Wednesday, September 5, 2012

Voice on a laser beam...


Sending voice over light is nothing new.  The first wireless voice communications system - using light - was the PhotoPhone, demonstrated in 1878 by Alexander Graham Bell - a full 25 years before Fessenden demonstrated the same feat using radio waves.  To be sure, optical communications has certain practical limitations, namely the blinding presence of the sun and the occasional opacity of the atmosphere due to weather, but it's still a fascinating and fun topic of discussion.

I'm one of those people who find wireless communications of any sort to be interesting and I have a particularly keen fascination with optical wireless communications - that is, using "radio waves" that I can see with my own eyes.

For short-range experimentation it's hard to beat a cheap laser pointer - and here is a bit of info on how one might go about this.

Modulating the laser pointer:

The laser pointer consists of a laser diode and like any diode, it has a maximum current rating that should be observed with more caution than its voltage.  What this means is that you cannot connect a laser diode to any sort of battery and expect it to work properly:  Too little voltage and it won't lase while too much voltage, it will never lase again!  What is needed is a simple circuit that limits the amount of current fed into the laser diode to a safe level and fortunately, cheap laser pointers always have something that does this.

Increasingly, cheap laser pointers simply rely on a combination of a simple circuit (or even a single resistor!) and the internal resistance of the battery powering it to keep the laser current at a safe level and since a laser pointer already has the necessary parts, why not use them?

In my opinion, one mistake that I often see on web pages that describe the modulation of a laser pointer is to attempt to modulate by varying the voltage/current of operation - typically using a transformer in series with the power source.  There are several things wrong with this:
  • It's not certain how far down in current one can go before the laser drops out of its "laser" mode or how high one can go before it gets "blowed up."
  • Laser current versus output isn't terribly linear which means that distortion can occur.
  • With the min/max current uncertainty, one can't fully modulate the laser's output safely which means that the audio on the beam will be somewhat "quiet" - something that reduces the efficacy of the link!
The better way to modulate a laser is to simply turn it on and off using Pulse With Modulation (PWM)  and taking advantage of the circuit already present to safely operate the laser from its intended power source - say, a pair of AAA cells (or 3.0 volts.)  While more complicated than simply putting the laser's power supply in series with a transformer, it's pretty much bulletproof and can sound pretty darn good!
 
 Figure 1:
Laser transmitter/receiver by K7RJ.
Click on the image for a larger version.

A simple circuit to do this may be found in the diagram in figure 1.

I won't take credit for this circuit which was thrown together by my friend Ron, K7RJ.  When built, this circuit was intended to be quick and easy and high performance was NOT in mind - just enough effort was put into it to make it work for demonstration purposes.

Contained within the diagram is enough information to connect your cheap, 3-volt laser pointer - just be sure to pay close attention to its positive and negative battery connections when you take it apart!

Also contained in this diagram is a very simple, low-performance receiver intended solely for across-the-room (or across-the-parking lot) testing of the transmitter to make sure that it works.  It should be emphasized that this receiver is not at all intended for longer-distance use - say, more than a few hundred meters at most, and its performance can be spectacularly enhanced with the careful installation of a small magnifying glass lens with the phototransistor at its focus.  Even when enhanced thusly, other optical receiver circuits will still run circles around it!  A link to a web page describing a far more sensitive circuit may be found at the bottom of this page.

At this point I'll make a few comments about laser safety and legality:
  • Make certain that your "laser range" is end-stopped - that is, when the beam goes beyond the receiver it does not cross a road or have any likelihood of being intercepted by aircraft in flight or landing/taking off where they can dazzle and distract!  In other words, the receive end should be against the side of a building or hill.
  • While cheap, red laser pointers are probably too weak to cause permanent eye damage, it's best not to stare into it or point it directly at people!  A standard, cheap red laser pointer will, at its worst, probably just dazzle and maybe cause a brief headache or eye pain as well as a temporary loss of night vision.  The farther you are away from it, the less dangerous it will be.
  • In some states and areas laser pointers are highly regulated or even illegal - including some U.S. and Australian states/localities - check your local listings!

  • It is NOT recommended that any but cheap, red laser pointers be used for this purpose.  Why?  First, they are the cheapest and secondly, they are fairly safe and low power.  It's also worth considering that typical electronic detectors respond far better (e.g. are more sensitive) to red light than green or blue!  Some of these "fancier" laser pointers of other colors have electronic circuits in them that can prevent them from being modulated effectively.

Figure 2:
Cheap laser pointer on a tripod
Click on the image for a larger version.
One thing that you'll immediately notice about laser pointers is that despite their name, they can be fiendishly difficult to aim them - particularly as the distance increases!  For this reason it's best to contrive a means by which a camera tripod can be used to hold a laser pointer - but even this can be tricky since even a fairly expensive tripod is quite "touchy"!  To the left you can see Ron's laser pointer mount with the pointer module itself being contained within a cheap project box from Radio Shack and connected by a short cable to the rest of the circuitry.

This brings up another point as well:  Do not put both the modulator electronics and your laser pointer in the same box.  By connecting them with a cable you will be able to make adjustments and turn the thing on and off without touching the tripod and possibly disturbing your carefully-aimed beam!

Another example of a laser pointer modified for such use may be seen on the right.  When I got this pointer I couldn't see how I could remove the laser module without the possibility of damaging it so I simply used it as-is:  A wooden dowel, the same diameter as AAA cells was used and at the inside end of the dowel was a small screw to which the minus (-) side connection was made.  The connection to this screw was made via a wire laid in a shallow groove along the length of the dowel and the positive (+) side was connected to the case of the pointer itself by using some copper foil wrapped around the end of the dowel opposite the screw.  The dowel was tack-glued into place, pushing against the internal battery spring and the laser's "on" button was simply taped down.  The entire pointer was then "hot-glued" to a cheap project box that itself has inside it a 1/4"-20 bolt glued into place to allow attaching to a tripod mount while electrically insulating the laser pointer's positively-connected case from the tripod.

Figure 3:
Minimally-modified pointer on a tripod mount.  This just happens to
be mounted atop an 8" telescope with a equatorial mount which allows
precise aiming - and it also includes a telescope!
Click on the image for a larger version.
A lot has been glossed over in this brief article - namely techniques about how to accomplish a laser communication over longer distances including links to descriptions of higher-performance gear and methods of precisely aiming - and if you are really interested, you can take a look at my page:

 "Using Laser Pointers for Voice Communications" (see the link below) for a lot more detail than can be covered here.

How far can a lowly laser pointer go?

Under clear-air conditions on a line-of-sight path and using the very same lasers pictured above I've had a 2-way laser pointer to laser pointer communications on a 107 mile (173 km) path with fairly good signals.  This was, of course, using high-performance receivers with orders of magnitudes better sensitivity than the one at the top of this page!

There are other problems with using lasers over distance, however, namely that of scintillation - the rapid fading or "twinkling" caused by the irregularities in the atmosphere.  While this affects all types of light sources the combination of the coherent laser light and the small diameter of the beam as it exits the laser greatly exacerbates the problem - but that's a topic for another article!

Links from the "Modulated Light" (link) web site:

2 comments:

  1. I would like to ask a a question what are kind of lights that affect radio waves

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  2. Your question is a bit ambiguous, but in general, light waves don't affect radio waves.

    Technically, light and radio waves are the same thing, differing mostly by frequency/wavelength, but because of their physical properties they are detected and produced in very different ways.

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