Step 6: How does that work? And where do I go from here?

This circuit works using amplitude modulation, exactly like AM radio, except using a visible light wavelength instead of a radio frequency. The audio signal leaves the iPod as a varying voltage which forces a varying current through the laser. Then the laser's varying brightness conveys the musical information. Finaly, the phototransistor varies in resistance as the brightness on it changes. The mic amp applies a small voltage to the phototransistor and amplifies the resulting current.

A problem with this system is that at each step there is a non-linear transfer function, that is, there is distortion that occurs because the brightness changes are not always proportional to the change in voltage applied. See the screenshot below for an example, and listen to the attached audio sample.

The next step in this project would be to use pulses (like fast, computer operated morse code) to convey digital information like text, crystal-clear audio, or even video. One could even network computers with laser beams in a manner similar to fiberoptics but in open air. I will post C code for my transmitting and recieving programs.
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<p>plzz give me audio transformer ratings....</p>
how to make a laser cutting gun
Why do i need audio transformer (i mean, how it works and what is the point of it) and where can i buy/find one (pls paste a link if you know a store or if you found it on ebay) tnx ahead, please reply quickly ;)
Why? Because it allows the input audio signal to modulate the laser power supply while also electrically isolating the two circuits. -- Basically, it transfers the signal from the stereo to the laser&nbsp;<em>magnetically</em>, without actually making an&nbsp;<em>electrical</em>&nbsp;connection between the circuits.<br> <br> This one featured at the link here will probably work:&nbsp;<br> <a href="http://www.mouser.com/ProductDetail/Xicon/42TU120-RC/?qs=sGAEpiMZZMv0IfuNuy2LUU2nVj3GMLUawQykJuAOLGA%3d" rel="nofollow">http://www.mouser.com/ProductDetail/Xicon/42TU120-RC/?qs=sGAEpiMZZMv0IfuNuy2LUU2nVj3GMLUawQykJuAOLGA%3d</a><br>
Can you please tel me the Voltage &amp; Current values required for the Audio Tramsformer ???? <br>And Link OR a Site to buy a suitable Audio Transformer??? <br> Hope quickly response as soon as possible....
please reply immediately!!!!!! <br>
Ok, so my projects very similair only it doesn't work. I think my problems with the wiring and I don't know how to fix it. <br> <br>I've got the laser wired up to the batteries and that turns on. If I test for sound now (even though no sound is hooked up) I'll hear a click noise every time I turnt the laser on and off. <br> <br>Now when I wire everything up with the transformer all I can get to come out is the click. No other audio. <br> <br>For the reciever I'm using a .5v 6watt solar cell. <br> <br>If anyone could help I'd be thankful. I can also upload pictures of the project if that would help.
Your problem might be the photocell. I recommend trying a photodiode or phototransistor, because the photodiode is more sensitive than the photocell. <br> <br>Another way to think about it is that the the photo cell is 8 W but your laser is in the milliwatts so it's your laser is way less than one watt to the signal from the laser is going to be very weak in comparison to the total signal from the photocell <br> <br>Sorry if this comment doesn't make any sense I'm typing it with Siri
Ok, I think I understand, so the voltages don't matter for this case? I know the laser is being ran on 4.5 volts and the photocell is only .5 volts. I got the idea to use a photocell from kipkay (youtube for makemagazine). <br> <br>I'm really thinking that my problem is the wiring on the audio transformer.
I just looked at the package again and it says that the power output is .5w and typical output voltage is 6v so I think the front of the package had the numbers mixed up.
Your transformer wiring could be an issue. Try changing it, and check the schematics from me or some other posters. You can also use headphones on the output to see if it's working. By this I mean you can hook up headphones directly to the transformer output pins with the laser and battery disconnected.<br><br>Cheers,<br> Chris
Thanks I just did that and the transformers working. I went from there and hooked the batteries back up and the laser and I vaintly heard the music! Then the batteries died =( I'll hook it back up once they're charged and thanks for your help! It's always the little things we overlook.
Edit: I tried again after charging the batteries and no luck. It doesn't work now. Could have to do with laser color?
Hey! I'm glad you got something, even of it was faint.<br><br>Are you using a mic input on the receiver side? A solar cell is self-powered, so it may not work well in a PC mic jack, which is expecting a varying resistance. Honestly I just don't know -- ive never tried it. <br><br>What you can do is, build or use a preamp after the solar cell but before the main amp. Be sure to use the &quot;line in&quot; jack on the main amp, not a mic jack.<br><br>If you want more help, I suggest you draw a schematic of your setup, and/or use pictures.
I figured it out. The solar cell I'm using was to big of an area for the laser. The dot from the laser wasn't big enough, luckily I was using a laser that also had a led light. The more area of light shown on the cell produced the sound, it's working quite well, I'll probably make something more about mine sometime. <br> <br>Again thanks for your help.
Ok I figured out!! The solar cell I have has a larger area so I think that means it needs more light hitting a wider area. The laser I'm using also has a flashlight and i turned it on and it started working! Thank you again for your help hope you have a good night!!
I assume you're using the transformer in that manner to &quot;step down&quot; the level of the input signal as well as modulate the DC going to the diode. Is that correct?
Is the audio transformer necessary? I forgot to buy one and don't want to go back out to radioshack to get one.
YA CAKE! btw the project is awesome
can you replace the laser with an led
That would be an Optoisolator...at least by the time you actually got the led close enough to eliminate noise and get an effective signal...but basically an optoisolator uses an LED coupled with a phototransisitor in order to electrically isolate the control circuit from the circuit being controlled.
i would seriously not recommend doing that will it work? yes is it effective? absolutely not an led is not a focused beam, cannot be used over long distances like a laser can, and it is more prone towards the distortion as the light is more spread out. The laser points light precisely at the photo diode, so that is the light which is converted to audio. Theoretically, using an led would require total darkness so as to eliminate that interference. Give it a shot and tell how it went!
Could you just hook a laser diode to the audio output and a reciever to a microphone input? IT seems like it would make sense.
Short answer, no.<br><br>This is mainly because laser diodes only conduct electricity in one direction - that is, they work with Direct Current (DC). However, (most) audio outputs have alternating directions of current (AC). So when you hook up a laser diode to the audio output, you only get half of the waveform.<br><br>Furthermore, laser diodes have a minimum voltage required to turn them on. So you will actually get less than half of your waveform if you attach the laser diode directly to the output.<br><br>That is why there is a battery pack involved: the batteries provide the direct current bias needed to keep the laser on. The audio output (via the transformer) subtly changes the strength of that direct current, and is therefore manifest in the laser's output brightness.<br><br>Hope that helps.
what is a duh? is it an iPod knockoff?
No. It is an IPod. It means that you need music. Your'e a duh! =P
actually: would ripping apart a wireless cheapy headphone not also work, and then replacing the leds with a laser? or is this a bad idea?
No the LEDs are in the transmitter, and they have a wide angle so your headset can receive from anywhere in the room. A laser transmitter has to point straight toward its receiver, in this case William Tell style
Could you please specify the type of audio transformer that you used? thanks..
would it be possible to make a two way radio sort of thing if you made two of these??<br />
&nbsp;Is the potentiometer absolutely necessary? &nbsp;I dont have one on hand so I want to know if I can use just a resistor. &nbsp;Can I use one of those photocells instead of the phototransistor?
Short answer:&nbsp;YES.<br /> <br /> Long answer:&nbsp;if you knew exactly what resistance(s)&nbsp;to use, you could substitute the potentiometer(s) for fixed resistor(s). However, these resistances depend on your laser pointer and the voltage of your battery pack, so it is really convenient to use the pots. (They are inexpensive too.... and useful in other projects, so I'd recommend buying/salvaging some.)<br />
&nbsp;What is the potentiometer for? &nbsp;I got it working without a pot or a resistor. &nbsp;Does the pot filter out static and background noise?
Lasers often have a non-linear response. That is, their brightness depends on the power-supply voltage in a complicated way. The potentiometer(s) along with a control of the input audio amplitude (mp3 player volume)&nbsp;allow you to select which section of the psv-brightness curve to use. So, you should be able to select a linear portion, and get great audio quality. Using a non-linear portion will result in nasty sounding distortion.<br /> <br /> Also, I realize now I&nbsp;didn't address your second queston &quot;can you use a photocell instead of a phototransistor&quot;. I&nbsp;assume you mean either a Cadmium-Sulfide passive cell or an active solar panel (generates electricity from light). The issue with the photo-cells are that they (typically)&nbsp;have a slower responce-times as compaired to a phototransistor or photodiode. The result is that a photo-cell-based circuit does not pick up on the higher frequencies as well, if at all. It might sound like music through a wall (only bass).<br /> <br /> But, you can always try origional designs! Do let me know how you get along with this!<br /> <br /> Chris<br />
&nbsp;Also how would I hook this up directly to a speaker instead of having to put it in a mic port?
nice project <br />
hey... i don't seem to be able to get any music across... is there any way of testing if the transmitter works???? and i'm using a 10k ohm potentiometer, could that be a reason???
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?
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
Can you post some pics of the sensor lab?
My laser uses 3 button cell batteries (4.5 volts) so can i replace the 3 volts in the diagram with 4.5 volts to give the laser a reasonable brightness?
Yes. Although remember that you want to maximize your signal to noise ratio (SNR), and that the raw brightness is not the signal strength. The signal strength is roughly proportional to the range of brightnesses the laser exhibits during operation. For a digital signal where the laser goes full-on and then full-off, the raw brightness is the brightness range. However, in an analogue scheme such as this one, one must closely consider the modulation to determine what parameters will maximize signal strength. Put briefly, a dimmer laser beam may result in better transmission because the laser is more sensitive in that brightness region (it can get dimmer and brighter from that mid-point). Hope that helps, Chris
Yeah i think it is actually better to use <em>less</em> voltage than the laser takes for normal operation (not in audio circuit) because it will be receiving extra voltage from the ipod which which would create a voltage greater than what the laser is build for. Since we are talking about lasers, overvoltage could damage the laser.<br/>
Yes, overvoltaging the laser could also be a concern! If you burn out a laser let me know! (I think it is harder than you might think)
Can you help me to understand how i should wire up my LM386 amplifier IC? Here is the schematic of the IC:
UbuntuNinja,<br/> I assume you wish to use the LM386 as part of an audio amplifier on the recieving end? I would recommend building an audio amp with it along the lines of MAKE's Crackerbox Amp: <a rel="nofollow" href="http://makezine.com/09/crackerboxamp/">http://makezine.com/09/crackerboxamp/</a><br/><br/>You will need to then use a battery in series with the phototransitor (unless you use a small solar cell, which provides its own power), and hook this up to the audio amp.<br/><br/>Viel Spass!<br/> Chris<br/>
hope this one works!!
That schematic is very helpful. I like how you correctly represented the different ways to wire the transformer. I was actually having difficulty figuring out which side of the transformer was supposed to be wired to what looking at the authors diagram.
Here is the schematic for the circuit without the black background, for easy and efficient printing: <a rel="nofollow" href="http://www.instructables.com/file/FYUF1DKFVW22CNV/">http://www.instructables.com/file/FYUF1DKFVW22CNV/</a><br/>Here is that same file in Inkscape SVG (scalable vector graphics) format:<br/><a rel="nofollow" href="http://www.instructables.com/file/FW4ZN91FVW22CNW/">http://www.instructables.com/file/FW4ZN91FVW22CNW/</a><br/>

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Bio: I am a student and hobbiest. I'm into physics, linguistics, sailing, electronics, hacking (not cracking), music, and the like. I post most of my ... More »
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