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I built a circuit similar to this one here and I can hear my breathing but not my heartbeat.

enter image description here

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.

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    \$\begingroup\$ If you are hearing breathing but not the heartbeat, you might need to low-pass filter the signal from the piezo. The piezo itself looks like it should work fine. I just hooked a similar, no-name piezo up to check, and with a low-pass of 100 Hz, it's working great. \$\endgroup\$ – Anindo Ghosh Dec 24 '13 at 6:19
  • \$\begingroup\$ ditto squared what anindo says. \$\endgroup\$ – Andy aka Dec 24 '13 at 10:51
  • \$\begingroup\$ @AnindoGhosh Also, what about physical construction: how about mounting the transducer into the throat of a small flared horn. Real stethoscopes have several inches of pickup area. \$\endgroup\$ – Kaz Dec 24 '13 at 17:18
  • \$\begingroup\$ @Kaz :-) I wasn't trying to make a real stethoscope, just checking how to eliminate breathing sounds and capture heartbeats! The piezos I have are 35mm diameter which seems to suffice to check that I'm not undead ;-) \$\endgroup\$ – Anindo Ghosh Dec 24 '13 at 17:49
  • \$\begingroup\$ Still nothing I tried putting a 150Hz low pass filter in. Then I tried increasing the output capacitance to 400uF. Finally I increased the gain of the 386 to 200. It must be the element, I don't see any heart beat like signal on my scope when I measure just the element across the 1Mohm either. \$\endgroup\$ – confused Dec 25 '13 at 18:21
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You clearly do not have enough gain.

In first attempt, you used MOSFET with V_gs(th) of 0.85 volt. This means the FET will not open at all as long as voltage is less than 0.85 volt.

In your second attempt, you have connected high-impedance piezo output to input LM386, which has input resistance of 50K. This put way too much load on the piezo -- they like the loads of 100K to 10M.

I advice you either get a real jfet (mot a mostfet!), or put an op-amp such as TL081/741 in front of your audio amp.

And I recommend to add two diode protection circuit -- see http://www.openmusiclabs.com/learning/sensors/piezos/ . Otherwise, if you drop the device on the floor, piezo might generate so much voltage it will burn out the amplifier input.

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