EPE Project: Stethocope - More Gain

Sep 10, 2015 5 Replies
M

This is the diagram of a stethoscope published in EPE magazine May



2013.

For a transducer, it uses a disk from a cheap piezo "buzzer" mounted in a diaphram type stethoscope head.



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I ordered the PCB from EPE and built it up according to the instructions. But the gain appears to be insufficient, even with all the trimmers cranked up.



When the piezo disk is tapped, there is a sound. But no heartbeat when applied to the chest.



I have already tried piezo's from two different suppliers.



Can anyone please tell me how to make this work?



Mike Towner


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C

Some thoughts:

The piezo buzzer must be the kind with no electronics, like you might get from a birthday card (cur the wires off at the chip). Did you use the existing wiring? It's quite hard to solder to these wafers without causing damage.

If you fitted LK1 (to the 1K electret-bis resister) then remove it. A piezo has a very high impedance and that 1K will kill the response.

The wafer produces a signal when it flexes, so support it around the rim and allow the movement to apply to the center.

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T

Don't try it on a vampire?

www.wescottdesign.com
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T

What's the impedance of a 15nF transducer at 1Hz? They're loading it with 1M.

NT

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B

Z = 1/ 2.pi.f. C = 10.6 Megohm

Probably not a great idea.

Bill Sloman, Sydney
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G

Hmm I guess what I would do is apply some other known signal to the input. (do you have a function generator or some such?) And then look at the signal along the chain, (Do you have a 'scope?) and make sure all the stages were working properly. It's your job to figure out what the signals "should be".

George H.

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