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07:30 - 19:00

Monday to Friday

Voice communication with infrared light and fiber optics

Voice communication with infrared light and fiber optics

Introduction: (Initial Observation)

Infrared light is very similar to visible light, and although it is invisible to the human eye, it is visible and detectable by some animals. Infrared is used in TV remote controls, wireless keyboards, and even in some laptop computers for wireless communication with printers. Devices for transmission of sounds via infrared are now available in many electronic stores.

This project investigates the methods and mechanisms of using fiber optics and infrared in transmitting voices. You will perform experiments and make models to demonstrate this technology.

This is two projects in one. You may choose to limit your project to fiber optics or to infrared communication.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Project plan/ outline:

A project plan or outline is a list of what you are planning on doing in your project. An outline can be as simple as a list of tasks that you have in mind. The following is my project plan for this project.

    1. Gather more information about Infrared, how it is made and its sources.
    2. Find an infrared emitter. I expect it to be something like a regular light bulb with no visible light.
    3. Find an infrared sensor. I expect this to be like a photocell or solar cells that I have seen on solar calculators. It also should be sensitive to infrared.
    4. Get a small radio and replace its speaker with a infrared emitter. In this way, the radio will create infrared waves instead of sound waves.
    5. Connect the infrared sensor to the microphone jack of a guitar amplifier or input of an amplified computer speaker.
    6. For the fiber optic line experiment, I will replace the infrared emitter by a diode or laser light. Light can enter the cable and get to the photocell. Everything else is expected to be the same. I will try to buy fiber optic cable for this experiment, but I may also try using Nylon fishing lines instead. I think it will work.
    7. My specific question for this project is “How does the strength of an infrared signal change by changing the distance?”

Information Gathering:

Find out about infrared and fiber optic communication. Read books, magazines or ask professionals who might know in order to learn about the mechanism of conversion of different waves or signals to each other. Keep track of where you got your information from.

The following are samples of information that you may find.

Keywords: IR Emitter, phototransistor

IR Light Beam Circuits

Use an IR LED and phototransistor pair to create a light beam switch. Point the components at each other to turn the switch on, then break the beam to turn the switch off. An IR LED/detector pair is exactly how your TV remote works. You can control your TV from across the room because the diode is pulsed briefly at a much higher current which gives off much more light.

IR LED: Radio Shack 276-143 ($1.69) or Jameco 106526 ($0.45). Connect like an ordinary LED using a 330 ohm series resistor to the +5 supply. Current draw is about 11 mA with a 330 ohm resistor. Current runs from anode to cathode. Flat on the case marks the cathode.

IR Phototransistor Radio Shack 276-145 ($0.99) or Jameco 112168 ($0.45). A phototransistor is just like a regular transistor, except the base lead is disabled or absent and light activates base current. The flat on the case marks the collector, the other lead is the emitter. Connect the collector to one end of a 10K resistor and connect the other end of the resistor to a +5V supply. Next, connect the emitter to the ground. To view the output, measure the voltage at the collector. The voltage should start out at +5V. When pointing the IR diode at the phototransistor, the voltage should drop down to near zero.

Telling them apart The IR LED and the IR phototransistor look alike. If you don’t know which is which, you can try them in the test circuit described above, and swap if nothing happens. If you have the Jameco IR pair, the IR LED has one lead embedded into the clear section that looks somewhat like a tiny upper case letter F. You must look carefully to see this.

Seeing Infrared

See like a snake, or an insect


Infra-red light is invisible, however; for this project it is good if you could somehow see the infrared light to verify your experiment setup. You can use digital cameras in order to see infrared. Digital cameras can see infrared and ultraviolet lights that are invisible to the human eye.


    • A digital camera with a view screen, still or digital video.
    • A television remote control.
    • A floppy disk (Blocks the visible but passes infrared.)
    • A black light.
    • Optional an infrared LED source.
      • Infrared LED
      • 300 ohm resistor
      • 9 volt battery and holder.
    • Optional, a filter which passes UV but blocks visible, look for Wood’s glass.
    • Optional, an ultraviolet filter for a camera.


Remove the floppy disk from its protective plastic cover.

Optional infrared LED source.

Hook up a 9 volt battery holder in series with the 300 ohm resistor and an infrared LED. Be careful that there is only one orientation in which the LED will glow. Make sure the longer leg of the LED is closest to the positive terminal of the battery.

To Do and Notice IR

Look at the output end of a television remote control. Press one of the channel change buttons and notice that you cannot see anything.

Turn on the camera and watch the view screen. Look at the TV remote control with the digital camera. Notice that you can now see a flashing light in the view screen.

Dissect a floppy disk. Remove the reddish floppy disk from inside its plastic protective sleeve. Look at the TV remote control through the floppy disk. Notice that you cannot see the remote control infrared light. Point the TV remote control at the camera through the floppy disk. Notice that you can see the flash of infrared light from the remote control through the floppy disk.


Plug a battery into the infrared LED source and point it at the camera.
Notice that the camera can see the LED glow but you cannot.

Make the room dark, use the infrared LED to illuminate a toy which you then photograph with the digital camera.

What’s Going On?

The CCD (charge coupled device), which converts light into an electrical signal inside a digital camera or digital video camera, is sensitive to infrared and ultraviolet light in addition to visible light.

You can use the camera to see normally invisible, infrared light, like that emitted by a television remote control.

The floppy disk blocks visible light but transmits infrared light.
Thus you can see the flashes of the remote control through the floppy disk filter with the digital camera.

Pit vipers such as the rattlesnake have two pits which can image infrared light. This lets them see the position of a warm blooded mouse even in complete darkness, since the warm mouse emits infrared radiation.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to see and experience how infrared communication works. I also want to know how distance affects the strength of infrared signals.

Another Purpose can be like this:

The purpose of this project is to see and experience how fiber optic communication works. I also want to know if plastic fibers can also be used to transmit signals.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

This is how you define variables for the first question/purpose.

The independent variable is the distance between the infrared emitter and infrared detector.

The dependent variable is the strength of signal detected by the infrared detector.

Constants are the voltage, IR emitter, IR detector, IR filter.

Controlled variables are outside light and condition of air between IR emitter and IR detector.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

This is a sample hypothesis:

The strength of IR radiation reduces by distance.

My hypothesis is based on my observation of Television Remote Controllers that fail to work after a certain distance.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”


Experiment 1:


In this experiment you will make an IR emitter and an IR detector and then measure and record the effect of distance on the strength of infrared signals.


Make an IR emitter by connecting an infrared LED using a 330 ohm series resistor to the +5 supply. Current draw is about 11 mA with a 330 ohm resistor. Current runs from anode to cathode.

As shown in the diagram (below – left), in an IR emitter, the wire starting the + sign of your 5 Volts battery or power supply, first passes through a 330 Ohms resistor and then enters the Anode of the infrared diode (the anode is the longer lead). The cathode or shorter lead of the infrared diode connects to the negative (-) of the battery or ground of the power supply.

Make an IR detector: Connect the collector lead of an IR phototransistor to one end of a 10K resistor and connect the other end of the resistor to a +5V supply. Connect the other lead (emitter) to the ground of the power supply or negative (-) of your 5 volts battery. To view the output, measure the voltage at the collector. The voltage should start out at +5V. When pointing the IR diode at the phototransistor, the voltage should drop down to near zero.

As shown in the diagram (above – right), in an IR detector, the wire starting the + sign of your 5 Volts battery or power supply passes through a 10K Ohms resistor and then enters the collector lead (shorter lead) of a phototransistor and from the emitter lead of phototransistor (longer lead) it connects to the negative (-) of the battery or ground of the power supply.

As you see a phototransistor looks like an LED.

Set the multimeter to DC Volts and connect it between the collector of your phototransistor (shown with X in the above diagram) and the negative of your battery (or ground of your power supply). Read the voltage.

Repeat reading the voltage while holding the infrared collector at different distances from the infrared emitter. Record your results in a table like this.

Distance Voltage Loss of voltage
(5 – Voltage)

Loss of voltage is an indication for the strength of IR source. You can use the distance column and the loss of voltage column of the above table to draw a line graph.

Additional Notes:

If you have ever made any electrical circuit or you have knowledgeable assistance, you should not have any problem on doing this experiment; however, following additional notes may be helpful to reduce your problems.

Resistors are used to restrict the electric current to an appropriate amount for your IR LED and phototransistor. Resistors are marked with color bands that indicate their resistance. If you can not find 330 Ohms and 10K Ohms resistors, just get the closest possible.

To identify a resistor using its color bands use the color code calculator.

Note that the leads of a LED are called Cathode and Anode while the leads of a phototransistor are called Collector and Emitter.

Do your experiment in a low light or dark room or cover your phototransistor with a piece of IR filter that you may even cut yourself from the plastic disk inside a floppy disk.

Experiment 2:


In this experiment, you will transmit sound via an IR emitter and receive it via an IR detector. You will need a medium or large size battery operated transistor radio for this experiment. (For safety purposes, we do NOT recommend using home electricity for this experiment. So if you have a radio that works either by battery or by electricity, you must use battery or you must be supervised by an electrician). You may optionally try this experiment with a bright LED instead of infrared LED so you can actually see the light waves/ blinking lights.


Set the radio to a local station and set the volume to loud. Temporarily turn off the radio. Open the radio and locate the speaker. Disconnect the two wires that go to the speaker and connect them to a bright LED. Turn on the radio again. LED must blink or show a vibrating light.

Connect a photocell to the input of an amplified computer speaker. Turn the speaker on and set it to loud.

Place the photocell in the path of the light coming from LED. You must hear the radio from the speakers.

Additional Notes:
If you are not successful in your first try, read the following notes for more details. The picture below shows a pair of infrared emitter and detector that may be used in this experiment.

Matched Infrared Emitter and Phototransistor Detector

Radioshack Catalog #: 276-142

Divide your experiment in two independent parts.

Part1: Produce infrared waves

Start with a device that has a speaker. Some examples of such device are computer speakers and transistor radios. If the speaker is able to produce sound, it means that an electrical wave is entering the speaker. A wave of electricity can be detected and measured by a volt meter set to AC volts. Such electrical waves must also be able to light up a visible LED (Light Emitting Diode). Therefore, as the first step of your experiment, connect an LED where you now have a speaker; it should light up. The light may blink depending on the sound that is being played. (In many cases, you don’t even have to disconnect the speaker).

If the light does not come up, try increasing the volume. If it still does not light up, it simply means that your device is not producing a voltage high enough for this experiment. For example a small transistor battery that operates on only 2 AA batteries will not produce enough electrical waves to light up an LED.

After you successfully light up a visible LED, you can then replace it with an infrared emitter. Since infrared is not visible, you must look at it through a digital camera to make sure that it is working. Digital cameras convert invisible infrared light to visible light.

Part 2: Get infrared waves

Connect the phototransistor in series with a battery to the input line of an amplified speaker.

Now you must test to see if your phototransistor is receiving light waves. Bring your phototransistor under the light of a fluorescent light bulb. At this time you must hear a sound in the speaker.

This sound is being produced because fluorescent light bulbs emit blinking lights.

If you can not hear the noise, switch the pins of your phototransistor or reverse the position of your battery. Your device must finally be able to get the light waves or pulses and convert them to sound waves or noise in the speaker.

Part 3: Pair them up

If you can hear the noise caused by the blinking fluorescent light and if your infrared can blink, you can now pair them up. Hold the infrared emitter faced to the phototransistor; you should be able to hear the sound from your sound device. Adjust the distance and eliminate outside light interference. You may also place an infrared filter on your phototransistor to filter visible lights.

Experiment 3:


In this experiment, you will transmit sound via a fiber optic wire. Electrical waves entering a speaker are redirected to a bright LED and converted to light waves. Light waves enter a clear optical fiber and exit the other end where a photocell is installed. Electrical waves produced in the photocell are then entered to an amplified speaker to be converted to sound waves.


This experiment is very similar to experiment number 2. The only difference is that you place an optical fiber between the LED and photocell. Light from the LED will enter the optical fiber (or group of optical fibers) to get to the photocell.

Additional Notes:

  • One fiber optic wire transmits only a small fraction of the LED light so the sound that you will hear in the other side is very weak. You may optionally use a lens to focus the light on the entrance of fiber wire or use a bunch of fiber to get stronger signal.
  • Instead of a photocell, you may use a phototransistor as described in experiment number one. The input of the speaker can be received from the Emitter pin and Ground (or Negative). I have shown this point with X in the diagram of experiment 1 and with “PIC Pin” in the diagram on the right.
  • This is the phototransistor circuit that worked for me. Note that if you hook up phototransistors in a wrong way, they will burn out. You need to identify the Emitter and Collector as your first stem and make sure that +5 volts current enters the Collector.
  • The piece I used from Radio Shack is number 276-145 (NPN Infrared Phototransistor).

In the above diagram, when the light (shown by two arrows) hit the photo transistor, the electricity from the 5 volt power source connected to C can travel down to E and go toward the ground. On its way, the electricity pass our PIC pin, where we connect our amplified speaker.

What is PIC?
PIC stands for Peripheral Interface Controller. PIC pin is a pin or a connection where you connect a peripheral device such as a speaker or a light bulb.

What are Collector and Emitter?
A transistor usually has three pins called Emitter, Collector and Base. Collector and Emitter are not internally connected; however when you apply a small flow of electricity to the base, Collector and Emitter connect. As you see transistors act like a switches. In a phototransistor, base is a light window. Light entering a phototransistor works like a small electricity at the Base.

Materials and Equipment:

List of material for experiment number 1

    • Phototransistor
    • Infrared L.E.D.
    • 5 Volts battery (6-volt Lantern battery works as well)
    • Multimeter or DC voltmeter (Analog or Digital)
    • 330 Ohms resistor
    • 10,000 Ohms (10K) resistor
    • IR filter (the plastic disk inside floppy disks)

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Your experiment results are the presented in the form of the results table described in experiment 1 and a graph for it.

Here I just include some pictures from my experiment.

I disconnected the wires that go to the speaker and connected them to the bright LED. I noticed that when I have two LEDs connected in parallel, they both have more light, so I also connected a red LED that is not being used for its light. Obviously, this additional LED is reducing the current to more appropriate amounts for both LEDs. I could also use a resistor in series with the LED instead of second LED.

Here the light from the bright LED hits the photocell. The photocell is connected to two wires of the input of an amplified computer speaker.

Photocell can be soldered to the wire; however, I just wrapped the wires around the leads of the photocell.

Here the light from the bright LED enters a fiber optic wire. The fiber optic wire that I used is shielded and has special connections or jacks on both ends.

Light from the other end of fiber optic is entering the photocell.

The sound heard was very low.


No specific calculation is required for this project.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Visit your local library and see some communication related books in the electronics section. Communications in the form of electrical waves, electromagnetic waves (radio waves) and light waves are all related topics that can be found in electronics books.

Following are some web resources:

Infrared communication

Infrared Communication

Introduction to Fiber optic communications

To find more web resources search the Internet for fiber optic communication, and infrared communication.