wow...thanks everyone!!
sorry for such a late reply, iv been havin trouble with my computer
i think i will end up doing something similar to the link aevans17 gave me, but does anyone know anything about frequency to voltage converters? my teacher thinks this is the best way to do it, to turn the audio frequency into a voltage and then use a comparator. a frequency to voltage converter is just essentially a fm demodulator (possibly using a monostable) anyone have any comments?
you'll probably hear from me again if i hit any problems
and will let you all know how i get on.
thanks for all your help!!
edit: just a question, Noggin, could you please explain what you mean by
'I'd get an AVR from Atmel with an ADC on it.
Take a few samples from the ADC and run a DFT on it.
Determine the frequency its at and compare it to what it should be.
Use PWM to light up 3 led's -> top led is too high, middle is just right, bottom is too low'
an AVR, a DFT and a PWM please? i'm not familiar with the abbreviations, thanks