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AMAZON multi-meters discounts AMAZON oscilloscope discounts Over 35 million people have seen a Laserium light show, and just about every one of those 35 million have gone home afterward wishing they could create the same kind of mind-boggling special effects. If you have a laser, you are already on the road to producing your own laser light shows. A small assortment of basic accessories is all you need to make dancing, oscillating shape on the ceiling, wall, or screen. This section details some basic approaches to affordable laser light shows. You’ll learn how to produce light shows using dc motors and mirrors that make interesting and controllable “Spirograph” shapes, how to make a laser beam dance to the beat of music, and how to make “sheet” and “cone” effects using mirrors and lenses. The Laserium “laserists” use advanced components and lasers costing many tens of thousands of dollars. A few of these more sophisticated components are detailed in the next section (there, you’ll discover the use of servos and galvanometers to control the laser beam, how to make exciting smoke effects, and ways to use argon, krypton, and other laser types to add more colors to the show). THE “SPIROGRAPH” EFFECT Imagine your laser drawing unique, “atom-shaped,” repeating spiral light forms, with you adjusting their size and shape by turning a couple of knobs. The “Spirograph” light show device (named after the popular Spirograph drawing toy made by Kenner uses three small dc motors and an easy-to-build motor speed and direction control circuit. Table 19-1. “Spirograph” Light Show Device Parts List
Depending on how you adjust the speed and direction of the motors, you alter the shape and size of the spiral light forms. And because the motors used are not constant speed, slight variations in rotation rate cause the light forms to pulse and change all on their own. A complete parts list for the “Spirograph” light show device is in TABLE 19-1. Mirror Mounting Got a penny? That and a little bit of glue is all you need to mount each mirror to a motor. The best motors to use are the 1.5- to 6-volt dc hobby motors that are made by Mabuchi, Johnson, numerous other companies and are sold by Radio Shack and most every other electronics outlet in the country. Measure the diameter of the shaft; it can vary depending on the manufacturer and original application for the motor. Then drill a hole in the exact center of a penny using a bit j slightly smaller than the motor shaft. Use a drill press to hold the penny in place and to prevent the bit from skipping. You’ll find drilling easier if you turn the coin over and position the bit in the middle column of the Lincoln Memorial (for a penny less than about 25 years old). Note that the newest pennies are easiest to drill. Don’t worry; the hole can be off a few fractions of an inch, but it shouldn't be larger than the motor shaft. If anything, strive for a press fit. File away the flash left by the bit so the surface of the penny is smooth. Next, apply a drop of cyanoacrylate adhesive (Super Glue) to a 1-inch-square or diameter mirror to the center of the penny. Best results are obtained when using a fairly thin mirror and gap-filling glue. The Hot Stuff Super “T” glue made by HST-2 (available at hobby stores) is a good choice. Wait an hour for the adhesive to dry and set. Repeat the procedure for the other three mirrors. Avoid gaps between the mirrors and pennies. Although a small amount of misalignment is desirable, a large gap will cause excessive beam displacement when the motor turns. You’ll see exactly why this is important once you build the Spirograph light show device. Finally, mount the penny and mirror on the end of the motor shaft, as depicted in ill. 19-1. Apply several drops of adhesive to the shaft and let it seep into the hole in the penny. Wait several hours for the adhesive to set completely before continuing. Alternatively, you can solder the penny to the shaft. This requires a heavy-duty soldering iron or small, controllable torch. Mount the penny on the motor shaft first, then tack on the mirror. ill. 19-1. Mirror and penny mounting detail for the dc motors used in the “Spirograph” light show device. Mounting the Motors Ideal motor mounts can be made with 1-inch plumbing pipe hangers, sold at a plumbing supply outlet or hardware store. The hanger is made of formed, u-shaped metal with a mounting hole on one end and an adjustable open end at the other (many other styles can also be used). Secure the motor in the hanger by loosening the bolt on the end, slipping the motor in, and then finger-tightening the bolt. Secure the hanger on an 8-by-24-inch piece of ¼-inch hardwood pegboard, as shown in ill. 19-2. Add wood blocks to the underside of the pegboard to make an optical breadboard, as explained in Section 7, “Constructing an Optical Bench.” Arrange the hangers as shown in ill. 19-3, and lightly secure the hangers to the pegboard using 10/24-by- ½-inch bolts and matching hardware. Use flat and split washers as indicated in ill. 19-2 to prevent movement when the motors are turning (and vibrating). Building the Motor Control Circuit The motor control circuit allows you to individually control each motor. You have full command over the speed and direction of each motor by flicking a switch and turning a dial. ill. 19-2. How to mount the motors to a pegboard base. The schematic for the motor control circuit's shown in ill. 19-4. The illustration shows the circuit for only one motor; duplicate it for the remaining two motors. The prototype used a 3½-by-4-inch perforated board and wire-wrapping techniques. Your layout should provide room for the electronics, switches, potentiometers, and transistors on heatsinks (the latter are very important). Lay out the parts before cutting the board to size. Power is provided by a 6 Vdc battery pack consisting of four alkaline “D” cells. ill. 19-3. Arrangement of the motors on the pegboard base. +6Vdc; Laser beam ill. 19-4. An easy-to-build pulse width modulation speed control and direction switch for a small dc motor Q1 must be mounted on a heatsink. The double-pole, double-throw switches allow you to control the direction of the motors or turn them off. The potentiometers let you vary the speed of the motors from full to about ½ to 2/3 normal. Different speeds are obtained by varying the “on” time, or duty cycle, of the motors. The more the duty cycle approaches 100 percent, the faster the motor turns. The design of this circuit does not allow the motors to turn at drastically reduced speeds which, in any case, isn't really desirable to achieve the spiral light-form effects. Alternative speed control circuits are shown in FIGs.19-5 through 19-7. Ill. 19-5 details a similar control circuit using 2N3055 heavy-duty transistors. This circuit also works on the duty-cycle principle (more accurately referred to as pulse width modulation). These transistors should be placed in a suitable aluminum heatsink with proper case-to heatsink electrical insulation. The 2N3055s and heatsinks require more space than the IRF511 power MOSFET transistors used in the schematic outlined earlier, so make the board larger. Ill. 19-7 shows a basic approach using 2- to 5-watt power potentiometers. Be sure to use pots rated for at least 2 watts, or you run the risk of burning them out. Bear in mind that the potentiometer approach consumes more power than the pulse width modulated systems. No matter how fast the motors are turning, a constant amount of current is always drawn from the batteries. Current not used by the motors is dissipated by the potentiometer as heat. ill. 19-5. An alternate method of providing pulse width modulation using a 2N2055 power transistor. ill. 19-6. How to connect a high-wattage rheostat or potentiometer to To motor control the speed of a dc motor. ill. 19-7. Hookup diagram for using a Sprague UDN-2950Z half-bridge motor driver IC Use the output of the circuits in ill. 19-4 or 19-5 to supply the Speed (PWM) signal. Ill. 19-7 shows speed and direction control using a Sprague motor control IC (others are available). You can obtain the IC through most Sprague reps as well as from Circuit Specialists (see Section A for sources). Use the pulse width modulation circuit shown in ill. 19-4 or 19-5 and apply to the speed pin (pin 5) of the UDN-2950Z IC. Parts lists for the three alternative speed control circuits are provided in TABLES 19-2 through 19-24. Mount the circuit board on one end of the optical breadboard using 10/24-by-½ bolts and 10/24 nuts. Make sure there is sufficient space between the bottom of the board and the wire-wrap posts and top of the optical breadboard. Mounting the Laser Cut a 3-inch length of 2-inch plastic PVC pipe lengthwise in half. Using 10/24-by-1/2-inch bolts and hardware, mount the two halves on the optical breadboard. Insert a 2½- to 3-inch diameter worm-gear pipe clamp under the PVC half before tightening the nuts and bolts. Use silicone adhesive to attach rubber feet above the four bolt heads. The rubber feet serve as a cushion between the laser tube and bolts, as well as to increase the height of the beam over the optical bench. More details on this and other laser- mounting methods can be found in Section 7. Table 19-2. Motor Control Circuit Parts List (Each Motor)
All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more. Table 19-3. Alternate #1 Motor Control Circuit Parts List (Each Motor)
All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more. Table 19-4. Alternate #2 Motor Control Circuit Parts List (Each Motor)
The PVC pipe laser holder is designed for a cylindrical laser head. If you are using a bare laser tube, install it in a suitable enclosure as detailed in Section 6, “Build a He Ne Laser Experimenter’s System.” Aligning the System Slip the laser into the holder and tighten the two clamps. The distance between the front of the laser and the motors isn't critical, but be sure that there is no chance that the mirror on the first motor will touch the end of the laser. Turn the laser on but don't apply power to the motor control circuit. Rotate the hangers on each motor so that the beam is deflected from mirror 1 to 2 to 3. Fine-tune the alignment by rotating each motor 90 degrees. The misalignment inherent in the mirror mounting should displace the beam on the mirrors. Avoid fall-off where the beam skips off the mirror. Beam fall-off causes a void in the spiral when the motors turn. If you cannot align the motors so that the beam never falls off the mirrors, check the gap between the mirrors and pennies. Place the motor with the largest gap at the end of the chain as motor number 3. If beam fall-off is still a problem, try mounting a mirror and penny on a new motor. Place all three switches to their center position and apply power to the motor control circuit. Flick switch #1 up or down and rotate the potentiometer. The motor should turn. If the motor whines but refuses to turn, flick the switch off, turn the pot all the way on, and reapply power. The motor should turn. Test the speed control circuit by turning the pot. The motor should slow down by an appreciable amount (you’ll be able to hear the decrease in speed). If nothing happens, double-check your work. A motor that won’t change speed could be caused by improper wiring or a bad transistor. A blown transistor could cause the motor to spin at about a constant 80 percent of full speed. Next reverse the motor by moving the switch to the opposite position. The motor should momentarily come to a halt, turn in its tracks, and go the other way. Turn the first motor off and repeat the testing procedure for the other two. After all motors check out, turn them back on and point the #3 mirror so that the beam falls on a wall or screen. Watch the spiral light form as all three motors turn. Do you notice any beam fall-off—if so, stop all the motors and readjust them. Note that the motors vibrate a great deal at full speed, and that can cause them to go out of alignment. When you get the motors aligned just right, tighten the hangers to prevent them from coming loose. Test the different types of light forms you can create by turning off the #1 motor and using just #2 and #3. Depending on how the direction is set on the motors, you should see an “orbiting atom” shape on the screen, as depicted in ill. 19-8. If the form looks more like constantly changing ellipses, reverse the direction of one of the motors. Adjust the speed control on both motors and watch the different effects you can achieve. Now try the same thing with motor #1 and #3 on. Try all the combinations and note the results. What happens if the light form doesn’t show up or appears very small, even when the screen is some distance from the light show device? This can occur if the mirror is precisely aligned with the rotation of the motor. Although this is rare using the construction technique outlined above, it can happen. You can see how much each motor contributes to the creation of the light form by turning on each one in turn. You should see a fairly well-formed circle on the screen. The mirror is too precisely aligned if a dot appears instead of the circle. Replace the mirror and motor with another one and try again. Note that the size of the circle does not depend on where the beam strikes the mirror. The circle is the same size whether the beam hits the exact center of the mirror or its edge. ill. 19-8. The “atom” laser light form made with the “Spirograph” device. Notes on Using and Improving the “Spirograph” Device Here are some notes on how to get the most from the spiral light-form device: * Keep the mirrors clean and free of dust or the light forms will appear streaked and blurred. * Never adjust the position of the motors when they are turning. The mirrors are positioned close together, and moving the motors could cause the glass to touch. The mirrors will then shatter and fragments of glass will fly in all directions. It is a good idea to use protective goggles when adjusting and using the spiral light show device. * Cheap dc motors like those used in this project make a lot of noise. You might want to use higher quality motors if you plan on using the “Spirograph” maker in a light show. Get ones with bearings on the shaft. You can also place the device in a soundproof box. Provide a clear window for the beam to come out. * The “Spirograph” device is designed for manual control. With the right interface circuit, you can easily connect it to a computer for automated operation. The motor direction and speed control circuit used in this project is similar to the robotic control schemes outlined in my book Robot Builder’s Bonanza (TAB BOOKS, catalog number 2800). Refer to it for ideas on how to control motors via computer. * You can obtain even more light forms by adding a fourth motor. Try it and see what happens. * Don’t be shy about turning some of the motors off. Some of the most interesting effects are achieved with just two motors. SOUND-MODULATED MIRRORS In the early seventies, during the psychedelic light show craze, Edmund Scientific Company offered an unusual device that transformed music into a dancing beam of light. The system, called MusicVision, was simple: Thin front-surface mirrors were attached to a sheet of surgical rubber. The rubber sheet was then pulled taut across the front of an 8- or 10-inch woofer. A projector was positioned off to one side so that it cast one or more beams on the mirrors mounted on the rubber. When the hi-fl was turned on, the speaker would move, vibrating the rubber sheet and causing the mirrors to bob up and down. The beam of light from the projector would follow the mirror, projecting an undulating and constantly changing pattern on a wall or the back side of a rear-projection screen. A filter wheel added color to the light shapes, which then colorfully bounced and jumped in time to the music. Imagine what would happen if you replaced the projector and color wheel with a laser. Point the thin beam of a laser at the mirror and you would get a projected image of it on the wall doing a dance. There are numerous ways to build sound-modulated mirror systems. Below are just a few of them; you are free to experiment and come up with some of your own. The Old Rubber Sheet over the Speaker Trick A simple yet effective light show instrument can be made using a small mirror, a sheet of rubber, and a discarded peanut can. This design, with parts indicated in TABLE 19-5, comes from laser light show designer and consultant Jeff Korman, who calls it “PeanutVision.” Using an all-purpose adhesive, mount a thin front-surface mirror— measuring approximately ½ inch in diameter—onto a 6-inch square sheet of surgical rubber. Surgeon’s gloves (available at many surplus stores) are a good source of surgical rubber, but a better choice is to use flat squares of the stuff. Check an industrial supply outlet and don’t be afraid to improvise. If the rubber dates some time back, however, check to be sure it’s still in its protective wrapper. The rubber dries out in time when exposed to air. In a pinch, you can get by using the rubber from balloons, but if possible, use thin-walled balloons. While waiting for the adhesive to dry, drill 5 to 10 small holes in the metal end of a Planter’s Peanuts can. Mount a 3-inch round speaker over the holes. Use small hardware, epoxy, or glue to hold the speaker in place. Stretch the rubber over the open end of the can. Pull the rubber tight while making sure that the mirror is placed in the approximate center of the opening. Wrap one or more rubber bands around the sheet to secure it, as indicated in ill. 19-9. Solder a pair of wires to the speaker terminals and connect the leads to a low-wattage stereo or hi-fl. Unless you use a high-capacity rated speaker, don’t connect it to a stereo system that delivers more than a few watts—otherwise you’ll burn out the voice coil in the speaker. Table 19-5. PeanutVision Parts List
ill. 19-9. How to build the Peanut Vision, using a peanut can, rubber sheet, mirror, and speaker. Connecting the light show speaker in parallel with the main speakers of the hi-fl reduces the chance of burnout, but it also changes the output impedance and might affect the sound. Instead of the usual 8 ohms of impedance of regular hi-fl speakers, adding the light show instrument in parallel reduces the output impedance to 4 ohms (assuming you use an 8-ohm speaker in the light show instrument). This generally causes no dam age, but the sound quality of the stereo could be affected. Use the sound-modulated mirror system by attaching it to a swivel mount and frame. Place the frame in front of the laser and adjust the swivel until the beam strikes the mirror and is reflected to a back wall or screen. The further the wall is from the instrument, the larger the beam pattern will be. You can also control pattern size by adjusting the volume. Again, be careful you don’t turn up the volume too much, or the speaker could be ruined. Enhancing the Light Show Your light shows look even better if you are the proud owner of an argon, krypton, green He-Ne, helium-cadmium, or other non-red, visible-light gas plasma laser tube. The argon and krypton lasers produce light with many distinct wavelengths. These are mainlines and can be separated by using an equilateral prism or dichroic filters. Send each beam to a different PeanutVision system. An argon laser with its 488.0 and 514.5 mainline beams separated can be used with two sound-modulated mirrors. Alter the appearance of the light forms by feeding the right channel to one mirror and the left channel to the other. Alternatively, you can add a filter to the sound output of your hi-fl and separate the highs from the lows. Route the high-frequency sounds to one mirror and the lows to the second mirror. Position the mirrors so that the beams converge on the screen. The colors will appear as if they are dancing with one another. Each has its own dance steps, but both are moving together to the beat of the music. Direct Mirror Mounting Instead of mounting the mirror on a sheet of surgical rubber, mount it directly onto the speaker cone. The best mounting location is in the center, above the voice coil. If the mirror is thin enough (0.04 inch or so), its mass won’t overload the speaker and it should vibrate in unison with the sound. Frequency response using this mounting technique is excellent—almost the frequency response of the speaker itself. Note that higher frequency sounds don’t move the speaker cone and mirror as much as low-frequency sounds so the visual beam pattern effect is more marked at low frequencies. If you want the visuals only and don’t want the speaker to emit sound, you can re duce its audio output by carefully cutting away the cone material. Use a razor blade to cut the cone at the outer edges. Next, cut out the inside where the cone attaches to the voice coil, but keep the spider—the portion attaching the voice coil to the frame of the speaker—intact. The electrical connection for the voice coil might be physically attached to the outside of the cone, so be sure to leave this part attached. The speaker will still produce sound, even with all or most of the cone removed. However, the sound level will be low and not generally audible when the room is filled with music from the main sound system. If sound from the speaker is a problem, mount it in a small wooden box. Fill the box with fiberglass padding (the kind made for speaker stuffing), and provide a clear window for the laser beam. Or, you can make the speaker and laser self-contained by making the box large enough for the tube. Keep the fiberglass away from the tube and add one or two small holes for ventilation. ill. 19-10. Adjustable-frequency sine-wave oscillator. For lower frequency tones, increase value of C1 and C2 (make them the same). R6 is a dual-ganged 1-megohm precision potentiometer. The soundproof box comes in handy if you are controlling the beam with an audio oscillator. The oscillator, operating under your control, produces a buzzing or whining noise that's distracting when accompanied with a music soundtrack. With the speaker systems stuffed in the box, the oscillator noise will be largely inaudible (unless you are standing right next to the box). Schematics for useful audio oscillators appear in FIGS. 19-10 and 19-11. Both oscillators produce sinusoidal ac signals that cause the speaker cone to move both in and out from its normal centered position (a waveform that's positive only moves the cone all the way out but not in). The op-amp circuit shown in ill. 19-10 is the cheapest to build but requires a healthy assortment of parts. The circuit in ill. 19-11 is based around the versatile Exar XR-2206 monolithic function generator. Many mail-/Internet- order electronics firms, such as Circuit Specialists, offer this chip. Its higher cost (about $9 to $12) is offset by the minimum number of external components required to make the circuit function. TABLES 19-6 and 19-7 include parts lists for the generators. Bell-Crank Mounting Both speaker systems outlined above cause the mirror to bob up and down, thereby moving the beam across a wall or screen. Moving the apex of the mirror back and forth—in an arc motion—produces better pattern effects. You can build a sound-modulated mirror system using a speaker, mirror, and a model airplane plastic bell-crank. The bell-crank and other mounting hardware are available at hobby shops that carry radio control (RIC) model parts. ill. 19-11. How to build a sine-wave generator using an XR-2206 function generator IC. You can use the speaker as-is or remove the cone, as described above. Devise the crank and mount as shown in ill. 19-12. Use a gap-filling cyanoacrylate glue to bond the parts to the metal speaker frame. Note: be sure the bell crank and other model parts are not made of nylon; these don’t adhere well to any glue. After the glue used to attach the mirror and mount has set (allow at least 30 minutes), secure the frame of the speaker to a tiltable stand. Position the speaker so that the laser beam glances off the mirror at a 45-degree angle. The mirror should rock the beam right and left (or up and down, depending on how the laser and speaker are arranged). Some “sloppiness” is inherent in this and other sound-modulated mirror systems. The beam won't trace a perfect line as it moves back and forth. The beam pattern using just one mirror is more-or-less one-dimensional. You can create two-dimensional patterns using two mirror/speaker systems. Position the two speakers at 90 degrees off-axis to one another, and aim the laser so that the beam strikes one mirror and then the next. ill. 19-12. One way to attach a mirror to a speaker. The bell-crank arrangement, when used with the proper lever/fulcrum geometry, can greatly increase the deflection of the mirror. Table 19-6. Op-Amp Sine Wave Generator Parts List
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. SOUND MODULATION OF AIRPLANE SERVOS Radio-controlled (RIC) model airplanes use motorized servo mechanisms for controlling such things as the rudder, ailerons, and landing gear. The servos, which connect to a central receiver on board the craft, consist of a miniature dc motor, a control board, a potentiometer, and a gear reduction system. All work together to provide closed-loop feedback, a system where the position of the servo arm is known and maintained at all times. How Servos Work The servo operates from a 4.5 to 8 volts dc source. That provides power to the motor and circuitry. To actuate the servo, the receiver (acting under command from the radio control transmitter), sends a series of pulses. The width of the pulses varies from 1.0 to 2.0 milliseconds and determines the direction and distance of travel. Table 19-7. Function Sine Wave Generator Parts List
All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more. When a pulse is received, the servo circuit actuates the motor, which turns the gearing system as well as an output potentiometer. The position of the potentiometer wiper indicates the position of the servo arm (connected to some linkage on the aircraft). The servo circuit monitors the position of the potentiometer and turns off the motor when the pot reaches a given point. Very fine movement—much less than 1 degree of revolution—is possible with most servo systems. Although RIC servos are meant to be used with the proper type of receiver, you can rig up your own actuating circuit using only a handful of components. By applying an audio input to the circuit, you can make the servos dance back and forth to the music. Laser light is deflected by a mirror mounted on the end of the servo arm. Note that servos are not as nimble as other light-show devices (such as the galvanometer described in the next section), but they can be used to create interesting “sweeping scan” effects. Using two motors placed at right angles to one another lets you create two-dimensional light forms. Building the Sound Servo Systems Ill. 19-13 shows the circuit for the sound-modulated servo (parts list in TABLE 19-8). The project is designed around the 556 IC, a dual version of the venerable 555 timer chip (two timers in one integrated circuit). One half of the 556 provides a series of pulses, and the other half varies the width of the pulses based on the voltage presented at the modulation input. Potentiometer R2 provides a threshold adjustment that lets you find a suitable “mid-point” where the servo arm swings both clockwise and counter clockwise when music is applied to pin 3, the modulation input. ill. 19-13. Schematic diagram for operating a model aircraft servo with amplified audio signals. Table 19-8. Sound Modulated Servo Parts List
All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more. Most RIC servos work the same, but a few odd-balls can present problems. The prototype circuit used Aristo-Craft Hi-Tek HS-402X servos, which are low-cost Korean copies of the popular Futaba servo motor. Capacitors C4 and C3, with resistor R3, determine the pulse width. If you don’t get the results you want with the servo you use, try varying the values of these components. The servo has three color-coded wires: red, white, and black. The red and black wires are the positive and ground leads, respectively. The white wire is the pulse lead, and connects to the output of the circuit. Build two identical circuits if you are controlling a pair of servo motors. Using the Sound Servo System Connect the output of an amplified music source to pin 3, the input of the circuit (a 500 mW to 1 watt amplifier provides more than enough power). Connect the circuit to the servo motor as indicated in the schematic. Turn up the volume on the amplifier and watch for a racking motion of the servo. If nothing happens or the servo immediately travels to the far end of its rotation and stays there, adjust R2 to modify the input volt age. If the servo moves to one extreme and makes a chattering noise, disconnect the power immediately. The chattering is caused by the gears in the gear train skipping. If allowed to continue, the gears will strip and the servo will be useless. When adjusted properly, the servo should move back and forth in syncopation with the music. The amount of movement depends on the relative sound level of the music. The servo tends to react more to low-frequency sounds, which generally have a higher power content than higher frequency ones. The higher the volume, the more the servo will wiggle back and forth. Note that the frequency response of the servo depends on the amplitude of rotation. The more the servo rotates, the lower the frequency response. If the servo is allowed to swing too far in both directions, the motor won’t respond to changes in the music of more than 8 to 10 Hz. When the motor is set so that it slightly vibrates, frequency response is increased to a more respectable 30 to 50 Hz. Mirror Mounting Your local hobby store should stock a variety of plastic and hardware items that can be used to mount a suitable mirror on the servo. The output shaft of the servo is de signed to accommodate a number of different plastic wheels, armatures, and brackets. You can glue the pieces together or use miniature 4/40 or 3/56 hardware (or whatever happens to be handy). Attach the mirror to the bracket using epoxy. Positioning the Servos Mount the servos on an optical breadboard (as discussed in Section 7) using the hardware provided with the servo or purchased separately. By mounting two servos at a 90-degree angle (one vertical and one horizontal) and positioning the mirrors so that the beam is deflected off one mirror and then the other, you gain complete control of the X and Y coordinates of the laser beam. If you provide each servo with a slightly different signal (left and right stereo channels, for example), you can create unusual lithesome patterns. Using active or passive filtration you can divert high-frequency sounds to one servo and low-frequency sounds to the other. Remember that the servo isn't really sensitive to frequencies, just the relative amplitude of the music generated by these frequencies. The servos respond best to such sounds as drums and bass and other low-frequency, short-duration instruments. Filter these out with a circuit that rolls off at about 300 to 500 Hz, and the servo will no longer respond to them but act on the amplitude of the remaining frequencies. One of the best advantages of the sound-modulated servo system is it doesn’t re produce the music—unlike the speaker/mirror light-show instrument detailed earlier. This is especially important if you’re putting on a light show. It can be disconcerting to an audience to hear the squeaky, raspy sounds of the speaker/mirror system along with the high fidelity of the auditorium audio system. SIMPLE SCANNING SYSTEMS Not all light-show effects are designed to bob with the music. Some effects are made by scanning the beam using prisms, mirrors, mirror balls, and other rotating reflecting optics. Depending on how you arrange the optical components and laser, you can create unique “sheet” and “cone” effects. A sheet is a one-dimensional scan where the pinpoint laser beam is spread out in a wide arc. When projected on a screen, the beam draws out a long, streaking line. A cone is a three-dimensional scan where the beam is moved both up and down as well as right and left. When projected on a screen, the beam draws a circle or oval. You need an extremely powerful laser (100 mW or more) to see the scanning effect in mid air, and then the beam is most visible when it shoots towards you, rather than away from you. As an example, light-show experts rig up mirrors of fiberoptics so the rays of laser light are directed toward the audience. Of course, the beams are aimed so that they don’t actually strike anybody but are deflected to “beam stops”—flat-black fabric or metal baffles that prevent the beam from bouncing around the room. Unless you fill the room with smoke or fake fog, the scanned beam from a 10 mW He-Ne is invisible, and even with the smoke it's extremely weak. Details on adding smoke can be found in Section 20, “Advanced Laser Light Shows.” Sheet Effects There are three basic ways to create sheet-effect scanned images (more sophisticated approaches are shown in the next section). * Reflect the beam off a mirror attached to the shaft of a motor, as shown in ill. 19-14. The arc of the scan is approximately 170 degrees with a one-sided mirror (silvered on one side only). ill. 19-14. Mount a mirror on the shaft of a small dc motor as shown to produce a sweeping scan effect. * Bounce the beam off a holographic scanner, which is a specialized mirrored wheel used in laser-based supermarket checkout systems. The scanner is a wheel with mirrored or flat, polished edges. The number of facets on the outside of the wheel determine the arc of the scan. * Pass the beam through a cylindrical lens. The lens expands the beam in one direction only. Fine angle of the arc is determined by the focal length of the lens. Most cylindrical lenses expand the beam to cover a 90- to 120-degree arc. In all approaches, the intensity of the beam is reduced by a factor determined by the arc of the scan as well as any time the beam is stopped or blocked. Beam intensity is reduced the most with the one-sided mirror techniques. If you were to slow down the motor spinning the one-sided mirror, you’d see that the beam isn't reflected for half the period of rotation (that is, when the beam strikes the back of the mirror). When the reflective side of the mirror faces the laser, the beam is directed outward in an arc. The beam intensity along the arc is only a fraction of what it's when the beam is stationary. Holographic scanners must be precisely mounted on the motor shaft. Wobble of the scanning wheel causes multiple scan lines when the beam is projected. The multiple lines might be desirable when using certain smoke effects because the width (not arc) of the scan is increased. The scan width increases from the diameter of the actual beam to the distance between the far right and left lines. The cylindrical lens does not suffer from excessive reduction in beam intensity or multiple scan lines. The beam is refractively widened into an arc so that no motors or mirrors need be used to provide the scanning action. The only requirement of the cylindrical lens is that its focal length must be carefully chosen if you desire a specific scanning arc. Cone Effects A cone effect is made by mounting a mirror off-axis on the shaft of a motor. A similar mounting technique was described for the “Spirograph” laser light show device, detailed at the beginning of this section. These mirrors are mounted slightly off-axis to produce a small circle shape on a screen. In the cone-scanning system, the mirror is mounted at a greater off-axis angle to produce a larger circle. Altering the Speed of the Scan With the exception of the cylindrical lens system, the scanning systems described here use motors that can be accelerated or decelerated as desired for a particular effect. Beyond a certain speed, the scanning rate isn't detectable to the human eye and further speed increase isn't necessary. This can help prolong the life of your motors as well as make the light show system quieter. Both the one-sided mirror and cone systems use optical components that could pre sent an uneven load on the motor shaft. That can lead to excessive noise and wear on motors that are not equipped with shaft bearings. You can reduce wear and noise by decreasing the speed of the motor without adversely affecting the visual effects of the scan. Use the motor speed circuits provided earlier in this section. Slow “sweeping” effects can be achieved by reducing the motor speed to a crawl. Most speed control circuits cannot slow down a motor beyond a certain point without stalling the motor or causing the shaft to jerk instead of turn smoothly. If you can’t get the motor to turn slowly enough, consider adding a gear reduction system to decrease the rotation of the mirror. Sweeping scans can also be created using R/C servos. Even at top speed, the scan of one servo is slow enough to see, so the beam appears as a comet with a streaking tail. For a repetitive sweep, the servo circuit described earlier requires a low-frequency sinusoidal waveform. The oscillators depicted back in FIGS. 19-10 and 19-11 serve as excellent tone sources for the servos. |
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