Ultra Sensative Amp

[Addohm]

I am working on another project at the moment. My wife is very pregnant, and I thought it would be cool to be able to hear our babies heart beat at our leasure, make recordings, etc. I put together a circuit, involving an IC TLC272 to pre-amp the mic, and IC LM386 to amp the head phones. My gain is set to about 90, using a 4.7µF cap across pins 1 and 8. I will be powering this by a 9v battery, or perhaps a wall wart if I need the additional power.

Both IC's are going to be controlled by pots, probably around 100k, until I know exactly what I need to aquire these sounds. I will draw up a schem tonight and post it, and hopefully get some good suggestions for making this work.

My current circuit works, but doesn't really pick up those ultra low(quiet) sounds. My mic is brand new, but I may need to put a cone around it to act as a sound collector, but then you easily risk outside noise distortion.

Adam

 

[Leftyretro]

I am working on another project at the moment. My wife is very pregnant, and I thought it would be cool to be able to hear our babies heart beat at our leasure, make recordings, etc. I put together a circuit, involving an IC TLC272 to pre-amp the mic, and IC LM386 to amp the head phones. My gain is set to about 90, using a 4.7µF cap across pins 1 and 8. I will be powering this by a 9v battery, or perhaps a wall wart if I need the additional power.

Both IC's are going to be controlled by pots, probably around 100k, until I know exactly what I need to aquire these sounds. I will draw up a schem tonight and post it, and hopefully get some good suggestions for making this work.

My current circuit works, but doesn't really pick up those ultra low(quiet) sounds. My mic is brand new, but I may need to put a cone around it to act as a sound collector, but then you easily risk outside noise distortion.

Adam

Hi Adam;

"My wife is very pregnant"

Just wait a little while, you will need no amplification to hear your new baby

The latest copy of Nuts and Volts (Dec) has a project called Big Ear (an audio telescope) that might let you compare with your design. It seems to be available as a kit also.

Lefty

 

[audioguru]

I used a thick plastic lid from a jar of peanut butter. I drilled a hole in its center, mounted a rubber grommet in the hole then mounted an electret mic in the grommet. The mic didn't touch the skin and the flange of the lid sealed out external noise and prevented acoustical feedback to headphones. A speaker might cause acoustical feedback.
I used a lowpass filter to attenuate background noises but pass heartbeats. I will post a pcb layout if you ask.

 

[Space Varmint]

I used a thick plastic lid from a jar of peanut butter. I drilled a hole in its center, mounted a rubber grommet in the hole then mounted an electret mic in the grommet. The mic didn't touch the skin and the flange of the lid sealed out external noise and prevented acoustical feedback to headphones. A speaker might cause acoustical feedback.
I used a lowpass filter to attenuate background noises but pass heartbeats. I will post a pcb layout if you ask.

Man, cat food cans are way better than peanut butter lids. What were you thinking?

 

[Mikebits]

What about two tin cans and a string...

Congrats on the baby...

 

[Space Varmint]

How would you amplify that?

 

[Space Varmint]

Well on a more serious note (here come the arguments) I would suggest putting a Darlington amp configuration on the mic input.

Now I will get lambasted....

But let me tell you what that does. By putting very high value resistors for R4 & R5 like 100K ohms, you will have a very high impedance input. If you match the impedance to rmicrophone you will get maximum power transfer, but that is not what you want. By having a higher impedance you will get maximum voltage transfer. It is the voltage amplitude variations in the baby's room you are interested in. The changes in audio levels. This will increase the sensitivity of your speech input amplifier system. R6 should match the following CE (common emitter) amplifier. Usually something between 470 to 2.2K will work. A CE amp impedance is roughly R1 divided by R2 in a voltage divider bias.

Another advantage of using a Darlington is the current gain is beta of one transistor times the beta of the other transistor. This gives you ample current gain which is often over looked but is an excellent source for successive CE amplifiers since power is equal to current times voltage. It also gives good isolation so that you are not loading down the amplitude variations coming from the microphone. That is why they are commonly referred to as buffer amplifiers.

Try it. I think will will see greatly improved sensitivity.

By the way, congratulations on the new member of your family. God bless.

 

[transistor495]

The product is already available
Fetal Heart Beat Monitor - (Ultrasonic Doppler) -* Compact Ultrasonic Dopplers
**broken link removed**

So the best way to monitor unborn heart beats are to use an ultrasonic transmitter/receiver. No available circuits are there apart from patent ideas. A good topic for research

Is that harmful to the babies?(no idea. take at one's own risk!)

 

[transistor495]

Well on a more serious note (here come the arguments) I would suggest putting a Darlington amp configuration on the mic input.

'quack,quack...'

 

[Space Varmint]

'quack,quack...'

OK inch worm. Inch your way in.

OP, though what I said is true, there is no better than I have ever met in my life, concerning audio, than audioguru. So when it comes to the nitty-gritty. What he tells you will be precise.

edit:

I don't know about the peanut butter lids though.

 

[Mikebits]

OK inch worm. Inch your way in.

OP, though what I said is true, there is no better than I have ever met in my life, concerning audio, than audioguru. So when it comes to the nitty-gritty. What he tells you will be precise.

I don't know about the peanut butter lids though.

Geez, get a room...

 

[Addohm]

Thanks everyone for your responses.

Yes, they are available, however mostly only seen posessed by OBGYN. That also means that they are a bit pricy. I can probably build a quality one for less then 10$. My wife and I purchased a stethescope, and it wasn't enough. We then were anxious enough (and dumb enough) to purchase the only one offered by Baby's 'R Us. This one does not work, and is in fact worse then the stethascope.

Anyhow, here is what my current Schema and PCB are. Attached is the ExpressPCB files. Any input is welcome.

**broken link removed**

**broken link removed**

I can see how the peanut butter lid would work, did you use a metal or plastic lid? I was thinking of a rubber trimmed cone of plastic that would be perfect enough to resonate the sound in to the mic. It shouldn't be too difficult to craft out of solid HDPE stock, with a dremel and some patience.

 

[Addohm]

Audioguru, I was actually thinking about going dual op-amp already, but now I think you pushed me across that like of thinking and doing

I already use a stethescope head for mic. The one that I had, that failed us, I cut the end off of it and shoved the mic in to the tubing.

Adam

 

[Mikebits]

I am really surprized the stethoscope did not work. Is it a good one?

 

[ericgibbs]

Just wait a little while, you will need no amplification to hear your new baby

I agree with Leftys post.

Next project you will want to build is a 500Watt noise cancellation system.

Putting your fingers in your ears and humming loudly .... dosnt work...

 

[Addohm]

Not sure really. Only paid 30 bucks for it on Ebay Guess that says it all though. But, then again, the mic I am using is very small, and really only meant for speaking in to. I already bought an electret mic and it is actually due in this morning. Will try that, just have to modify the stethescope head to mount it.


Dual opamp might do the job though, I might put together my own rendition of it in to ExpressPCB and on to a breadboard when I get home from work.

 

[Mikebits]

I agree with Leftys post.

Next project you will want to build is a 500Watt noise cancellation system.

Putting your fingers in your ears and humming loudly .... dosnt work...

Oh boy, how true that is. 3AM wakeups. Hehe, better him than me...

 

[Addohm]

I agree with Leftys post.

Next project you will want to build is a 500Watt noise cancellation system.

Putting your fingers in your ears and humming loudly .... dosnt work...

Haha, no doubt. She is actually due on the 2nd of December. But for now I would like to keep this project going. There is nothing in stores, currently, that is affordable and actually does what is expected of it. Most anxious couples, I would assume, do not like to wait for their (god forsaken) appointments to see\hear whats going on with their baby

 

[Addohm]

Oh boy, how true that is. 3AM wakeups. Hehe, better him than me...

Lucky for me, I am at work at 3am!

When my wife goes back to school though, blarg @ the 2pm wake-ups! hehe

 

[ericgibbs]

Haha, no doubt. She is actually due on the 2nd of December. But for now I would like to keep this project going. There is nothing in stores, currently, that is affordable and actually does what is expected of it. Most anxious couples, I would assume, do not like to wait for their (god forsaken) appointments to see\hear whats going on with their baby

hi,
As you have no point of reference for understanding the sounds you may hear, you could easily get yourself stressed out for no reason.

I would recommend that you build a A/V baby monitor for the babies bedroom.