Introduction: Sending Music Through a Frequency-Modulated (FM) Infrared (IR) LED

By Optical Interface Design Club
Hello Instructables community! Welcome to my first project upload. This was actually my Freshman Electrical/Optical Engineering Final Project at the University of Alabama in Huntsville. It was actually categorized as a higher level project--for junior or senior students--but my group decided that all the suggested freshman-level project options weren't challenging enough. This is actually a modification of MaxmCarter's High Fidelity Laser Audio Transmitter (http://www.maxmcarter.com/lasrstuf/lasermodulator.html). During the process of the build, we had to change the design around a bit so that we could directly connect it to an ipod. We also found a couple of mistakes and redundancies that we had to modify. Nevertheless, we still use the same Intergrated Circuits (ICs). If you are looking to transmit your voice over an FM laser maxmcarter's design might suit you better. However, if you want to transmit your music through FM by optical methods, look no further!

So why FM?

Well . . . that is a very good question. In highschool I attempted to transmit music through amplitude modulation with a cheap laser pointer and the noise was unbearable. Turns out that ambient light created a sort of feedback that made my project very inefficient. If the lights weren't out, the reciever wouldn't play my signal. Even with the lights out, only one song was audible--"Hey Soul Sister" by Train. So the purpose of this project was to fix that bug. We wanted to be able to send music in the middle of the day as clear as possible. With FM, we could set up a series of oscillations that would fix that. The circuit forms a free-running FM oscillator/modulator operating at twice the operating output frequency. This is the core of the modulator. The output is linear ramping voltage of either positive or negative slope.


And why an IR LED?


We originally planned to use a laser pointer and a photodiode. However, it turned out that we bought the wrong photodiode--we had a blue photodiode receiving red laser light. We realized this at the very last minute and had to change to parts we already had--an IR phototransistor paired with an IR LED. If you want to use a laser pointer and a correct photodiode, you can simply replace these components.


Either way, this was the Intro and I hope you will enjoy building this project. It's a bit challenging but I will try to make it as simple as possible.  

Step 1: Parts List for Modulator/Transmitter

So here it is, the parts list.

For the transmitter:

 Capacitors

 .1uF ………………………….5

  4.7uF (25V)………………2

  270pF……………………….1

  1uF (35v) ………..……….2

  .001uF………………….…..1

  .01uF…………………….….1

  10uF (25v)…………………1

  100uF (16v)………………1

Diodes

  1N5817 (Schottky rectifier)……….2

  IR LED………………………………………….1

  1N914…………………………………………1

 Resistors

  10k…………………………………………….5

  2.7M……………………………………….…1

  270k…………………………………………..2

  100k…………………………………………..4

  2.2k………………………………..………….1

  33k…………………………………………….1

  560 Ohm………………………………..….1

  240 Ohm……………………………….…..1

  2 Ohm………………………………………..1

 ICs

  LF347 (Quad JFET op-amp)……………………1

  CD4069 (Hex CMOS inverter)………………………..1

  CD4013 (Dual CMOS D flip-flop)…………………….1

  TC1411 (HS driver)(Digikey TC1411CPA)………..1

  LM317 (Voltage regulator)…………………….……..1

Potentiometers (trimmers)

  100k (trimpot)……………………….…..1

  10k (trimpot)………………………….…..1

  500 Ohm (trimpot)……………………..1

you also need a Breadboard and many connectors/wires

I suggest you purchase the ICs through Digikey.com, The Pots and diodes through Radioshack, and the rest through Jameco. Doing this, my modulater cost me about $45-60. I bought most parts in exccess. I suggest Part no. 20812 for breadboard. You can also use a regular breadboard if you wish to work under a budget.

Step 2: Parts List for Demodulator/Receiver

Here are the parts. Same suggestion as before.

 Capacitors

 .1uF………………………….......................7

  100uF (16v electrolytic)………………….1

  220pF (mica or metallized film)……..1

  2200pF (mica or metallized film).….1

  .001uF (2% polypropylene)…………..1

  .01uF (metallized film)……………………2

  .001uF (metallized film)………………….2

  .05uF (ceramic)…………………………..….1

  270uF (16v electrolytic)………………….1

  1uF (35v)………………………………….…….1

Diodes

IR Phototransistor………..1

1N914………………………….1

Inductor/choke

4.7 mH (molded choke)…………….1

Transistor

2N2222…………………………..1

Resistors

   1k………………………1

  470 Ohm…………..1

  2.2k……………..…..1

  820k………………….1

  33k…………………..4

  100k……………………..4

  9.53k……………………..1

  10k ………………….…...5

  39k………………….…….1

  10 Ohm…………….…..1
3.3k............................1

ICs

  LF347 (Quad JFET op-amp)…………………………1

  CD4069 (Hex CMOS inverter)…………….………1

  CD4013 (Dual CMOS D flip-flop)…………………1

  LM386-4 (Audio amp.)………………..……………..1

Potentiometer (trimpot)

10k (Trimmer)…………………………………………….1

Breadboard and wires/connectors


Step 3: Building the Modulator Part 1 (m1)

Ok lets get to the actual building. Here are the schematics for the modulator. I have divided it by parts to ease the build process. I advise you to build it all on one breadboard. Remenber to: Cut capacitor and resistor leads before placing in the board. We need the least amount of interference possible. also, keep the circuit tight and with as short connections as possible. Review the schematic many times before powering to avoid damage--most of the ICs are easily damaged. I have uploaded the schematic but you will have to click on the image and view it as "original file". sorry guys.


Basically, m1 is just the modulator. U2 pins 7,11,13 are grounded and pin 14 goes to VDD as well. U3 pins 4,6,7,8,9,10, and 11 are grounded while pin 14 goes to VDD. U4 pins 4 and 5 are grounded. no VDD for U4, it operates through oscillating signal.


In case needed:

U1 is the LF347
U2 is CD4069
U3 is CD4013
U4 is TC1411
U5 is LM317


Step 4: M2 the Diode Current Regulator

This circuit makes sure the diode does not overload. The deal is the same with the image.

Step 5: M3 the Line Amplification

This is where the ipod will connect to the circuit.

Step 6: The Modulator

The modulator is finished here is what it should look like. The oscilloscope readings pertain to pin 8 on U2 and pin 1 on U3.

Step 7: The Demodulator (d1)/Receiver

So this is the easier part--and the part where the most mistakes could be made. The demodulator simply receives, amplifies and converts the signal for the speaker to play it. This part will aslo be divided for your benefit. Lets get started!

I have also uploaded the schematics.

In case needed:

U1 is LF347 (pin 4 to VDD and ground pin 11)
U2 is CD4069B (Pin 14 to VDD and Ground pins 7,9,11,13)
U3 is CD4013B (Pin 14 to VDD and Ground pins 6,7,8,9,10, and 11)
U4 is LM386 (Pin 6 to VDD and Ground pin 4,2)


This first section is the Bandpass Amplifier

Step 8: D2 the Limiter

make sure you build these by the steps

Step 9: D3 the Integrator

Here is the Schematic for the Integrator and its connections to the rest of the Demodulator.

Step 10: D4 the Audio Output and Bias

Here is the final part. When you finish, this is what the project should look like. But you can't use it yet! there's still the need for linearization. You need to configure the Demodulator to receive the Transmitter. This process can be long.

Step 11: Linearization

So here are the Linearization steps:

1. Remove Ipod (or other audio input) and speaker.
2. Remove LED and Phototransistor.
3. Connect the pins where the positive and negative leads of the LED would be placed together via a 100  ohm resistor
4. Connect this via 10M ohms resistor to the pin where the phototransistor would connect to the 220pf capacitor.
5. Connect the Modulator's Grnd to the Demodulator's Grnd.
6. Power the boards.
7. Configure the 100K pot on the Modulator and the 10k pot on the Demodulator until you hear something.
8. Replace components.

Now you can have fun with your FM IR Transceiver!

Here is a short video of mine working.

The noise in this transmission is actually the speaker. I waited till last minute and had no other choice but to use a broken speaker from a previous lab project--hence the noise and need for an amplification tool (i made a paper megaphone). A new speaker would make the quality many times better.


So there you have it. You may be required to be a member of this website to be able to construct this project with my guide; this was the only way I could upload the schematics from Multisim. Hopefully yours goes as well as mine. Have Fun!