Laser Torch Based Audio Transmitter and Receiver


Laser as a communication medium can provide a good substitute for the present day communication systems as the problem of interference faced in case of electromagnetic waves is not there and high deal of secrecy is available. Laser communications offers a viable alternative to RF communications for inter satellite links and other applications where highperformance links are a necessity. High data rate, small antenna size, narrow beam divergence, and a narrow field of view are characteristics of laser communications that offer a number of potential advantages for system design.

            The present paper involves the study of wireless, open channel communication system using laser a carrier for voice signals. Using this circuit we can communicate with your own neighbours wirelessly. Instead of RF signals, light from a laser torch is used as the carrier in the circuit. The laser torch can transmit light up to a distance of about 500 meters. The phototransistor of the receiver must be accurately oriented towards the laser beam from the torch. If there is any obstruction in the path of laser beam, no sounds will be heard from the receiver.


Fig 2.1 block diagram



            It is also called a capacitor or electrostatic microphone. Condenser means capacitor, which stores energy in the form of an electric field. Condenser microphones require power from a battery or external source. Condenser also tends to be more sensitive and responsive than dynamic, making them well suited to capturing subtle nuances in a sound. The diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance. Specifically when the plates are closer together capacitance increases and a charge current occurs and this current will be used to trigger the transmitting section.


            A laser diode needs a certain value of current, called the threshold current, before it emits laser light. A further increase in this current produces a greater light output. The relationship between output power and current in a laser diode is very linear, once the current is above the threshold, giving a low distortion when the beam is amplitude modulated. For example, the 65Onm 5mW laser diode used in this project has a typical threshold current of 3OmA and produces its full output when the current is raised by approximately 1OmA above the threshold to 4OmA.

            Further increasing the current will greatly reduce the life of the laser diode, and exceeding the absolute maximum of 8OmA will destroy it instantly. Laser diodes are very fragile and will not survive electrostatic discharges and momentary surges! However, if used within specifications, the typical life of one of these lasers is around 20,000 hours.

In the transmitter circuit (Fig.1) the laser diode is supplied via an adjustable constant-current source. Note that the metal housing for the laser diode and the lens also acts as a heat sink. The laser diode should not be

powered without the metal housing in place. The increasing the voltage at VR1 reduces the laser current. The setting of VR1 determines the quiescent brightness of the laser beam, and therefore the overall sensitivity of the system. The electric microphone is powered through R1 and is coupled to the non inverting input of 1C1 a via capacitor. This input is held at a fixed
DC voltage to give a DC output to bias.

            Here we use the light rays coming from laser torch as the medium for transmission. Laser had potential for the transfer of data at extremely high rates, specific advancements were needed in component performance and systems engineering, particularly for space-qualified hardware. Free space laser communications systems are wireless connections through the atmosphere. They work similar to fibre optic cable systems except the beam is transmitted through open space. The laser systems operate in the near infrared region of the spectrum. The laser light across the link is at a wavelength of between 780 - 920 nm. Two parallel beams are used, one for transmission and one for reception.


            The transmitted signal is picked up by the photo detector diode in the receiver .The output voltage of this diode is amplified by the common emitter amplifier around T4. This amplifier has a gain of 20 or so, and connects via VR2 to IC2, an LM386 basic power amplifier IC with a gain internally set to 20.This IC can drive a speaker with a resistance as low as four ohms, and 35OmW when the circuit is powered from a 9V supply. Increasing the supply voltage will increase the output power marginally. Incidentally, the photodiode

used for this project has a special clear package, so it responds to visible light, and not just infrared.

            A loudspeaker (or "speaker") is an electro acoustic transducer that converts an electrical signal into sound. The speaker moves in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water.



Fig 4.1. Transmitter


Fig 4.2. Receiver



            An op amp is a high-gain, direct-coupled differential linear amplifier whose response characteristics are externally controlled by negative feedback from the output to the input. OP amps, widely used in computers, can perform mathematical operations such as summing, integration, and differentiation. OP amps are also used as video and audio amplifiers, oscillators, etc. in the communication electronics. Because of their versatility op amps are widely used in all branches of electronics both in digital and linear circuits. OP amps lend themselves readily to IC manufacturing techniques.

            Improved IC manufacturing techniques, the op amp's adaptability, and extensive use in the design of new equipment have brought the price of IC ops amps from very high to very reasonable levels. These facts ensure a very substantial role for the IC op amp in electronics. Fig shows the symbol for an op amp. Note that the operational amplifier has two inputs marked (-) and (+). The minus input is the inverting input. A signal applied to the minus terminal will be shifted in phase 180° at the output. The plus input is the non-inverting input. A signal applied to the plus terminal will appear in the same phase at the output as at the input. Because of the complexity of the internal circuitry of an op amp, the op amp symbol is used exclusively in circuit diagrams.

Fig 5.1 symbol of op-amp

            An operational amplifier often referred to as op Amp, is a very high gain high performance amplifier designed to amplify ac and dc signal voltages. Modern integrated circuit technology and large-scale production techniques have brought down the prices of such amplifiers within reach of all amateurs, experimenters and hobbyists. The Op Amp is now used as a basic gain element, like an elegant transistor, in electronic circuits.
                The availability of two input terminals simplifies feedback circuitry and makes the operational amplifier a highly versatile device. If a feedback is applied from the output to the inverting input terminal, the result is a negative feedback, which gives a stable amplifier with precisely controlled gain characteristics. On the other hand, if the feedback is applied to the non-inverting input, the result is positive feedback, which gives oscillators and multivibrator. Special effects are obtained by combination of both types of feedback.

Fig 5.1.1 symbol of IC741

Fig 5.1.2 Negative feedback control circuit
            The above figure shows the basic circuit, including the negative feedback loop of an op amp. The output is fed back to the inverting input terminal in order to provide negative feedback for the amplifier. The input signal is applied to the inverting input. As a result, the output will be inverted. It is possible to operate the op amp as a non-inverting amplifier by applying the signal to the plus input. In this circuit the feedback network is still connected to the inverting input.
5.2 VR (potentiometer/resistance variac/trimmer):


fig 5.2 symbol

The potentiometer is a resistor of variable resistance. It has three terminals; a fixed resistance is found between two of the terminals and the third terminal slides along the fixed resistor. Often, it is used to control the volume in an audio amplifier.


            The capacitor plays a crucial role in electronics -- it stores electrons for when they're needed most. Capacitors consist of two conducting plates placed near each other. Inside the capacitor, the terminals connect to two metal plates separated by a dielectric. The dielectric can be air, paper, plastic or anything else that does not conduct electricity and keeps the plates from touching each other.

fig 5.3. Ceramic capacitor

            They can store electric charge for later discharge. Direct current through a capacitor will charge the capacitor for a short time, and then stop flowing. Alternating current, because of the changing electric fields it generates, can “flow” across a capacitor.

      5.4 BATTERY (9 VOLT)

            If you look at any battery, you'll notice that it has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative. In an normal flashlight batteries, the ends of the battery are the terminals. In a large car battery, there are two heavy lead posts that act as the terminals. Electrons collect on the negative terminal of the battery. If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can (and wear out the battery very quickly -- this also tends to be dangerous, especially with large batteries, so it is not something you want to be doing). Normally, you connect some type of load to the battery using the wire.

Fig 5.4: 9V Battery

            Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts.


            For this project we have removed the laser assembly from a small laser pointer. The power supply circuit is the green board attached to the brass laser head. We carry similar laser pointers in our catalog that are easily disassembled for this project. The power supply circuit came conveniently marked with a plus and a minus next to two holes in the board. We solder the black negative lead from the battery clip to the hole marked minus. We solder one of the coil leads to the hole marked plus. We solder the red positive lead of the battery clip to the other lead from the coil.

Fig 5.5. Laser torch


            Sound is an amazing thing. All of the different sounds that we hear are caused by minute pressure differences in the air around us. What's amazing about it is that the air transmits those pressure changes so well, and so accurately, over relatively long distances. It was a metal diaphragm attached to a needle, and this needle scratched a pattern onto a piece of metal foil. The pressure differences in the air that occurred when you spoke toward the diaphragm moved the diaphragm, which moved the needle, which was recorded on the foil. When you later ran the needle back over the foil, the vibrations scratched on the foil would then move the diaphragm and recreate the sound. The fact that this purely mechanical system works shows how much energy the vibrations in the air can have! All modern microphones are trying to accomplish the same thing as the original, but do it electronically rather than

mechanically. A microphone wants to take varying pressure waves in the air and convert them into varying electrical signals. There are five different technologies commonly used to accomplish this conversion. We use condensermic in our project.


             A condenser microphone is essentially a capacitor, with one plate of the capacitor moving in response to sound waves.

            The standard symbol of a phototransistor, which can be regarded as a conventional transistor housed in a case that enables its semiconductor junctions to be exposed to external light. The device is normally used with its base open circuit, in either of the configurations shown in fig. 5.10.2, and functions as follows.

Fig. 5.7 Phototransistor symbol.

            In practice, the collector and emitter current of the transistor are virtually identical and, since the base is open circuit, the device is not subjected to significant negative feedback. The sensitivity of a phototransistor is typically one hundred times greater than that of a photodiode, but is useful maximum operating frequency (a few hundred kilohertz) is proportionally lower than that of a photodiode by using only its base and collector terminals and ignoring the emitter, as shown in fig.


            Phototransistors are solid-state light detectors with internal gain that are used to provide analog or digital signals. They detect visible, ultraviolet and near-infrared light from a variety of sources and are more sensitive than photodiodes, semiconductor devices that require a pre-amplifier. Phototransistors feed a photocurrent output into the base of a small signal transistor. For each illumination level, the area of the exposed collectorbase junction and the DC current gain of the transistor define the output.responsive, is measured in nanometers (nm). Rise time, the time that elapses when a pulse waveform increases from 10% to 90% of its maximum value, is expressed in nanoseconds (ns). Collector-emitter breakdown voltage is the voltage at which phototransistors conduct a specified (nondestructive) current when biased in the normal direction without optical or electrical inputs to the base. Power dissipation, a measure of total power consumption, is measured in milliwatts (mW).


            In all of the laser communicators on this page, the laser light is amplitude modulated. This simply means that the amount of light the laser emits varies over time. To understand what is going on, it helps to consider how a loudspeaker makes sound. A loudspeaker is a paper cone attached to a coil of wire that sits in a magnetic field from a strong permanent magnet. When an electric current flows in the loudspeaker coil, the coil becomes an electromagnet, and it moves toward or away from the permanent magnet. As it moves, the paper cone pushes on the air around it, compressing the air in front of it, and expanding the air behind it. Waves of compressed and expanded air travel to your ear, and cause your eardrum to move in time to the movements of the paper cone.

            The laser communicator adds two components to the loudspeaker concept. We take the electrical signal that goes to the loudspeaker, and connect it instead to the laser, so the laser gets brighter and dimmer as the electric current varies. The second component is the receiver, which converts the light back into an electric current. This current varies in time with the first current, because the amount of light that it receives
is varying in time. This second electric current is used to move the paper cone of a loudspeaker, just as before. However, now the loudspeaker can be quite a distance away from the original electric current, without any wires connecting the two.



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Fig 8.1 Transmitter

Fig 8.2 Receiver

The PCB manufacturing process involves use of expensive equipments, but homebrew PCB fabrication is less expensive .It requires Intel Pentium PC,600-1200dpi laser printer with premium-quality paper or butter-paper and miscellaneous items like single side copper laminated board, Lacquer thinner, sand paper and others. The various steps involves in PCB fabrication are
                                    9.1. PC-based artwork
                                    9.2. Printing on a laser jet printer
                                    9.3. Transfer of pattern to copper plate using cloth iron
                                    9.4. Etching and Drying
                                    9.5. Drilling and cleaning
                                    9.6. Caution
            The PC based artwork consists of drawing the conductor pattern. For putting artwork on the component side of the board, flip the whole image before or while taking the print. When the pattern has been drawn, take the print out in 600 to 1200 dpi on a translucent or butter   paper. Keep the paper side on which the toner is deposited facing down over the copper laminated boards copper side and then when the board is turned component side up, the pattern on the conductor will be found properly aligned with the components. Finally we take the printout of the PCB.     

Take the printout of the circuit layout from a laser printer. The idea is to use a coated paper so that the toner comes loose when heated which would transfer a sharp black print on to copper laminate. Print for each of the required layers should be taken on separate paper.

Scrub The Copper Side Of The Copper Clad Laminated Used For The PCB Board With A Sponge. The Scrubbing Involves Removes Oxidation, Stains, Etc. And It Also Makes The Copper Surface Some-What Rough Which Helps The Toner To Adhere To The Copper Surface. The Next Step Is To Degrease The Board Thoroughly Using A Paper Towel Soaked With Acetone Solvent. Keep Doing It Until No More Discoloration Is Seen On The Paper Towel. Rub Hard And Keep Switching To Clean Parts Of Towel. Place And Align The Paper On The Copper Side, Using An Iron Box To Maximum Setting On The Back Of The Paper For At Lest Half A Minute. If You Don't Apply Enough Heat, The Film Or Toner May No Stick Or Be Dark Enough. The Removal Of Paper From PCB Is Done By Putting It Into Hot Water For 10 Or More Minutes. Check Whether It Has Transferred Properly Onto Copper Plate.
Dig The Bristles On The Tip Of A Smooth Tooth Brush Into The Holes, Remove The Paper Part From The Tight Areas Like Drill-Holes. Now Cut The PCB To Required Size By Using A Hacksaw.
Etch the unwanted copper from the board using the ferric chloride solution for 20 or more minutes. One pint can etch at least 3.6 sq. meters of the 28gm board. Heating the etchant will speeds up the etching process. The PCB is attached to a wooden piece and dip in to the solution. Lift the PCB up and

Check whether all the unwanted copper is removed. Then it is immersed in to cold water to clean. When etching is complete, board is removed from the solution and rinse it under running tap water .Acetone or lacquer thinner is used to remove the toner .Lacquer thinner is used as a solvent in painting industry. Wash the board in lacquer thinner solvent, rubbing with a paper towel, to remove the toner instantly.
In this we had used a PCB hand drill .Use 0.8mm PCB drill bit to drill out all of the component holes. After drilling the holes scrub sponge is used to clean before soldering .After drilling and cleaning, wash the board in cold water and then dry it.
Lacquer thinner is extremely volatile, inflammable and explosive. Acetone can irritate eyes and respiratory system .Ferric chloride is corrosive, so avoid skin and eye contact.

Soldering is the process of joining metals by using lower melting point metal to wet or alloy with the joined surfaces. Solder is the joining material. It melts below 427’C. Soldered joints in electronic circuits will establish strong electrical connections between component leads. The popularly used solders are alloys of tin and lead that melt below the melting point of tin.
In order to make the surfaces accept the solder readily, the component terminals should be cleaned chemically or by abrasion using blades or knives. Small amount of lead coating can be done on the cleaned portion of the leads using soldering iron. This process is called tinning. Zinc chloride or ammonium chloride separately or in combination are the most commonly used fluxes. These are available in petroleum jelly as paste flux. A solder joint can at first glance to be okay, but under close examination it could be a ‘Dry Joint’. A dry joint is when either the circuit board or the leg of the component has not been properly heated to allow the solder to flow between the surfaces freely. This creates an intermittent or no electrical connection. This can also be caused by a lack of flux or if you reuse old solder.
            Quite often, reheating a bad join will cure the problem but in a lot of cases, the old solder will need to be removed and some new solder applied. The residues, which remain after the soldering, may be washed out with more water, accompanied by brushing.
Soldering iron is the tool used to melt the solder and apply at the joints in the circuit. It operates in 230V mains supply. The iron bit at the tip of it gets heated up within a few minutes. 50W and 25W soldering irons are commonly used for soldering purposes.

  1. Make a layout of the circuit.
  2. Straighten and clean the component leads using blade or knife. Apply a little flux on the leads. Take a little solder on soldering iron and apply the molten solder on the leads.
  3. Mount the components on the PCB by bending the leads of the components using nose-pliers.
  4. Apply flux on the joints and solder the joints. Soldering must be done in minimum time to avoid dry soldering and heating up of components. Wash the residue using water and brush.


            First of all, it's most important that you don't look directly into the laser beam. If you do, it could cause permanent eye damage. Also, you are responsible for the safety of others near the laser, which means you must stop others from also looking into the beam, and take all necessary safety steps. This is covered by legislation. Both the receiver and the transmitter can be powered by separate 9V batteries or suitable DC supplies. Before applying power to the transmitter PCB, set VRI to its halfway position, to make sure the laser current is not excessive.

            To be totally sure, you could set VRI fully anticlockwise, as this setting will reduce the laser current to zero. Then apply power to the board. If the laser doesn't produce light, slowly adjust VRI clockwise. The laser diode should emit a beam with an intensity adjustable with VRI. At this stage, keep the beam intensity low, but high enough to clearly see. If you are not getting an output, check the circuit. You won't see the laser beam intensity change with the modulating signal. To check that the system is working, place the two PCBs on the workbench, spaced a meter or go apart. You might need to put a sheet of paper about 2Omm in front of the photodiode to reduce the intensity of light from the laser beam. Set the volume control of the speaker to about halfway. If the volume control setting is too high you'll get acoustic feedback. Move the laser diode assembly so the beam points at the receiver's photodiode. It's useful to adjust the beam so it's out of focus at the photodiode, to make alignment even easier. You should now be able to hear the speaker reproducing any audio signal picked up by the microphone. LED is not focused, and simply spreads everywhere, so a reflector might help the sensitivity. Warnings The laser diode in this project is a class 3B laser and you should attach a warning label to the transmitter.. Remember that, as for any hazardous device, the owner of a laser is responsible for its proper use.


1.      Less costly
2.      Circuit can be easily constructed
3.      High data rate
4.      No communication licenses required.
5.      The laser transmission is very secure because it has a narrow beam.
6.      There are no recurring line costs.
7.      Compatibility with copper or fiber interfaces and no bridge or router requirements.
8.      Lasers can also transmit through glass, however the physical properties of the glass have to be considered.
9.      Narrow beam divergence
10.  Laser transmitter and receiver units ensure easy, straightforward systems alignment and long-term stable, service free operation, especially in inaccessible environments, optical wireless systems offer ideal, economical alternative to expensive leased lines for buildings.


             To avoid 50Hz hum noise in the speaker, keep the phototransistor away from AC light sources such as bulbs. The reflected sunlight, however, does not cause any problem. But the sensor should not directly face the sun.


1. Using this circuit we can communicate with our neighbors wirelessly
2. It can be used in inaccessible areas.
3. In future it can be commissioned in satellites for communication.
4. It can be used in conference halls.


Some of the possible ways are as follows:-
            Instead of the short range laser, high range lasers can be used which range a few
hundred meters. Provisions have to be made for cases when there is no heavy traffic.


            After the successful working of the project, it can be concluded that this project is suitable for easily communication. There can be further up gradations in the project which could lead to a much better system for communication.


  General Purpose Single Opeartional Amplifier

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