Free-Air Laser Light Communications

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Because light is at such a high frequency in the electromagnetic spectrum, it's an even better medium for communications than radio waves. Lasers are perfect instruments for communications links because they emit a powerful, slender beam that's least affected by interference and is nearly impossible to intercept.

This section explains the basics of laser light communications using both helium- neon and semiconductor lasers. You’ll discover the different ways light can be modulated and cajoled into carrying an analog signal from a microphone or FM radio. The following section details advanced projects in laser light communications.

LIGHT AS A MODULATION MEDIUM

Higher frequencies in the radio spectrum provide greater bandwidth. The bandwidth is the space between the upper and lower frequencies that define an information channel. Bandwidth is small for low-frequency applications such as AM radio broadcasts, which span a range 540 kHz to 1600 kHz. That’s little more than 1 MHz of bandwidth, so if there are 20 stations on the dial, that’s only 50 kHz per deejay.

Television broadcasts, including both VHF and UHF channels, span a range from 54 MHz to 890 MHz, with each channel taking up 6 MHz. Note that the 6 MHz bandwidth of the TV channel provides more than 100 times more room for information than the AM radio band. That way, television can pack more data into the transmission.

Microwave links, which operate in the gigahertz (billions of cycles per second) region, are used by communications and telephone companies to beam thousands of phone calls in one transmission. Many calls are compacted into the single microwave channel because the bandwidth required for one phone conversation is small compared to the overall bandwidth provided by the microwave link.

Visible light and near-infrared radiation has a frequency of between about 430 to 750 terahertz (THz)—or 430 to 750 trillion cycles per second. Thanks to the immense bandwidth of the spectrum at these high frequencies, one light beam can simultaneously carry all the phone calls made in the United States, or almost 100 million TV channels. Of course, what to put on those channels is another thing!

Alas, all of this is theoretical. Transmitters and receivers don’t yet exist that can pack data into the entire light spectrum; the current state of the art cannot place intelligent information at frequencies higher than about 25 or 35 gigahertz (billion cycles per second). It might take a while for technology to advance to a point where the full potential of light beam communications can be realized.

Even with these limitations, light transmission offers additional advantages over conventional techniques. Light isn't as susceptible to interference from other transmissions, and when squeezed into the arrow-thin beam of a laser, is highly directional. It is difficult to intercept a light beam transmission without the intended receiver knowing about it. And, unlike radio gear, experimenting with even high-power light links does not require approval from the Federal Communications Commission. Businesses, universities, and individuals can test lightwave communications systems without the worry of upsetting every television set, radio, and CB in the neighborhood (however, CDRH regulations must be followed).

On the down side, light is greatly affected by weather conditions, and unlike low frequencies such as AM radio, it does not readily bounce off objects. Radar (low-band microwave) pierces through most any weather and bounces off just about everything.

EXPERIMENTING WITH A VISIBLE LED TRANSMITTER

It’s easy to see how laser lightwave communication links work by first experimenting with a system designed around the common and affordable visible light-emitting diode. The LED provides a visual indication that the system is working and allows you to see the effects of collimating and focusing optics.

The LED communications link, like any other, consists of a transmitter and receiver. An LED is used as the transmitting component and a phototransistor is used as the receiving component. To facilitate testing, a radio or cassette player is used as the transmission source. You listen to the reception at the receiver using headphones. In Section 14, “Advanced Projects in Laser Communication,” you’ll learn how to transmit computer and remote-control data through the air via a laser beam.

Just about any LED will work in the circuit shown in ill. 13-1, but if you want to operate the link over long distances (more than 5 or 10 feet), you should use a high- output LED, such as the kind described in Section 4, “Experimenting with Light and Optics.” After you test the visible LED, you can exchange it with one or more high output infrared LEDs to extend the working distance. However, you enjoy the greatest range using an infrared or visible laser (we’ll get to that shortly).

Building the Transmitter

The transmitter, with parts indicated in TABLE 13-1, is designed around a 555 timer IC. The 555 generates a modulation frequency upon which the information you want to send is placed. The output frequency of the 555 changes as the audio signal presented to the input changes. This modulation technique is commonly referred to as pulse frequency modulation, or PFM, and is shown diagrammatically in ill. 13-2. The signal can be received using a simple amplifier, as shown later in this section, but for best response, a receiver designed to “tune “to the PFM signal is desired. Advanced receivers are discussed later.


ill. 13-1. Schematic diagram for the pulse frequency modulated LED transmitter. Adjust frequency by rotating R1. With components shown, frequency range is between 8 and 48 kHz.

  • IC1 LM555 timer IC
  • R1 100k-ohm potentiometer
  • R2 10k-ohm resistor
  • C1 0.1 F disc capacitor
  • C2 33 uF electrolytic capacitor
  • C3 0.0015 uF mica or Hi-Q disc capacitor
  • LED 1 Light-emitting diode (see text)
  • S1 SPST switch

All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more, unless otherwise indicated.


ill. 13-2. Comparison of input voltage and width of the output waveform.

Construct the transmitter in a small project box. Power comes from a single 9-volt transistor battery. The switch lets you turn the circuit on and off and the potentiometer allows you to vary the relative power delivered to the LED. In actuality, adjusting the pot changes the modulation frequency, which in turn changes the pulse width, which in turn changes the current delivered to the LED. Got that?!

In any case, the entire range is beyond human hearing and above the audio signals in frequency that you will be transmitting. You can readily increase the modulation frequency to the upper limit of the components used in the transmitter and receiver, but lowering them into the 20-to 20,000-Hz region of the audio spectrum causes an annoying buzz. Any sourcebook on using the LM555 timer IC will show you how to calculate output frequency for astable operation.

Mounting details are provided in FIGS. 13-3 and 134; parts are shown in TABLE 13-2. Solder an LED to the terminals of a ¼-inch phone plug jack, and mount the jack in the base of a ¾-inch PVC end plug, as shown in ill. 13-3. If the plug is rounded on the end, file it flat with a grinder or file. Lightly countersink the hole so that the shaft of the phone jack is flush to the outside of the plug. Countersinking also helps the shaft of the jack to poke all the way through the thick-walled PVC fitting.

The transmitter and LED connect via a ¼-inch plug that's mounted so that it extrudes through the project box, as detailed in ill. 134. Use a 5 18 nut to hold the plug in place. The ¼-inch mini plug used in the prototype is threaded for 5 18 threads, but not all plugs are the same. Check yours first.


ill. 13-3. How to mount the LED in a PVC end plug. The same approach is used for the receiver phototransistor.


ill. 13-4. The project box, shown with ½-inch mini plug for connecting to the LED.

Table 13-2. Plug and Box Transmitter Parts List

  • 1 ¾-inch schedule 40 PVC end plug
  • 1 1/8-inch miniature phone jack
  • 1 ¼-inch miniature phone plug
  • 1 ea. Project box, knob for potentiometer, 6 Vdc battery holder (4 “AA”).

Attach the transmitter into the LED by plugging it in. Install a 9-volt battery and turn the transmitter on. The LED should glow. You won’t be able to test the transmitter circuit until you build the receiver.

Building the Receiver

The receiver, shown in ill. 13-5, is designed around the common LM741 op amp and an LM386 audio amplifier. See TABLE 13-3 for a parts list. Power is supplied via two 9-volt batteries (to provide the 741 with a dual-ended supply). A switch turns the circuit on and off (interrupting both positive and negative battery connections) and a potentiometer acts like a volume/gain control.

You can listen to the amplified sounds through headphones or a speaker, or you can connect the output of the receiver to a larger amplifier. A good, handy outboard amplifier to use is the pocket amp available at Radio Shack. The pocket amp accepts an external input and has its own built-in speaker.

Construct the receiver in a plastic project box. The one used for the prototype measured 2¾ by 4¼ by 1 inches and was more than large enough to accommodate the circuit, batteries, switch, potentiometer, and output jack.

The receiving phototransistor is built into a PVC end plug in the same manner as the transmitter LED, described above. Mount the phototransistor as shown in ill. 13-3, being sure to note the orientation of the transistor leads and jack terminals. Although the circuit will work if you connect the phototransistor backwards, sensitivity will be greatly reduced.

Connect the receiver to the phototransistor, install two batteries, plug in a set of headphones, and turn the power switch on (but don’t put the headphones on just yet). Adjust the potentiometer midway through its travel and point the phototransistor at an incandescent lamp. You should hear a buzzing sound through the headphones (the buzzing is the lamp fluctuating under the 60-cycle current).


ill. 13-5. The universal laser light detector. The output of the LM386 audio amplifier can be connected to a small 8-ohm speaker or earphone. Two 9-volt batteries provide power. Decrease R1 to lower sensitivity; increase R3 to increase gain of the op amp (avoid very high gain or the op amp might oscillate).

Table 13-3. Universal Receiver Parts List

  • IC1 - LM741 operational amplifier IC
  • 1C2 - LM386 audio amplifier IC
  • R1 - 220 k-ohm resistor
  • R2 1 k-ohm resistor
  • R3 10 k-ohm resistor
  • R4 10 k-ohm potentiometer
  • R5 - 10 ohm resistor
  • C1 - 0.1 j disc capacitor
  • C2 - 220 F electrolytic capacitor
  • C3 - 10 j electrolytic capacitor
  • C4 - 100 uF electrolytic capacitor
  • Q1 - Infrared phototransistor
  • S1 - DPDT switch

All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more.

If you don’t hear the buzz, adjust the volume control until the sound comes in. Should you still not hear any sound, double check your wiring and the batteries. Even with the phototransistor not plugged in you should hear background hiss. No hiss might mean that the circuit's not getting power or the headphone jack isn't properly wired.

The receiver can be used with the LED lightwave link as well as all the other communications projects in this section (as well as most of those in the remainder of this guide). Its wide application makes it an ideal all-purpose universal laser beam receiver. When I refer to the “universal receiver,” this is the one I’m talking about.

Using the Lightwave Link

Once the receiver checks out, you can test the transmitter. Switch off the lights or move to a darkened part of the room. Turn on the transmitter source (radio, tape player) and aim the transmitter LED at the receiver phototransistor. Adjust the controls on the receiver and transmitter until you hear sound. You might hear considerable background hiss and noise, caused by other nearby light sources. If you use the communications link outdoors in sunlight, the infrared radiation from the sun might swamp (overload) the phototransistor, and the sound could be drastically reduced or cut off completely. The transmitter and receiver works best in subdued light.

Test the sensitivity and range of the communications link by moving the receiver away from the transmitter. Depending on the output of the LED, the range will he limited to about 5 feet before reception drops out.

Extending the Range of the Link

Most all phototransistors are most sensitive to infrared light. The peak spectral sensitivity depends on the makeup of the transistor, but it's generally between about 780 and 950 nm in the near-infrared portion of the spectrum. A red LED has a peak spectral output of about 650 nm, considerably under the sensitivity of the phototransistor.

A solar cell offers a wider spectral response and can provide greater range. The best type of solar cell to use is the kind encased in plastic like the phototransistor (many have a built-in lens). Connect the cell in the circuit as shown in ill. 13-6.


ill. 13-6. How to connect a solar cell to the input of the universal laser light detector. The cell provides better sensitivity in the visible light range than an infrared phototransistor.

The solar cell is sensitive to a wide range of colors. The light spectrum above or below the red radiation from the LED isn’t needed for reception, so block it with a red filter. Test the effectiveness of the filter by temporarily taping it to the front of the solar cell.

You can also use the filter with the phototransistor to help limit the incoming radiation to the red wavelengths. Even though the phototransistor is designed to be most sensitive to near-infrared radiation, it can still detect light at other wavelengths, especially red. One or two layers of red acetate placed over the phototransistor can increase the range in moderate light conditions by several feet.

The best way to increase the working distance of the communications link is to add lenses to the LED and /or the phototransistor. The PVC end plug makes it fairly easy to add lenses to both transmitter and receiver components. Mount a simple double-convex or piano-convex lens in the end plug. The PVC rings hold the lens in place and let you easily adjust the distance between the lens and phototransistor. If the lens has a focal length of more than 10 to 15 mm, attach a coupling to the end plug and stuff the lens in the coupling. Again, use PVC rings to hold the lens in place.

Be sure that you position the lens at the proper focal point with respect to the junction of the LED or phototransistor. If you don’t know the focal length of the lens you are using, test it following the instructions provided earlier in this guide.

You can see the effect of the lens on the transmitted light by pointing the LED against a lightly colored wall. At close range and with the lens properly adjusted, you should actually see the junction of the LED projected on the wall (assuming you are not using an LED with a diffused case). You might also see a faint halo around the junction; this is normal and is caused by light emitted from the sides of the LED.

With the lens(es) attached, try the lightwave link again and test its effective range. With extended range comes increased directionality, so you must carefully aim the transmitter element at the receiver. A simple focusing lens on both receiver and transmitter should extend the working distance to a hundred feet or more. Test the real effectiveness of the system at night outside. The darkness will also help you better aim the transmitter. At 100 feet, the light from the LED will be dim, but you should be able to spot it if you know where to look for it.

Note that the plastic case of the LED and phototransistor acts as a kind of lens, and that can alter the effective focal length of the system. Experiment with the position of the lens until the system is working at peak performance.

ACOUSTIC MODULATION

In 1880, Alexander Graham Bell, with his assistant Sumner Tainter, demonstrated the first photo-phone, a mechanical contraption using sunlight or collimated artificial light to transmit and receive voice signals over long distances. Its operation was simple. The system used a lightweight membrane similar to reflective Mylar as a voice diaphragm. A bright beam of light, typically from the sun, was pointed at the diaphragm, which vibrated when a person talked into it. The vibration then caused the light to fluctuate in syncopation with the sound. A receiver, located some distance away, demodulated the fluctuating light levels and turned the beam back into the talker’s voice.

Bell had great hopes for the photophone, and in fact had predicted that it would be a bigger hit than the telephone. But the problems of poor range in inclement weather doomed the photophone as just another scientific curiosity. Had Bell used a laser with his photophone, he would have been able to greatly increase the range of the device. Of course, clouds, fog, and heavy rain would have still reduced the working distance of the laser photophone, limiting it to a clear-weather communications device.

You can easily duplicate Bell’s photophone, adding the laser as a high-tech improvement. The process of transmitting low-frequency audio signals via a photophone like device is more accurately termed acousto-modulation. You can use a stretched membrane as the acoustic vibrating element or adapt a surplus speaker as a “light switch.”

Stretched Membrane Modulator

Thin reflective Mylar is a fairly common find among the mail order surplus outfits, as well as local army/navy surplus shops. Reflective Mylar, or a reasonable facsimile made with generic acetate, is used to produce parachutes for radiosonde equipment, the thermal layer on camping blankets, high-tech jewelry, radar jammer streamers, and lots more. Price is reasonable. A small 2-by-2-foot square sheet of reflective (or “aluminized”) acetate costs about a dollar on the surplus market and a little more when you buy it from a commercial dealer. One small square is all you need.

Refer to TABLE 13-4 for a parts list for the stretched membrane modulator. Secure the Mylar sheet inside a 4- to 6-inch diameter embroidery hoop as shown in ill. 13-7. The hoop allows you to open the two halves, insert the material, and pull it tight as the two halves are tightened together. The idea is to pull the Mylar as taut as possible.

Mount the hoop on a wooden or plastic base. Set up a speaker behind the hoop and direct a laser beam at the Mylar. Activate the speaker using a radio, tape player, or amplified microphone. As the speaker vibrates, it oscillates the Mylar and thus modulates the laser beam.


ill. 13-7. Basic arrangement for the Mylar speaker modulator. (A) Front view with Mylar stretched in embroidery hoop; (B) Side view with speaker behind the hoop.

Table 13-4. Mylar Hoop Modulator Parts List

  • 1) 4- to 6-inch diameter circular wood or plastic embroidery hoop
  • 1) 4- to 6-inch diameter hill-range speaker
  • 2) 3-by-3-inch wood block (for base and speaker mount); ½-inch plywood or pine
  • 2) 1-by-½-inch corner angle bracket
  • 4) 8/32 by 1½-inch bolts, nuts, flash washers
  • 1) ¼-inch 20 nut and washer (for tripod)
  • 1) Portable camera tripod

The modulation isn't electronic or electromechanical, but positional or geometrical. You can see the effect of the modulation by positioning the laser so the beam strikes a distant target. When the Mylar vibrates, the beam is displaced at the target, making squiggles and odd shapes. Position a receiving element such as a solar cell at the target and you can register the movement by sensing the varying intensity of the beam (actually, the intensity falls off as the beam moves off- axis to the center of the cell).

The universal laser receiver, described earlier in this section, can be readily used to capture and demodulate the signal transmitted over the beam. The solar cell is connected to the receiver as shown in ill. 13-6.

The ideal size for the solar cell depends on the divergence of the beam and the distance between the laser and receiver. Beam divergence with most helium-neon lasers is only about one milli-radian (less on high quality tubes). Placing the target 50 meters away produces a spot of about 50 mm across (about 2 inches). That means you can use a silicon solar cell that’s 2 inches in diameter and capture all or most of the beam. Modulation that causes the beam to wander off-axis to the cell generates a change of voltage.

You can readily calculate the approximate beam spread at any distance by multiplying the divergence in radians by the distance in meters. E.g.:

Divergence (radians) = 0.001

Distance = 200

0.001 times 200 equals 0.2, or 200 millimeters.

Another example: What is the spread at 1 km using a laser with a divergence of 1.2 mrad? Answer: 1.2 meters. That’s a small amount considering that the beam travels over half a mile. You can reduce beam divergence by adding collimating optics to the output of the laser. Section 8 provides details on building laser collimating optics.

Speaker Cone Modulator

An interesting effect used in many light shows is created by mounting a mirror in front of a speaker (the mirror can also be mounted directly on the speaker). A laser beam, reflected off the mirror, bounces around on the wall or screen in time to the mu sic (various mirror/speaker mounting techniques are discussed more fully in Section 19).

You can use the same technique to transmit audio information over the air. Simply place a receiving element at the spot where the beam lands. For best results, keep the amplitude of the speaker at a low level so the beam doesn’t deflect more than a few degrees. Use a 2-or 3-inch diameter silicon solar cell as the receiver element. You can use the speaker to transmit music from a radio or tape player, or rig up the speaker to an amplifier and microphone and broadcast your own voice.

Even with the speaker turned down low, wide deflection of the beam becomes a problem when transmitting over long distances. The beam covers a larger area at the target as the distance between the receiver and transmitter is increased. It is generally impractical to enlarge the sensing area by more than 4 or 5 inches in diameter, so another approach is recommended. This idea comes from Roger Sontag at General Science and Engineering. Instead of bouncing the light off of a mirror, cut an edge off the cone of a speaker and use it as a “shutter”. As the speaker cone vibrates, it alternately passes and cuts off the laser beam. The system requires careful alignment, but the deflection of the beam at the receiver is minimal.

The best speakers to use are those that measure 4 to 6 inches in diameter and have a deep taper. Avoid using a speaker where the cone lies flat in the frame.

Mount the speaker in a swivel mount so that you can adjust its height and angle. Place a helium-neon or cw diode laser to one side of the speaker so that the beam skims across the top of the cut portion of the cone. Energize the speaker with a fairly powerful amplifier (but don’t exceed the wattage rating of the speaker), and watch for the cone to move in and out in response to the sound. Now look at the target and watch it flicker as the speaker moves. Tithe cone doesn’t block the beam, or blocks the beam entirely, readjust the position of the speaker as needed.

You can use the universal laser beam receiver described earlier in this section to capture the signal on the modulated beam. The intensity of the beam could swamp the phototransistor, so place a set of polarizers in front and vary their rotation to reduce the beam intensity to a usable level.

ELECTROMC MODULATION OF HELIUM-NEON LASERS

Agreeably, using a sheet of plastic or a dissected speaker does not represent a high- tech approach to laser modulation. Although it might appear otherwise, it’s fairly easy to modulate the beam of a helium-neon laser, and using only a handful of parts at that. Two approaches are provided here: both have an effective bandwidth of around 0 Hz to 3 kHz, making them suitable for most voice and some music transmission schemes.

Transformer

A transformer placed in line with the high-voltage power supply and cathode of the tube can be used to vary the current supplied to the tube. This causes the intensity of the beam to vary. This is amplitude modulation, the same technique used in AM radio broadcasts.

Although you can use a number of transformers as the modulating element, Dennis Meredith of Meredith Instruments suggests you use a public address power output transformer. It’s ideal for the job because of its high turns ratio—the ratio of wire loops in the primary and secondary. You wire the transformer in reverse to the typical application: the speaker terminals from a hi-fl or amplifier connect to the “output” of the transformer and the laser connects to the “input.”

PA transformers are available from almost any electronics parts store, including Radio Shack, who offers a good one for under $5. PA transformers are rated by their voltage, usually either 35 or 70 volts. Get the higher voltage rating. There are several terminals on the transformer. Connect the speaker terminals to the common and 8-ohm terminals; connect the laser cathode, as shown in ill. 13-8, to the common and one of the wattage terminals (parts list in TABLE 13-5). Experiment with the wattage terminal that yields the most modulation. The prototype seemed to work best using the 5-watt terminal.

The cathode passes some current, so touching its leads can cause a shock. Isolate the transformer and wires in a small project box, like the one shown in ill. 13-9. Five- way binding posts (fancy banana jacks) are used for the cathode connections; the audio input is a ½-inch miniature phone jack.

Transistor

Who wants to lug around a bulky and heavy transformer when you can provide modulation to the He-Ne tube using a simple silicon transistor? This next mini-project provides a seed that you can use to design and build an all-electronic analog or digital laser communications link. I have not fully tested the upward frequency limits of the transistor modulator, but I successfully passed a 4 kHz tone through the prototype circuit using a 2 mW He-Ne tube.

Table 13-5. Transformer Modulator Parts List

  • 1) 70-volt PA transformer
  • 1) ¼-inch miniature jack
  • 2) 5-way binding posts (25-amp)
  • 1) Project box
  • 1) Audio input


ill. 13-8. (right) Wiring diagram for the He-Ne laser transformer modulator. (left) ill. 13-9. Wiring diagram for the He-Ne laser transistor transformer. Experiment with different transistors and test the results. Both the transistor and transformer modulation schemes require a well-amplified audio signal.

You can employ just about any transistor, but I found the common 2N2222 signal transistor to be adequate. Connect the transistor between the high-voltage power sup ply and cathode of the laser tube, as shown in ill. 13-10. Heatsinking isn't required in most applications; after an hour of testing the transistor, it remained cool. The transistor is all you need for the basic setup, but you might want to add a 390-ohm resistor to the base of the transistor. To make the “circuit” more permanent, mount it on a small piece of pen board or wire it into one of your He-Ne laser enclosures.


ill. 13-10. The finished transformer modulator, enclosed in a project box with insulated binding posts added.

Apply a well-amplified signal to the base of the transistor and aim the laser at the universal laser light receiver. You should hear sound. If the sound is weak, double-check your wiring and try turning up the volume. You might need one or two watts of power to produce a measurable amount of modulation.

You can build a completely portable He-Ne laser modulation system using a Walkman cassette player, IC amplifier, helium-neon tube, and 12-volt power supply. Suitable amplifier circuits appear later in this section.

ELECTROMC MODULATION OF CW DIODE LASER

A cw diode laser can be modulated using the .circuit provided earlier in this section for the LED transmitter. Although it’s always better to limit current to the laser using feedback from the monitor photodiode, this system provides a safety net because the laser is driven with pulses at the modulation frequency of about 40 kHz.

Laser diodes exhibit a great deal of divergence, so collimating optics are necessary if you want to use one in a lightwave communications project. Many surplus cw diode lasers come with collimating optics or have suitable optics available for them (most are pulled from existing equipment, such as compact disc players or bar-code scanners). Alternatively, you can build your own collimator using a simple bi-convex lens, as de scribed in Section 8, “Laser Optics Experiments.”

Mount the battery pack, modulating circuit, and laser in a project box. The box used in the prototype measured 6¼ by 33/4 by 2 inches. Construction details for the project are shown in ill. 13-11; the parts list is included in TABLE 13-6.


ill. 13-11. Layout diagram for the laser diode transmitter. Use “C” or “D” size batteries for long-life performance.

Be aware that aiming a laser diode is tough at best. Although the laser emits a deep red glow, the visible illumination isn't enough to see in anything but absolute darkness. Looking directly into the laser for any length of time is decidedly a bad idea: the collimating lens acts to focus the light in a narrow beam. Don’t let the red glow of the laser fool you. Your eye loses its sensitivity as it approaches the near-infrared band, but its susceptibility to damage from radiation isn't lessened.

You can try aiming the laser using trial and error but the results might be frustrating. Another approach is to use an infrared imaging card, such as that sold by Kodak. The card is coated with a substance that’s sensitive to infrared light. Directing an infrared source at the card causes it to glow. Before you use the card, you must first “charge” it under the white light of the sun or a desk lamp.

Still another approach is to use an infrared inverter tube. These are expensive “see- in-the-dark” devices used by police, soldiers, and voyeurs. The tube—usually mounted in a pair of binoculars, goggles, or glasses—blocks most visible light and amplifies infrared light. They are normally used with a separate infrared light source, but in this instance, the diode laser provides the needed JR radiation.

The phototransistor used with the receiver could be swamped by the power emitted by the diode laser. Use a pair of polarizers, as shown in ill. 13-12, to control the amount of infrared radiation that reaches the phototransistor. A filter placed in front of the phototransistor can also increase sensitivity and reduce background noise. Infrared filters are available at most photographic stores; surplus is another good source. Many IR filters may appear dark red or purple or even completely black. You might not be able to see through the filter, but it's practically transparent to near-infrared radiation.


ill. 13-12. The output beam of the laser can be controlled by adding polarizing films to a connector and tube, as shown. The same method can be used on the receiver phototransistor.

Table 13-6. CW Modulated Laser Diode Transmitter Parts List

  • Laser diode with collimating optics
  • Driver board
  • Control potentiometer and knob
  • SPST switch
  • Battery holder ”C” size
  • Project box (approximately 3¾ by 6¼ by 2 inches)
  • First polarizer
  • RG-59 video connector (or alum, tube)

Notes:

Replace R4 with 10k pot to vary center tuning frequency of 565. OK to use other PLL chip.

Increase overall gain of circuit by increasing R3 (up to 1MQ).

With components shown, center free- running frequency of 565 PLL is 39.75kHz.

Calculate PLL free-running frequency with the formula: R4C2 Rink


ill. 13-13. Circuit schematic for the 555-based PLL laser light PFM receiver. Although R4 is shown as a resistor, you might want to substitute it with a 10k precision potentiometer so that you can “dial in” the center frequency of the transmitter. Experiment with the value of C1 for best high-frequency response.

Note that circuit's functionally identical to the laser light detector/receiver shown in ill. 13-5 but with the addition of the 565.

OTHER MODULATION AND DEMODULATION TECHNIQUES

Amplitude modulation is susceptible to interference from changing light levels. This can lead to noise and poor system response. The pulse frequency modulation technique used with the visible LED and diode laser system rejects noise and isn't as sensitive to changes in the intensity of the source beam.

The universal laser light receiver can be used to capture the signal transmitted over an AM or PFM modulated beam. A better approach is the circuit shown in ill. 13-13. It uses a LM565 phase-locked loop (PLL) adjusted so that its center frequency matches the center frequency of the transmitter—about 40 kHz.

An audio signal impressed upon the 555 in the transmitter changes the center frequency. This change is detected by the PLL as an error signal. The amount of error signal is proportional to the frequency of the original audio signal. Therefore, tapping the error signal pin on the PLL chip and then amplifying it retrieves the audio that was transmitted over the beam.

Note: With components show, center frequency is 40.31 kHz; replace R1 with 100K pot to adjust center frequency.


ill. 13-14. Circuit diagram for the laser diode transmitter.

The circuits for the transmitter and receiver appear in FIGS. 13-13 and 13-14 (see TABLES 13-7 and 13-8 for a list of required parts). The transmitter is virtually the same as the one presented earlier in the section but with no provision for adjusting the center frequency. The receiver uses the 565 PLL (other PLLs can be used) and a trim pot to adjust the circuit for the exact center frequency of the transmitter. You can connect the transmitter as discussed above, or amplify it and apply the signal to the laser diode. With the circuits complete and set up, aim the laser at the phototransistor and provide an audio signal for transmission. Adjust the trim pot on the receiver until you hear the audio carried over the light beam. The components used in both receiver and transmitter can drift, so you might need to touch up the trim pot control to re-align the center frequency.

Table 13-7. Pulse Frequency Modulator Receiver Parts List

  • IC1) LM741 op amp IC
  • IC2) LM565 PLL IC
  • IC3) LM386 audio amplifier IC
  • R1) 220 k-ohm resistor
  • R2) 1 k-ohm resistor
  • R3) 10 k-ohm resistor
  • R4) 6.8 k-ohm resistor
  • R5) 10 k-ohm potentiometer
  • R6) 10 ohm resistor
  • C1) 0.1 uF disc capacitor
  • C2) 0.001 uF silvered mica capacitor
  • C3) 0.001 uF disc capacitor
  • C4) 0.047 uF disc capacitor
  • C5, C6) 10 uF electrolytic capacitor
  • C7) 220 uF electrolytic capacitor
  • C8) 100 uF electrolytic capacitor
  • Q1) Infrared phototransistor
  • S1) DPDT switch

Table 13-8. Pulse Frequency Modulator Transmitter Parts List:

  • IC1 LM555 timer IC
  • R1 56 k-ohm resistor
  • R2 10 k-ohm resistor
  • C1 0.1 uF disc capacitor .
  • C2 33 uF electrolytic capacitor
  • C3 470 pF silvered mica capacitor
  • S1 SPST switch
  • Laser

Table 13-9. Eight-Watt Audio Amplifier

  • IC1) LM383 audio amplifier
  • R1, R2) 2.2 ohm resistor
  • C1) 10 uF electrolytic capacitor
  • C2) 470 uF electrolytic capacitor
  • C3) 0.1 uF disc capacitor
  • C4) 2000 uF electrolytic capacitor
  • SPKR 8-ohm speaker

Table 13-10. Sixteen-Watt Audio Amplifier

  • IC1, 1C2) LM383 audio amplifier
  • R1, R3) 220 ohm resistor
  • R2, R4) 2.2 ohm resistor
  • R5) 1 megohm resistor
  • R6) 100 k-ohm potentiometer
  • C1, C7) 10uF electrolytic capacitor
  • C2, C5) 470uF electrolytic capacitor
  • C3, C4, C6) 0.2uF disc capacitor
  • SPKR 8-ohm speaker


ill. 13-15. An 8-waft audio amplifier, designed around the LM383 integrated amp. The IC must be in stalled on a suitable heatsink.


ill. 13-16. A 16-watt audio amplifier, designed around two LM383 integrated amps. The ICs must be installed on a suitable heatsink.

AUDIO AMPLIFIER CIRCUITS

The universal receiver has a built-in LM386 integrated amp. The sound output is minimal, but the chip is easy to get, it’s cheap, and it can be wired up quickly. It’s perfect for experimenting with sound projects.

If you need more sound output or must amplify the audio input for the transformer or transistor modulator, try the circuit in ill. 13-15 (parts list in TABLE 13-9). You can use it instead of the LM386 in the receiver or in addition to it. The circuit's designed around an LM383 8-watt amplifier IC. The IC comes mounted in a TO-220-style transistor package, and you should use it with a suitable heatsink. FIG 13-16 shows a higher output 16-watt version using two LM383’s (parts list in TABLE 13-10). Note that the LM383 IC is functionally identical to the TDA2002 power audio amplifier.

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