Raghu,

I,m not thinking in a heartbeat monitor, I was think in a digital holter. But since I don't know why there are 4 sensors and how it's outputs are combined to generate the cardiogram graphic I'm stuck.

A friend of mine has a brother that is a cardiologist, so he'll ask him for clarifications. But since he's a doctor I don't know if he has the technical knowledge that I need. But if he can indicate the sources that will be ok.

I could find only a few facts:

The sensor isn't a piezoelectric, it doesn't sense the beat, it senses the electrical stimulus of the heart

The stimulus has a peak to peak amplitude around 1mv (I assume that for a "normal" heart in "normal" situations)

The time scale in the graph paper uses 0.2s per division

but nothing too accurate... but a bit of information here, a bit there and soon I'll have the DigiHeart 2000 :-)