an Arduino based electronic stethoscope

Here’s a quick little update on my continuing attempts to build a clot monitor for Jamie.

The thermistor sock worked well as a temperature monitoring device, it’s currently living a second life monitoring the temperature of my living room, but it wasn’t the greatest for trying to detect clotting. There’s a lot of variation in the temperature of muscle as it works (details can be found here) and after that test we decided to go after a different potential indicator.

Sound may be a better option. If you were to monitor the sound of the veins in her leg near where the clot initiated last time once a week or so you could watch for changes over time that may indicate some sort of issue that could be checked up on by people who do medicine for a living.

Just strapping a microphone to her leg wouldn’t do though. Your veins don’t make much of an audible sound, though they do vibrate as they pump blood through the body. A stethoscope works by turning these vibrations into audible sound. For my purposes I wanted to record that sound and compare it to previous examples, which means using an electronic stethoscope.

You can find them online for a few hundred bucks but I’m cheap and like to tinker. I’d also like to be able to get into the source code and play around, something which is usually discouraged by larger companies afraid of reverse engineering by curious hackers. Instead of a nice new unit I bought an Arduino Uno, two Electret microphones and a stethoscope for $25.47.

Good things come in static dissipating packages.

The microphone amplifier chip in use here is the MAX4466. After looking around the net this appeared to be the best chip for audio analysis. A great discussion of small microphone amplifiers and a how to for spectral analysis with an Arduino was written by Arik Yavilevich here.

The next thing to do was to solder up the microphone and attach it to the stethoscope tubing. For this I used a bit of heatshrink with glue backing. I was a tad bit concerned about roasting the microphone and tubing with a heat shrink gun, but both turned out to be fine after a bit of testing. The stethoscope tubing in particular must be silicon or similar, because it fared quite well with the heat.

The electret microphone

This type of microphone is particularly easy to use. One wire for voltage, one for ground, and one more for signal is all you need. When wiring to an Arduino it’s best to wire the VCC to the 3.3V output. The 5V power can be noisy because of draw down on other pins of the Arduino and especially when powered off USB. 3.3V is provided by an onboard linear regulator and provides a much cleaner signal at the expense of loss of range.

The doctor will see you now

For code I used the “ReadAnalogVoltage” example code from the Arduino IDE. This was just a quick test for functionality. I would recommend using something a bit quicker for actual sampling as the analogRead() command is quite slow, But for this project it was more than enough.

And that, ladies and gentlemen, is my heartbeat.

A quick little project and there’s a lot of work yet to be done, but hopefully useful for things to come.

-Jonathan

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