PWM Comms LASER Transmitter

Hi All,

I have been working on this project for a few months, it basically a Pulse width modulation LASER comms system constructed around a triangle wave oscillator, microphone input w/ amplification, comparator and LASER.

I've got the oscillator and mic and comparator running fine, can't seem to get the LASER to turn on by driving it directly from the output of the comparator. The comparator output is 3V which is the correct operating voltage (2 x AA batteries - which I've removed and instead tapped wiring directly into the LASER). The current drive from the comparator is however far too small I've discovered. I'm getting around 10-20mA when under ideal conditions the LASER can draw up to 140mA before its current limiting kicks in.

What then is the best method of configuring the driver circuit for the LASER. I've read that the comparator output can drive a MOSFET switch which connects/disconnects the power supply from the LASER. If this is the case I'm unsure of how to implement it with correct voltage and current (stepping down from a 9V battery). Would I use a fixed voltage reference and a constant current source of around 100mA?

Here's a circuit diagram - if you need any more information let me know.

Thanks,
Rueffy.

If you're trying to power a laser with a driver already built in, it may not work well with this circuit due to capacitance in the driver that will attempt to smooth out the signal you are trying to transmit.

The best laser for use with this circuit is a pure diode laser, DPSS lasers are unstable and take a small amount of time to turn on, enough to disrupt the operation of this circuit.

The way I would proceed would be to use a plain NPN transistor to drive the laser diode directly, you would use a resistor inline with the laser diode to limit current to a safe level for the laser diode, then connect the transistor base to the output of the modulation circuit with a series resistor to limit current but still ensure full saturation of the transistor (you will need to calculate that from the transistor gain and drive current of the LM311

Thanks for your help, I've ordered in a simple LASER diode assembly that can be configured how I want it.

MarioMaster said:

The way I would proceed would be to use a plain NPN transistor to drive the laser diode directly, you would use a resistor inline with the laser diode to limit current to a safe level for the laser diode, then connect the transistor base to the output of the modulation circuit with a series resistor to limit current but still ensure full saturation of the transistor (you will need to calculate that from the transistor gain and drive current of the LM311

Click to expand...

As the output of the comparator is really a large voltage with a fairly low current, would a MOSFET perhaps be a more suitable choice instead of an NPN? Or would either be sufficent?

Thanks again.

At the frequencies you're running it at, either will work well. You'll need a resistor in series with the
base/gate in either case, but with a MOSFET it should be a lower value. For a bipolar, you would select a
resistor that just brings the transistor into saturation. For a MOSFET, use one that will almost max out
your op-amp, but not less than 10Ω.

Thanks guys, all is set for when my LASER diode arrives now, I've jacked up the sampling frequency to 49kHz so I should be able to get some nice sounding audio at the receiver hopefully. I made a video of the waveforms in action.

PWM (Pulse Width Modulation) of Music - YouTube

Neat project, and it's good to see new members doing stuff like this.

Something else to try for experimentation: how does the audio quality compare with directly outputting the analog signal to the laser rather than modulating it with PWM?

Another thing you could try is using pulse code modulation, like they do in fiber optics audio. You may need a special transmitter/receiver pair though.

Make sure the voltage for the laser diode is nice and stable. Any overshoot will shorten its life or kill it
outright. And make sure NONE of the resistors are wirewound!

Thanks guys,

Making great progress now, I've got the audio system up and running - sound is being reproduced from PWM with surprising good audio quality. This is however without the use of the LASER yet as it hasn't arrived in the post, but I don't expect any degradation in quality as I've already tested the system can receive LASER from my original LASER pointer and it worked fine.

Two things I am having issues with however,

1) Focusing my LASER beam at the receiver end, i.e. it is very difficult to target a photodiode with a laser beam and any significant distance, I need a lens of some sort that gives me a larger target area, I tried using a solar cigarette lighter which is basically a parabolic reflector that you point at the sun, it focuses the light onto the cigarette and lights it, but it sent the light off in all directions when I tried it with a LASER. Anyone know a solution to this problem?

2) The current draw when I plug in the speaker goes from 47mA to 400mA.. The system works fine when powered from the computer and also when powered from 9V batteries, but I don't expect much life drawing 400mA from 9V batteries, infact I was suprised it can handle that current at all. I'm using the receiver circuit provided here,

http://modulatedlight.org/optical_comms/LASER_COMM_simple.gif

The only modifications are a more advanced photodiode circuit (which introduces another comparator and a threshold to eliminate ambient light) and a small 0.25W 8ohm speaker instead of the connector jack.

Cheers,
Rueffy.

If your mount is stable enough, plain old divergence should take care of the rest. It's also easier to set up
the laser first and then move the target into the beam. Worse comes to worse, you could use an x-y
mount.

Thank you to all those that helped contribute to the development of my circuit. The project is now fully functional and I've made a video as part of my presentation to a class, the video shows the system in operation with a full description of signal flow etc.

http://www.youtube.com/watch?v=CCAkpJ-jG6A

rueffy.

i have an old heathkit hene laser tx-rx setup. thanks for explaining just how it works. good job on your project!

It looks really nice. I can't seem to end up with 49kHz with that triangle integrator formula there though no matter how I slice it. You have R1 R2 and R3 in there, but in the schema it is R3, R4, and R5. But I tried different combinations of the 3 values as drawn and always ended up with either 75kHz or 5kHz, or some other value which didn't make sense.

The Lightning Stalker said:

It looks really nice. I can't seem to end up with 49kHz with that triangle integrator formula there though no matter how I slice it. You have R1 R2 and R3 in there, but in the schema it is R3, R4, and R5. But I tried different combinations of the 3 values as drawn and always ended up with either 75kHz or 5kHz, or some other value which didn't make sense.

Click to expand...

Yes your calculating correctly, using my resistor/capacitor values it will come out at 75k. I'm not 100% certain why this is the case, but I think it has something to do with the op amps - I tried a range of different op amps using the same resistor/capacitor combinations and the frequency would vary, even swapping in one TL071 for another would vary the frequency. I started using LM741's but the slew rate was terrible, it produced something closer to a sinusoidal wave. However that formula is supposedly correct, I have the handout still, it was given to me by my oscillator teacher.