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.