How to receive and reassemble the audio signal ?

[tonytrackstar83]

led audio link

Hi everyone.
I'm trying to build an infraded audio link that can transmit an audio signal 3m. Basically I'm using a PWM setup to transmit, but I'm having trouble figuring out how to receive and reassemble the audio signal. Ideas??

 

[Jack// ani]

infraded audio link

why are you modulation it in PWM?? try to modulate the IR led direclty by you audio signal....it will make things easy, and demodulation on the reciever side as well!!

good luck

 

[Buriedcode]

Re: infraded audio link

Hi,

PWM is fairly complicated, although, with a 555 timer (NE555?) you could use FM. just set it up to oscillate at, say, 200Khz, then connect your audio input to pin 5. This will pull the frequency off a bit.

AM (amplitude modulation) is probably the easiest. But...if you directly modulate an IR led with audio, and demodulate it at the reciever, you'll pick up the 50/60Hz hum of any mains lights near the Rx. So, with AM, I'd say its best to modulate a carrier, of say, 100-500Khz. This is easily done, with a voltage controlled amplifier and an oscillator (again, 555 timer ).

Receiving is fairly straight forward. The only thing that makes it more complicated than direct modulation is a bandpass filter. This will attenuate frequencies above and below your carrier, but most importantly, get rid of the mains hum form lights. Then just run this into a lowpass filter to extract the audio (eliminating the carrier). I've done this myself, with basic circuits (555 timers, caps, opamps) and the sound quality is surprisingly good. Also, because you're using a carrier, you can amplify the signal at the reciever a great deal (because we're only 'looking' at our carrier frequency) increasing range...

Good Luck,

BuriedCode.

 

[electronicus]

Re: infraded audio link

http://www.electronic-projects.net/Electronic-Projects/projects/remote_light/index.shtml

 

[PaulHolland]

Re: infraded audio link

Hi, Use FM modulation using a 555 IC at say 200 KHz as suggested and use an LM567 (PLL) to demodulate the signal. works perfect and cheap.

regards,

Paul.