It was quite a surprise that small schematics with LEDs and atmega would attract such a huge noise. Initially there was a simple transistor on the PCB that opens for a short time when each heart beat is detected. But when we attached this contraption, noise became overwhelming.

To some extent we decreased it using optocoupler on the output instead of transistor. The result is far from perfect - but at least it more or less detects heart beats.

My hand is really needed there: it blocks significant part of mains noise and thus makes everything much more reliable. But - that video was shot before we developed our new filtering algorithm! Soon we'll see how it works with it and with proper input buffer on the new PCB :)