Everything for Electronics
Posted in: Developing Perspectives (October 2011)

Biology and Electronics

By Bryan Bergeron

One of the easiest projects to start with is an electronic stethoscope. You can buy a commercial version on eBay for a few dollars if you’re not looking for the thinnest, lightest model available. Or, you can build your own. You’ll need an inexpensive, acoustic stethoscope head, a microphone element, and an audio amplifier. Once you’ve learned what your heart, lungs, and intestines sound like, you can use an analog-to-digital converter and your favorite microcontroller to create a heart rate or breath rate display. Another option is to pick up and teardown an inexpensive blood pressure monitor from your local drug store. Can you figure out how they’re determining pressure? You may have to search the Web for details, such as how the sound of blood flowing through arteries changes as external pressure is applied.

Then, there’s the kit approach. I’ve seen several electrocardiogram (ECG) kits on the market. Ramsey ([url=http://www.ramsey.com]http://www.ramsey.com[/url]) sells one for about $50. If you’re familiar with op-amps, want a great way to learn how to work with op-amps and low level, low frequency signals, then build your own ECG monitor. I’d take the precaution of NEVER using an AC powered supply, even one of those low voltage bricks. In addition, use an optical isolator to minimize the chance that a voltage could be applied to your body as you’re testing the circuit.

Of course, there are experiments you can perform in which signals are intentionally sent to your body, as in Galvani’s experiment. Again, have a look on eBay for one of those abdominal toners that send pulses of electricity to your ab muscles, causing them to contract. At the other extreme — which I don’t recommend — are high voltage tasers designed to temporarily stun.

If you’re into exercising and fitness, then you probably know about the heart rate monitor pickups that work with Polar watches, and many of the treadmills on the market. This is another source of ‘teardown’ material that’s relatively inexpensive — especially if you’re willing to tear open one of the sealed sensors and replace a battery.

If you’re into optics and LEDs, there are dozens of experiments that you can perform with an op-amp, microcontroller, LED, phototransistor, and a handful of discrete components. The simplest is a heart rate monitor based on the reflection or transmission of red light through your fingertip or earlobe. With each contraction of your heart, red blood cells rush through the capillaries in your fingertip. A phototransistor/op-amp circuit can detect the resulting change in transmission/reflection of red light. Once you’ve mastered heart rate, you can try your hand at blood oxygen saturation (the more oxygen carried by a red blood cell, the more red the cell). You’ll have to do a little research to determine which wavelengths of light to use.

I’ve been working with a few simple sensors — a piezo microphone element and a few resistive pressure sensors — and an Arduino on a number of experiments. I use the piezo element in my shoe to determine each time my shoe strikes the ground — similar to the method used by the popular Apple/Nike running sensor. The pressure sensors — mounted at different locations under my running shoe insert — record the changes in pressure distribution as I run. My goal is to record changes in pressure distribution at different running paces. Runners at the Olympic Village use similar systems, but these sell for around $50K. I’ve amassed most of my sensor collection from teardowns of devices I’ve picked up from Ebay. I’ve also purchased a few sensors from SparkFun Electronics and Parallax. Amazon is a convenient source for ECG electrodes, but you’ll have to buy in bulk. If you want to experiment with a Polar heart rate monitor sensor strap, then consider the Polar interface from SparkFun. A Polar chest strap sensor is about $50 on Amazon. As a final note, think about what you’re doing. Electricity is potentially dangerous, in part because the salty fluid in your body is an excellent conductor. Don’t use AC power and DC — at significant current — can burn. If you’re not familiar with adhesive electrodes and other ‘direct’ connections, start out safe with optical sensors. Even then, consider that you could damage your eyes if you stare long enough into an active LED. Have fun on your journey. NV