Citizen Scientist

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Schematic for a simple instrument to monitor one’s electrocardiogram (easily made at home).

amplifier using three RadioShack op-amps and a handful of 100K resistors. (See the complete schematic on the next page.)

Some simple circuitry supports our instrumentation amplifier. A two-stage resistor-capacitor (RC) filter weeds out frequencies higher than about 50Hz. As filters go, this one is pretty wimpy, but it works well enough to do the job. I used a four-wire phone cord to carry the signals between my body and the amplifier, but you only need three of the wires. The side of my project box sports a phone jack for easy connection and disconnection.

THE ELECTRODES

I fashioned my first electrodes out of quarters smeared with a conducting layer of shampoo, taped firmly to my body, and connected to wire leads. They worked, sort of. Then I discovered anyone can buy bags of the real thing — the peel-and-stick electrodes used by cardiologists. The cost is about $13 for 50. (Google “ECG electrodes” for a host of suppliers.) I used alligator clips to connect the signal wires to the metal nipples on the backs of the electrodes.

158 Make: Volume 11

Connect the instrumentation amp’s negative lead to just under your subject’s left armpit and its positive lead just under the right armpit. You must also connect a ground lead for the circuit to the left shin just above the ankle. Without the leg as a ground, bad things happen to the signal, and it’s a great little experiment to record an ECG this way for about ten minutes and see the problems that creep in.

LOGGING THE DATA

To examine the ECG signal, you’ll need to digitize and record it on your computer. This requires an ADC or data logger device that can sample at 200Hz. (The Nyquist Theorem states that reading an oscillating signal requires sampling it at a minimum of 2x its frequency.) I’ve tried many data loggers, and my favorite is the Go Link from Vernier Software, which has 12 bits of resolution and samples at up to 200Hz. Add the matching voltage probe unit, and you’re ready to rock-and-roll literally hundreds of other science projects. I’ve negotiated a deal with Vernier especially for MAKE readers: $67 for both logger and voltage probe; see sas.org/make.html for details.

Illustrations by Tim Lillis