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I am an undergraduate and new to electronics. I wanted to know if it is possible to send audio over an infrared LED?

I plan to use a 555 timer to generate PWM output with carrier at around 44 Khz to satisfy the Nyquist rate. On the receiver side I would use a one or two stage active low pass filter to get the output. Is it feasible? What is the maximum range I can get?

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Make sure you have an IR filter over the receiver – user32885 Aug 15 '14 at 14:25
I am not intimately familiar with the IR-protocol, like with television remote controls. Is it the time of the on/off pulses that equate to binary values? – sherrellbc Aug 15 '14 at 15:50

That was - early in the history of LEDs - pretty common, even without the PWM stage. Depending upon the angle of the IRLED, you can get a VERY long distance out of one... but aiming gets rapidly more difficult with longer distances. A very carefully aimed IRLED with a very narrow beam can transmit audio reliably over several miles in clear weather.

The greater the distance, though, the greater the likelihood of incidental noise from reflection, refraction, and air movement.

The old Popular Electronics magazine published several articles long ago about transmitting unmodulated audio with IRLEDs. Back then, LED beam angles were nearly all pretty broad, so theirs required focusing/collimating lenses. Today, you can easily find IR laser LEDs so there's little need for collimating lenses except arguably at the photodetector end.

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Some years ago, I built a very simple AM transmitter and receiver using a red laser pointer and CdS photodetector. The practical range for that was around 100 meters at night. It probably could have worked at a longer range, but aiming accurately was quite difficult even with a tripod and lens on the transmitting side. The receiver had no optical components.

You might be intrigued by Alexander Bell's photophone invention which transmitted audio via modulated sunlight.

Your basic idea is sound, but if you're new to electronics, you might find that it's a bit easier to design and build a passive low pass filter, followed by an amplifier.

Also, if you plan to use this indoors, you might wish to consider optical filters to eliminate interference from fluorescent lighting. For your proposed system, the key to maximizing range will be the precision of aiming and the optics used. I'd recommend doing some experiments first with a simple system using, say, just a sine wave driving an IR LED or laser and the receiver at some distance away.

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If you want accuracy and repeatability, then PWM is not my favourite way to do it (due to the emitter and receiver asymmetrical rise and fall times and their tendency to change with temperature). I would use a voltage to frequency converter to drive the LED and a frequency to voltage converter to demodulate the receiver.

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I would expect PWM to have a fairly significant range in which it would behave linearly. A 50% ratio may be nowhere near the midpoint of a 5% ratio and a 95% ratio, but it should be pretty near the midpoint of 25% and 75% ratios, and very near the midpoint of 33% and 67%. – supercat Aug 15 '14 at 17:02

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