During the first year of study in Metropolia UAS, we had to make a so-called Introductory Project in Electronics. No one in our team did have any experience about practical application of amplifiers. Still, we managed to assemble and test the Heartbeat indicator. My task was to manufacture the printed circuit board. Later, for demonstration purposes, same PCB was made portable with alkaline battery pack. Example circuit has been taken from Embedded Lab:

Signal amplifier for the heart rate measurements

After two amplification stages, heartbeat signal should have been feed into microcontroller or at least some logical chain in order to calculate heart rate. But, at that time, field was so complex to us that we have decided to take on automated calculations later.