Heartbeat Monitor (HeMon)
Here is a circuit from Keith Wilson in Germany for monitoring your heart. The text that follows is his:
The heartbeat monitor was originally designed many years ago....
One day whilst walking through the town centre in Manchester (probably on the way to Maplin!), I came upon a heartbeat measuring machine - here, you had to place a finger into a "thimble"-like sensor, and hey presto, your hearbeat was measured and shown on a large LED display! Back then, I only had a very vague idea as to how something like that actually worked, but after looking in magazines, and tinkering around, I came up with a small circuit.
For HeMon, the original circuit has been preserved, the PIC micrcontroller and LED display were added several years later.
Heartbeat Monitor (HeMon) Schematic