>I saw an interesting circuit in one of these Circuits Encyclopedias. It
>consisted of an IR detector/transmitter pair set an angle. Human skin
>reflects IR and the presence (or not) of blood enhances it to some degree. I
>think it's the pressure causing the skin to move closer to the IR LEDs.

is this not what they do with the clip they put on a finger? I understood they
shone the IR through the finger, and the blood was opaque enough that it
modulated the light beam as the heart pumped.