Hello! I'm new to the forums, currently in College with several electronics classes. I'm currently working on a project in my free time which I'd give to gain some insight from those smarter than I. The purpose of the project is to construct a free space communication device via modulated laser diode. The first 'milestone' goal is to use modulate a song over the beam to a photo-detector to speaker. The following is my current progress.

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Step #1 : Power the laser - The first purpose of this circuit was to provide constant current to a laser diode. I achieved this using the 50kpot as the input to U1A-pin3. By adjusting the pot I was able to limit the current flowing through the led regardless of voltage changes at the VR1-collector.

Circuit [source] | LM324n (U1A) [datasheet] | Tip122 [datasheet]​

Step #2 : PWM Test - Moving forward I replaced U1A's input with a [myDaq] PWM output signal. The output signal was generated from a 8bit .wav file. I replaced the led with a small speaker and was successfully able to play an 8bit mono wav file through the circuit and listen to it via the speaker. I've attached the LabView file.

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Step #3 : PWM Modulate The Laser - I'm currently having issues providing enough voltage to power the laser. The signal coming from the DAQ is 1v, is it possible to somehow bias the laser to run at low intensity and then input the same signal resulting in a laser powered 100% of the time that vary's intensity across a photo-detector?


Any help / thoughts / suggestions / comments etc would be greatly appreciated and welcome! I'm also willing to hear suggestions on more efficient ways to accomplish this task.




Step #4 : Capture laser data - coming soon
Step #5 : Play Song - coming soon

 

1. Is this a homework or personal hobby to do something? (if this is a homework then it should be move to where it belongs)
2. You have to get used to put the ground at the lower position as up is positive and low is negative.
3. Does the led driver circuit was designed by yourself?

 

Hello ScottWang,

1. It isn't homework it's a personal hobby / project though it is related to my field of study.
2. Will do!
3. No I didn't and I've provided the link to the source in the write up!

Thanks

 

I was tried to shared a circuit to other forum in Taiwan, so I wrote him a Email before, he is very famous in youtube with ee videos .

I can't find your circuit in his video, and I saw his constant current is quite normal, but your has a little strange, are you sure that the circuit was you duplicated from that video?

The LASER diode driver.

 

I'm not using the circuit from the link you provided, I'm using the circuit from the link I referenced. At this time point he's discussing the advantages of this circuit over using LM317 which I commonly see used for laser-drivers & here he discusses using the circuit with PWM.

 

If your application maybe doesn't needs a constant current, because whatever you using pwm or sound to send, the current will not the same, so what you want is to limiting the max current to protect the lazer diode and the circuit was showed in the video.