Check out Electronics Design, Dec 15, 1997 Design Ideas.  W. Stephen Woodward of the University of North Carolina has a complete circuit design for the IR pulse sensor.  It outputs digital pulses as well as an analog voltage that is proportional to the heartrate.  It uses an IR LED, Phototransistor, and several op-amps.

I am thinking of taking that same basic idea with the IR LED and phototransistor and applying it directly with a microcontroller with an analog comparitor.  The basic idea is this:

Phototransistor output goes to the analog comparitor (-) input.
Drive the analog comparitor (+) input from a PWM source.

In firmware, change the PWM which will change the comparator reference voltage, until the comparitor starts giving pulses.
Now count the pulses over a time period to determine the heartrate.

I haven't tried this yet, but it seems like an idea to explore.  I am working with an Atmel AVR, but I know that lots of the PIC controllers have analog comparitors and PWM generators.  PWM can also be done in software.

Good Luck,
Brian



Brian Hammill -  Sr. Software Design Engineer
Network Systems -  Connectivity Products
Exide Electronics (919)878-6058
mailto://hammill@email.exide.com
http://www.exide.com