I built a circuit similar to this one here and I can hear my breathing but not my heartbeat.
Now I used this piezo from digi, and I used a FDV301n fet instead of their JFET. Eventually I removed the emitter follower and just put the capacitively coupled output of the piezo across the 1M right into the LM386.
So the thing is if I tap the piezo it comes out loud over the speaker, and then if I hold it to my chest and hold the speaker to my ear I can hear myself breathing lightly but can't hear my heart beat.
Ignoring the possibility that I'm some kind of undead monster I figure I must be missing one of two things. One I don't have nearly enough gain, or more likely I did not pick the right piezo element.
Could someone help me understand if I've picked a bad element and if so what a good one to use is and what the criteria I'd look for in a datasheet would be? I don't have a lot of experience using piezos like this. Or maybe I just need to pump up the gain.
Unless someone sees something else that's wrong here. My end goal is build a circuit that can record a clear heart beat that I can play back later.