Robot dancing to music - low-pass filter

Not sure if any of you have seen the unfortunately named spazzi robots - simple solonoid-based robots. I was thinking of making something similar for my wife's birthday, and thought it would be fun if I could make it 'dance' to music.

Since it would be driven by the bassline of the music, I'm thinking I'd need to build a low-pass filter (my electronics is a bit rusty), outputting a digital signal - a high pulse for each beat.

Any thoughts/suggestions please? Is this feasible, or does it not tend to work in pratice? Any ideas on a basic circuit?