[SOLVED] Electret Microphone Help (MCE-200)

[kenlfg]

Hi guys I've a problem with my MCE-200 electret microphone. I'm not sure what values am I supposed to use to drive the microphone. The IC I'm using is the MCP605(can't be changed) and a 2.5 supply voltage(can't be changed also). My circuit consists of an electret microphone -> a 2nd order butterworth HPF-> an amplifier. I'm trying to construct a circuit to pick up sounds from the heart, but for some reason the waveforms shown on the oscilloscope are just noise. However, the microphone responds when I tapped it. Hence, I'm not sure where the problem lies. An image of my circuit is shown below. Any help would be appreciated, thanks in advance

 

[BradtheRad]

Do you place your mic as close to the heart as the doctor holds his stethoscope, which is as close as possible?

Does your filter eliminate the sound of the heartbeat? Did you do preliminary tests with no filter?

The beats might occur at 50 to 150 x per second (bass region of the spectrum), but the sound of the heartbeat itself might be in a higher region.

 

[kenlfg]

Thanks for the quick reply! I'll check my circuit again.
Edit: I've checked the frequency of the heart beats and found that their frequency is about 1Hz. With such a low frequency, won't a High Pass Filter be useless?

 

[BradtheRad]

I have been experimenting by placing a microphone over my heart, to see what kind of signal shows on the oscilloscope.

I see mild 60 cycle hum. As for my heartbeat, it is too faint to be picked up.

My voice comes through fine. The mic generates up to 40 mV amplitude. I tried a couple of mics.

Your mic signal will need to be amplified. The mic cable will need to be shielded. You'll need to minimize hiss, etc.

Yes, there's also the question what kind of filter to send it through. Hi-pass or low-pass.

You want to filter out clothes rustling, or hands bumping the mic, etc. However these do not necessarily occupy a particular range in the frequency spectrum. They may be more in the nature of transient waveforms, which need to be clipped so they don't overload ears, or instruments.

 

[kenlfg]

Wow okay thanks a lot Brad! But is there anything wrong with my bias network for the mic?

 

[BradtheRad]

The biasing is correct for an electret mic.

Normally you can omit the 30k.

One of the two capacitors in series can be removed.

I've checked the frequency of the heart beats and found that their frequency is about 1Hz

The overall period might be 1 Hz...
however the sound itself is short, maybe .1 to .3 second. Then silence. Hence if you run it through a filter it should admit up to 10 Hz.

Link to article showing heartbeat sounds on oscilloscope:

https://www.ymec.com/hp/signal2/heart9.htm

 

[kenlfg]

Thanks a bunch Brad! I'll try reconstructing my circuit!