by anhingus » Sat Sep 25, 2010 9:19 pm
withe an infrared transmitter/receiver pair feeding a 741 opamp or whatever. shape the input with a schmitt trigger for pulse counting.
as i recall, this rig is called a pulse plethysmometer.
since this works on reflected infrared, sticking a finger between source and receiver does not do well. try mounting the ir pair at 90 degrees.
the infrared beam ends up modulated by the expansion and contraction of blood vessels due to heartbeat.
you could also use a microphone or a small speaker and go by sound or possibly a flex sensor or ultrasound or . . . lots of possibilities.