    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
  13. I was surprised they'd only use a resistor, however Sam knows his stuff
    so I assume he's run across them.

    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
  14. :)

    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.


    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.
    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:

    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:
    Repair | Main Table of Contents:
    +Lasers | Sam's Laser FAQ:
    | Mirror Sites:

    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.
  15. 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
  16. Bob May

    Bob May Guest

    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.
  17. 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:
    Repair | Main Table of Contents:
    +Lasers | Sam's Laser FAQ:
    | Mirror Sites:

    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.
  18. Guest

    Guest Guest

    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

    Images on the following link ( )
    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?


  19. John Nagle

    John Nagle Guest

    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

    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
  20. James Waldby

    James Waldby Guest

    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.

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