Arduino source code
I've attached a wiring diagram created in Fritzing but the image may be too small. A larger version and source file can be found in the open source repository above.
Trick or treating, she was somewhat of a local celebrity and as you can see she got some bonus candy from people who wanted to try out the candy detector some more.
Full feature prototype on my desk, pre-packaging in costume:
Special thanks to Lee's Electronic in Vancouver! Awesome support and customer service.