I'm planning how I will make a MIDI heartbeat sensor, probably using my arduino.
So far I've only been gathering resources!
- Recotana made one and has a flickr set and schematic, uses infrared emitter and detector: http://www.flickr.com/photos/recotana/sets/72157603962786453/
- Discussion on Arduino.cc: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1221941583
- A schematic using a PIC microcontroller (also photo pickup): http://web-ee.com/schematics/medical/heartbeat-monitor/
- Another great example: http://dmd08.wordpress.com/2008/05/28/heartbeat-ir-sensor/