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transmit analog data through IR - possible ?

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tresca

Member
So I want to do a project (I think its cool) where you can send audio through an IR led. Im sure its possible (omitting quality, distance and what not - just somewhat clear transmission).

But here is my problem. I dont really have any specs on my IR leds nor my photodiodes. I got them to work in the sense that when I control the intensity of the IR diode, I can get some proportional change on my photodiode. Also, I dont have a scope or anything fancy to really see whats going on.

--all I basically did was have a potentiometer across my IR led to change its intensity, and reverse biased my photodidode with resistor and red led in series to detect the change ---

However, I think to do this project, I would need to look at the small signal analysis of both ir led and photodiode (assuming that because they are diodes they are also non linear devices).

Without the tools and much information other than doing small experiments to detect changes, is this project feasible ?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It's feasible if you want to do it - generally such systems used an FM modulated carrier (or two of them) to transfer audio to wireless headphones.

However, because of it's obvious limitations, it was mostly dropped and replaced by RF methods years ago.
 

tresca

Member
IF the both TX and RX are centimeters apart, is modulation required ? If I have as an example, a line out from my computer, with the TX biased slightly before its Vf, would that still work ?
 

tcmtech

Banned
Most Helpful Member
I have heard this actually works for turning an analog audio signal into a PWM data stream of sort ans then reconverting it all in simple analog.

Use a standard class D type audio amplifier IC. Instead of having the outputs run though a Pi filter and speaker just drive the IR LED with it to make a IR transmitter.

On the receiver end you use the IR photo transistor to pick up the PWM signal and then just feed it back through an amplifier and Pi filter circuit to get the audio back out.
 

Sceadwian

Banned
It would work through longer distances over cheap plastic fiber optic cable, but if you're gonna do that just run a wire. If the TX and RX are that close what's the point?
 

tcmtech

Banned
Most Helpful Member
Some days 'just because' is a good enough reason! :D

We all learned to do by doing it! Or at least I did. :)
 

Tesla23

Member
So I want to do a project (I think its cool) where you can send audio through an IR led. Im sure its possible (omitting quality, distance and what not - just somewhat clear transmission).

But here is my problem. I dont really have any specs on my IR leds nor my photodiodes. I got them to work in the sense that when I control the intensity of the IR diode, I can get some proportional change on my photodiode. Also, I dont have a scope or anything fancy to really see whats going on.

--all I basically did was have a potentiometer across my IR led to change its intensity, and reverse biased my photodidode with resistor and red led in series to detect the change ---

However, I think to do this project, I would need to look at the small signal analysis of both ir led and photodiode (assuming that because they are diodes they are also non linear devices).

Without the tools and much information other than doing small experiments to detect changes, is this project feasible ?

You generally need to modulate on some sort of carrier to remove the ambient noise at in your frequency range. For example, the IR your receiver gets increases as you walk past. Here is an example of a reasonable quality audio link using PWM.
 
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Sceadwian

Banned
"Just because" is never a reason. Because I want to see what happens for myself is. It's all in the wording =)
 
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tcmtech

Banned
Most Helpful Member
Nope!
'just because'
:D

It was it was my reason.:p :D:)
 

tresca

Member
Id like to avoid the PWM, if I have to modulate fine, but id like to 'try' and keep true to complete analog. Might be harder, but its something to do, just because ;)
 
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