# Do-It-Yourself Optical Signal Transmitter(Double Frequency) - do it yourself

We live in a time in which we have grown accustomed to the convenience of wireless gadgets all around us. At some point I believe we all have wondered how these wireless gadgets work. Even though the science behind it can be easily found online, theory doesn't let you appreciate the greatness or the complexity of the idea the way practical application does. These instructions will give you a small taste of that. The purpose of this project is to show how to create a portable two frequency optical signal transmitter using DC voltage supply. I will explain the theoretical reasoning behind each element of the project so you are capable of customizing your own Signal Transmitter.

Required Knowledge: Due to the complexity of components used and the level of circuitry involved, this project will require technical understanding in circuitry of college level or higher.

Estimated Time: Once the components have been collected, setting up the project should take around 30 minutes to one hour.

Estimated Cost: The components are easily bought online or at a nearby RadioShack store. The total cost of the components will be around \$20-\$30.

## Step 1: Components Required - I

We are going to break down the component requirements based on the layout of the circuit into two sections: Transmitter and Receiver. The requirements per section are listed at the end of the explanation for each component. The number in the bracket indicates the quantity.

In the figure above, the top row shows the circuit symbols for each of these components. The bottom row shows their real photos. The components are arranged in the same order as the following explanations.

1. Three-way switch: A switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. The three way switch when switched to the left connects the left terminal to the central terminal, when switched to the right connects the right to the central terminal and is 'OFF' when in the middle.

Transmitter: 1

2. Infrared Light Emitting Diode(LED): It is a two-lead semiconductor light source that resembles a basic pn-junction diode, except that an LED also emits light. The wavelength of light emitted by an infrared LED is in the range of 700nm to 1mm.

Transmitter: 1

3. Operational Amplifier: It is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op-amp produces an output potential (relative to circuit ground) that is typically hundreds of thousands of times larger than the potential difference between its input terminals.

4. Capacitor: A capacitor is a passive two terminal electrical component that is used to store electrostatic energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electric conductors separated by a dielectric.

Transmitter: 100nF(2), 10nF(2), 440?(2), 220?(3) Receiver: 1uF(3), 10nF(1), 100nF(1), 20nF(1)

## Step 2: Components Required - II

In the figure above, the top row shows the circuit symbols for each of these components. The bottom row shows their real photos(Note: Breadboard does not have a circuit symbol). The components are arranged in the same order as the following explanations.

5. Resistor: A resistor is also a passive two terminal electrical component that implements electrical resistance(as given by Ohms Law) as a circuit element.

The resistors are color coded. For further information regarding how to read the resistance of a resistor click here.

Transmitter: 2.2k?(2), 3.2k?(2), 440?(2), 220?(3) Receiver: 1k?(2), 3.3k?(1)

6. Light Emitting Diode(LED): The principle involved is the same as that for an infrared LED. The only difference is that the light emitted is in the visible range of the electromagnetic spectrum.

7. Diode(LED): A diode is a two-terminal electronic component with asymmetric conductance; it has low (ideally zero) resistance to current in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p-n junction connected to two electrical terminals.

8. Connecting Wires: They are conductors(usually copper) covered by an insulator(usually plastic) that are used to connect different components of a circuit.

9. Breadboard: It is usually a construction base for prototyping of electronics.Wires and components are simply pushed into the holes to form a completed circuit and power can be applied. For connections and more on breadboard click here.

Source: www.wikipedia.org/

## Step 3: Equipment Required

The electrical components usually are not of the exact value as the one displayed on them. There are a lot of reasons for why this happens. To achieve better accuracy, we are going to use their actual values which we will find using these components.

1. DC Power Supply: To power our circuit we need some kind of source that can deliver DC voltage. This can be anything from a laboratory grade DC Power Supply to a battery, however it must be able to supply 0-12V of DC voltage.

2. Multimeter: A multimeter is used to measure values of a wide range of electrical components. We will use this to figure out the exact values of the circuit elements.

3. Oscilloscope: It projects the AC signal from the circuit on a display screen and its parameters. It is also possible to compare signals at two or more points on the circuit at the same time. The figure above shows a typical Oscilloscope.

## Step 4: Software Simulation

Multisim is a Simulation Program with Integrated Circuit Emphasis(SPICE). It is used in designing, prototyping, and testing electrical circuits virtually. We are going to use it to create a layout for our circuit. If there is a problem it will serve as a reference for troubleshooting the circuit.

An alternate option is to use PSpice. Unlike Multisim, PSpice is free of cost but it is not as user-friendly.

## Step 5: Software Simulation: 1. Installing the software

2. Purchase the version that you think best works for you. Note: Students from most of the colleges in the USA are offered a discounted price.

## Step 6: Software Simulation: 2. Creating the circuit layout

1. Start Multisim.

2. If you are not familiar with Mutisim, read the basics from the 'Help' menu.

3. On a new spreadsheet, create the circuit for the transmitter and receiver circuits(see figure).

Optional: When implementing the circuit on a breadboard, the chances of making a mistake are high as there are a lot of components and wires in a small amount of space. To minimize this, I would recommend using the circuit simulation layout as a map. To do that, begin with rearranging the layout to the way you are planning on implementing it on your breadboard. For example, if your breadboard is not big enough to hold the either the transmitter or receiver circuit on one board, you will have to break it down into two parts.

Figure(above) is my layout for the circuit created in Multisim.

## Step 7: Building the Circuit

Let's move our circuit from our computer screens to a breadboard. The following steps will guide you through it:

1. Use the Multimeter to find the exact values of the resistors and capacitors.

2. Using the layout you created as a reference, implement the circuits(transmitter and receiver) on two breadboards.

3. Connect the Power Supply.(Warning: Do NOT turn the Power Supply on while making connections).

That's it! Your two frequency signal transmitter is complete. The two frequencies being transmitted are of 500Hz and 7kHz.

The photograph(above) is of the completed project. If you look at it carefully, there are two breadboards. The breadboard on the top is the receiver circuit and the one on the bottom is the transmitter circuit.

## Step 8: How does it work?

Now that you have created your own signal transmitter, let's try understanding the science behind it. First we will talk about the transmitter, then the receiver.

## Step 9: Transmitter Circuit - Overview

The main parts of the transmitter circuit are the two Wien Bridge Oscillators. This is where the generation of AC signal from DC input takes place. The two oscillators generate sine waves of frequencies 500Hz and 7kHz, respectively.

The reason I chose two frequencies with a high difference so that I could safely use passive filters(who have a -20dB/decade fall rate on a Bode plot). If the difference between the frequencies is not sufficient, there will be an area overlap between the two filters. This will cause both the LEDs to turn ON irrespective of the frequency selected, defeating the purpose of the project.

I couldn't go much higher than the 7kHz frequency because the Wien Bridge Oscillator practically works only up to a frequency of 10kHz. As for the lower range, the signal detection becomes very erroneous if the frequency is too low.

The figure above shows the circuit diagram for a Wien Bridge Oscillator and gives the formula to calculate its frequency in Hertz[Hz].

## Step 10: Transmitter Circuit - Voltage Plots

These plots(above) are taken from the receiving terminal of the two Wien Bridge Oscillators as observed from an Oscilloscope. The top image is from the oscillator generating the 500Hz frequency. The bottom image is from the oscillator generating the 7kHz frequency.

1. The receiver is a photo diode whose negative terminal is grounded and the positive terminal is connected to an inverting amplifier's inverting terminal. The negative feedback resistor is a 1M? resistor, which makes the gain really high.

2. This is followed by a voltage buffer/voltage follower(Above figure). I used a voltage follower so that the components connected after the buffer do not affect the voltage before it.

3. At this point the circuit branches out into two parts. The first part of the circuit starts with a passive low pass filter with a critical frequency of 5kHz. The second part instead uses a passive high pass filter but with the same critical frequency of 5kHz.

4. After the passive filter there is a peak detector in both parts of the circuit. A peak detector is just a diode followed by a capacitor in parallel with an LED whose other end is grounded. The low pass has a green LED and the high pass has a red LED.

## Step 12: Receiver Circuit - Voltage Plots

The above plots are of the AC signal as it is received by the Photodiode for the 500Hz and 7kHz frequencies, respectively.

## Step 13: Conclusion

This is a video showing the working for the project. For the sake of simplicity here I have only one frequency set up.

This is it! Although the project only blinks LEDs, which is not that amazing, it is only to show and help you understand how the project works. You can replace the LEDs to use it as your self created remote control! I used this to create a remote for my Roomba robot that I created for another class. I decided to keep this simple by using LEDs because I wanted to keep it generalized. If I would have used something more sophisticated, it would have made this project very specific to its use. I wanted to teach the idea behind the project with a more hands on approach for which I think simplicity is the key. I hope I was able to achieve that! :)