There are plenty of bio-feedback circuits (for example) on the internet for you to build. Just type into your web browser: medical circuits
From there you should be presented with many links that will lead you to a number of projects that should suit you. There, now wasn't that easy?
this one is quite interesting, i build it last year for yearly (mini) project
i made some changes to adapt it, mainly due to light sensor used being different
**broken link removed**
( too simple if you try to do something elaborated )