looking for a sensor that activates when hit by a certain note
First thing that comes to mind is a tuner. The one I have has an LED that lights up when the string you're tuning is exactly at the right pitch.
However this is of no use in your situation because it's way too precise and you're stuck to the basic notes. But perhaps modifying such a device could make it work the way you want. There are very cheap ones available on eBay.
http://r.ebay.com/ajjx60