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

It might be sufficient to use an audio O/P transformer with the O/P winding in series with the laser supply, but if you're using proper current limited feeds this probably won't work! It may be better to use a coupling cap to the base of the current regulator transistor.

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:

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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.

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

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.

Have a look at:

pretty much tells you step by step on how to do it - have fun (I did!)

What's a Opticon-buster ..?

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

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

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

what do you mean, current limiting resistor? thats included in the battery....

most of the not-ultra-cheap (ie laser + battery) pointers I have dismembered have had lasers with monitor photodiodes, and a crude control loop: npn drives laser, PD goes to base of pnp with pulldown, pnp emitter to battery, collector to npn base. the pnp then controls the laser intensity (in theory. best not to think about accuracy .)

(too lazy to draw sch).

an open-collector transistor will happily OOK the laser, if connected to npn base.

Cheers Terry

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..

snipped-for-privacy@spamnuke.ludd.luthdelete.se.invalid wrote in news:447247fe$0$491 $ snipped-for-privacy@news.luth.se:

I understand there are systems like this, using infrared with a modulated code. The hacker then stands on the corner near a fire station with their universal remote set on 'learn' and waits. After a fire truck goes by they now have the coded pattern. All that's necessary is modding the remote with a higher output. Now they have a remote control for the traffic signals. :)

Every technology is "dual use".

Brian

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

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

:)

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:

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:

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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

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.

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