|
 I
have never been so terrified as the day I thought I was going to lose
my wife. I blew through all the red lights on our way to the hospital,
as Michelle rapidly grew weaker in the passenger seat. It all started
just 10 minutes earlier when I confirmed with my homemade electronic stethoscope
(see the October
1997 column) that her heart was beating abnormally. By the time we
barreled into the hospital parking lot, she could hardly speak at all.
I had to carry her into the emergency room.
It turned out to be an easily treated side effect of a prescription steroid
she was taking. We were home in six hours. But the experience scared me
so much that I built Michelle an automated heart-monitoring device to
give us more warning should the symptoms ever reappear. That monitor was
acoustical. Although such a sensor makes it easy to detect an irregular
rhythm, you can learn much more by recording the heart's electrical signature.
So I decided to upgrade to an electrocardiograph, or ECG (also abbreviated
EKG). It can be built in an afternoon for about $60.
Figure
1. Click to enlarge. |
The heart's
strong pumping action is driven by powerful waves of electrical activity
in which the muscle fibers contract and relax in an orchestrated sequence
[see "Surgical Treatment of Cardiac Arrhythmias," by Alden H. Harken;
Scientific American, July 1993]. These waves cause weak currents
to flow in the body, changing the relative electric potential between
different points on the skin by about one millivolt. The signals can change
sharply in as little as one fiftieth of a second. So boosting this signal
to an easily measured one-volt level requires an amplifier with a gain
of about 1,000 and a frequency response of at least 50 hertz.
You signal jockeys may be thinking about using an operational amplifier.
But two vexing subtleties make most op-amps unsuitable. First, when two
electrodes are placed at widely separated locations on the skin, our epidermis
acts like a crude battery, generating a continuously shifting potential
difference that can exceed two volts. The cardiac signal is puny in comparison.
Even worse, your body and the wires in the device make wonderful radio
antennas, which readily pick up the 60-hertz hum that emanates from every
power cable in your home. This adds a sinusoidal voltage that further
swamps the tiny pulses from your heart. And because these oscillations
lie so close to the frequency range needed to track your heart's action,
this unwanted signal is hard to filter out.
Both problems generate equal swells of voltage at the amplifier's two
inputs. Unfortunately, op-amps usually can't reject these signals. If
we want to ensure that this "common-mode" garbage (whose amplitude, remember,
can be over 1,000 times greater than that of our signal) adds no more
than a 1 percent error to our voltage measurement, we need a common-mode
rejection ratio (CMRR) of at least 100,000 to one (100 decibels). This
precision eludes most op-amps.
When an application calls for both high gain and a CMRR of 80 dB or greater,
experienced experimenters often turn to special devices called instrumentation
amplifiers. When set to a gain of 1,000, the AD624AD from Analog
Devices offers CMRRs exceeding 110 dB. It can be purchased on-line
from Pioneer Standard Electronics
for $23.50. Gadgeteers may wish to experiment with less expensive options,
such as the AD620AN.
The AD624AD makes it easy to monitor your heart. A gain of 1,000 is selected
by shorting certain pins together as shown. The two-stage RC filter weeds
out frequencies higher than about 50 hertz. I used a four-wire phone cord
to carry the signals between my body and the amplifier. You'll need only
three of the wires. The side of my project box sports a phone jack for
easy connection and disconnection.
I fashioned my first electrodes out of quarters that had been smeared
with a conducting layer of shampoo, taped firmly to my body and connected
to wire leads. They worked. Then I discovered that anyone can buy bags
of 50 of the real thing: the self-sticking electrodes used by cardiologists.
The cost is about $13 on-line from www.medicalbuyer.com
(part no. 9641). Just peel and stick. I terminated the signal wires with
alligator clips to grip the metal nipples on the backs of the electrodes.
Connect the negative lead to your subject's left wrist, the positive lead
to the right wrist and the ground lead to the left shin just above the
ankle. It proved a bit cumbersome to operate test equipment with my wrists
wired up, so I attached the right and left leads just below my armpits
and ran the wires under my clothes and out just above my belt buckle.
The bouncing ECG trace makes a delightful display for any oscilloscope.
Oscilloscopes are essential for any electronics hobbyist, so if you don't
already own one, you should. Dealers of electronics surplus equipment
sell two-channel scopes with sweep speeds of 100 megahertz--far faster
than you need for a simple project such as this--for about $250. And you
may be able to find even better bargains through on-line auction sites
like eBay or at ham radio flea markets, called hamfests, run by local
ham clubs. If you're on a tight budget, you could build a cheaper monitor;
for suggestions, check out the discussion about this project on the Society
for Amateur Scientists's Web site.
If you want to digitize your heart traces for computer analysis, you'll
need an analog-to-digital converter that can sample at 100 hertz or better--that
is, at least twice the largest signal frequency. Both Vernier
Software and National
Instruments sell such units. Serious amateurs should consider NI's
extensive line of software, which offers turnkey solutions to just about
every data acquisition and analysis challenge there is. Though pricey,
these packages can change your life as an experimenter.
The Society for Amateur Scientists
(SAS) is a nonprofit research and educational organization dedicated to
helping people enrich their lives by following their passion to take part
in scientific adventures of all kinds.
The Society for
Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800
Internet: http://www.sas.org/
American
Science & Surplus
offers a unique mix of industrial, military and educational items, with
an emphasis on science and education. We supply a wide range of unusual
and hard to find items (some say bizarre stuff) to the hobbyist, tinkerer,
artist, experimenter, home educator, do-it-yourselfer, and bargain hunter.
American Science
& Surplus
P.O. Box 1030
Skokie, IL 60076
847-982-0870 Voice
800-934-0722 Fax
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