Flash an LED to music

Im trying to flash an LED to music. I have tried to follow this article and calculate values but cannot get it to go. Basically what i think i need is:

• audio in
• through a low pass filter (isolate frquencies <200hz)
• output to a peak detector or Schmidt trigger of some sort
• trigger a pulse monostable (eg 555) for duration.

My trouble is i cannot work out a useful low pass and peak circuit. surely this has been done before!

• Your lab homework instructions tell you exactly how to go about the design of both the filter (pp 1-7) and the peak detector (pp 7-8). Is there some reason that you can't do your own homework? – HikeOnPast Oct 18 '12 at 4:49
• Sorry for the confusion, this isn't my homework, just the first result in google for "flashing an led with low pass filter". this sheet is a very good resource, i have covered this when i was at uni and have worked out the maths and values, but it isn't working. – jme Oct 21 '12 at 2:35

As @DeanB said, the assignment shows exactly how to do what you are asking. It seems you are just stuck on some sort of mental block, it happens to everyone!

For the low-pass circuit:

You are given the following schematic:

And an equation that describes its transfer function, then they even solved for the cutoff frequency for you. Which comes out to be as follows: $$\omega_c = \frac{1}{\sqrt{R_1R_2C_1C_2}}$$ From here it is just some simple math to figure out what values you need for the low pass circuit elements.

For the peak detection circuit:

You are given the following basic circuit:

This circuit is a little bit less intuitive than the low pass filter and I can see how this could be a bit confusing. Conceptually, since you have already put your signal through a low pass filter, the peak detector is looking for quick rises in the input signal. These quick rises correspond to bass hits. If you have a bigger input signal, your output capacitor C1 will charge more fully and your light wil stay on longer.

Your design factor here is essentially just the decay time of the peak detection circuit. Which is determined by: $$\tau_{td} = R_1C_1$$ So if your $$\tau_{td}$$ is too small the light won't stay on very long, and if it is too big you will miss bass hits.

This is where the engineering comes in and it is up to you, as the circuit designer, to play around with the values and create a useable circuit. Best of luck, and let me know if you have any more questions.