I'm curious what is the part on right side, down the middle in round shape. The must be a ferrite, not necessary as big as in LW radio (10 cm) but as small as 1 cm installed vertically could bring up sensitivity 1000x times. Higher amplitude of signal at the input of the receiver, would lower requirements on next stage amplification, and consequently lower noise / interference susceptibility.
I'd suggest not to use analogRead at all, after pulses get logic level right after OPA you can use comparator (or build-in atmega comparator) to father process signal by digital input, using input capture or external interrupt feature, in order to determine freq., pulse width, etc.
Look here for some examples and libraries:http://interface.khm.de/index.php/lab/experiments/
The link which you gave has several interesting projects and I bookmarked it.
AnalogRead is a temporary solution to give flexibility in experimenting with different voltage ranges. In fact, I tried both input capture and external interrupt feature already, and they work as expected.
Instead of continuing with the above inductive short range (-40 mm) signal transfer from the belt to the pencil coil I'll try now
the same principle as the belt uses to send signals to the wrist device, radiowaves of 5.3 kHz ("very low frequency radio waves, vlf"). Perhaps I try with the high component (coil?) you mentioned. I have never built anything to receive radiowaves simply, and all tips are welcome.
When searching with the keywords "crystal receiver wiki" there were several astonishingly simple receiver schematics. Even one with a wire as an antenna, one germanium diode and high impedance earphones, nothing else! When the signal from the belt resembles a morse beep, and it is probably very strong (due to the short distance) compared to any other radio signals (?), the circuit to receive something to an adc-converter is probably very simple. A short wire as an antenna, ferrite coil with inductance L, parallel with a capacitor of capacitance C so that f = 1.0 / (2.0 * pi * sqrt(L*C)) = 5.3 kHz? No diodes, no amplifiers, no filters , ... or could it be even simpler than that?
Addition on thursday june 28:
Coils, wires, radio waves of 5.3 kHz make me puzzled. When I put the commercial wrist part (the radio receiver) into a kitchen kettle and the cover on, I was sure to have all the radio waves from the belt blocked completely. The kettle was supposed to be a Faraday's cage, right?
But the receiver inside the kettle happily received the radio waves and showed the correct heartbeat! I had another commercial receiver outside the kettle and both showed the same numbers (+-1) even after exercising. So at least 0.6 mm of metal (aluminium? steel?) did not prevent the waves penetrating. With another thicker walled kettle the same thing. Changing belts and receivers: the same thing. Obviously 5.3 kHz radio waves penetrate deep into the metal (googling: yes, they do, several millimeters!). Thus they go through the cable shield also (?).