Step 3: Create Transmitter Circuitry

Picture of Create Transmitter Circuitry
Use the schematic below as a guide in soldering togeather the transmitter circuit. Everything to the left of the laser is the transmitter circuit.

Tape or glue the compents down to a piece of cardboard, or use a breadboard.
Check out the photo of my finished board. I used an 1/8th inch female jack to make connections easier.

To test the circuit, turn up the iPod volume to MAX, play some music with alot of bass, and turn the potentiometer all the way down. The laser dot should appear to pulsate with the music, since this is an amplitude modulated (AM) circuit.
NGinuity3 years ago
I assume you're using the transformer in that manner to "step down" the level of the input signal as well as modulate the DC going to the diode. Is that correct?
ep03174 years ago
Could you please specify the type of audio transformer that you used? thanks..
Firecrow6 years ago
when i point my laser beam at the reciever i get no sound.. Does this mean that my laser beam is not good enough? and if so could i just connect it to a infrared led?
navaburo (author)  Firecrow6 years ago
Firecrow, You can test the function of your receiver by holding the laser close to the receiver and quickly passing the laser beam across the photodetector. You should here a low-pitched WompWompWomp. If you are using the mic port on a computer to receive and amplify the signal, you can use a recording program (like Audacity) to see if the laser is being detected. If you hear/see the signal from flicking the beam across the detector, then you know the detector works, and you have to work on the transmitter. Best of luck and let me know how it goes! - Chris
brian1088326 years ago
sooo. i am completely new to this... i have only the basic understanding of circuitry so please excuse my question but. on the schematic it shows the output from the mp3 player with 3 lines. i understand the left and right but what is the ground?
navaburo (author)  brian1088326 years ago
There are several ways to answer that question, I will give two of them:

1) The academic answer: Voltages are only defined between TWO points. A single point only has a voltage insofar as a reference (ground) has been established. Any device "communicating" with the mp3 player needs to have a reference to make sense of the left and right signals.

2) The simpler but not totally correct answer: The signal needs to "return" to the source to complete the circuit. The ground allows for this completion.


Electronics is a great hobby to get into. I would recommend getting yourself a copy of the classic intro "Getting Started in Electronics".
chewie50006 years ago
does any one recommend a particular phototransistor.
navaburo (author)  chewie50006 years ago
I have used the IR one that comes with the RadioShack matched set (http://www.radioshack.com/product/index.jsp?productId=2049723) and it works fine for red. - That one i a good start, and you can use the transmitter for other projects too. If you want to work with a color other than red or IR however, you should find a different reciever (check its spectral responce before buying to make sure that it will respond to the peak frequency of your xmitter).

Best of luck!
DanDMan1116 years ago
i havn't made this yet but I'm pretty certain that if you have a y-splitter you dont need to make the comp. receiver.... but i'm not positive.
navaburo (author) 7 years ago
Here is an improved schematic:<br/><br/><a rel="nofollow" href="http://personal.stevens.edu/~cmerck/downloads/music_laser.png">http://personal.stevens.edu/~cmerck/downloads/music_laser.png</a><br/><br/>also the source file: <a rel="nofollow" href="http://personal.stevens.edu/~cmerck/downloads/music_laser.sch">http://personal.stevens.edu/~cmerck/downloads/music_laser.sch</a><br/><br/>(due to some glitch I cannot post images)<br/>
hrf34208 years ago
hmm.. I think the same method could be used to listen in on people's conversations in other buildings. But instead you would only need to bounce a constant laser beam off a windows and back to the phototransistor.