I built one during second year of B.SC. Electrical Engineering, as a student project. But I remember that I just used a 555 IC to generate a timing pulse and an OP-AMP to amplify and then detect the peak. The resulting pulse was passed on to a simple pulse counter for the duration defined by the 555. I used to take a sample of 5sec and then using an EPROM I would multiply it with 12 to get the beats per minute.
I just wanted to give you one possible approach which I implimented.

Regards,