Image: DANIELS & DANIELS

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 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.

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 AD620A

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.