Citizen Scientist

DIY ECGs

TRACK YOUR TICKER WITH A HOMEMADE ELECTROCARDIOGRAM MACHINE. By Dr. Shawn

THE HEART’S PUMPING ACTION IS DRIVEN by powerful waves of electrical activity that cause weak currents to flow in the body, changing the electric potential between different points on the skin by about one thousandth of a volt (one millivolt, 1mV). Hidden within that activity is an enormous amount of information about what the heart is doing, and anyone who can detect it can peer into the workings of this incredible organ.

Fortunately, you don’t have to be a cardiologist with expensive equipment to pick up and decipher that signal. Anyone can do it with this homemade electrocardiogram (ECG) device, an analog-to-digital converter (ADC) to digitize the signal and send it to a computer, and a remarkable book that I’ll tell you about later.

You can assemble the circuit itself in an afternoon for about $40. The ADC will cost a bit more, between $50 and $300. But these devices open a universe of opportunities to the home-based experimenter, and so every citizen scientist should invest in one. (I’ve negotiated a great deal on one of these devices especially for MAKE readers. Read on.)

The experimental challenge is that the signal we’re looking for measures only about 1mV, it can change in as little as ¹/100 of a second, and it’s embedded in a noisy environment. To keep up with the signal and boost it to a digitizable 1V level, you need an amplifier with a gain of about 1,000 and a frequency response of at least 100Hz. But standard operational amplifiers (op-amps) like RadioShack’s 741 won’t work because of the surrounding noise.

When electrodes are placed far apart on the body, our skin acts like a crude battery and generates an irregular potential difference that can exceed 2V, dwarfing our 1mV heart signal. Even worse, your body and the wires that connect to the electrodes make wonderful radio antennas that pick up the 60Hz hum emanating from every power cable in your home. This adds a sinusoidal voltage,

which further swamps the tiny pulses from your heart, and because its frequency lies close to the 100Hz resolution we need to track your heart, it’s hard to filter out.

Now, you electronics types might think this shouldn’t matter because op-amps are “difference amplifiers” — that is, they subtract out any voltage that runs equally to both inputs. Unfortunately, op-amps don’t do that job perfectly, and when the swells are thousands of times bigger than the signal, as they are here, you’re sunk. To ensure that this “common-mode” garbage adds no more than a 1% error to our measurement, we need what’s called a common-mode rejection ratio (CMRR) of at least 100,000 to 1. In electronics parlance, CMRR is measured in decibels (dB), where every factor of 10 increase in voltage is equivalent to 20dB. This makes our required ratio 105, which equals 20* 5 or 100dB — a precision beyond that of most op-amps.

THE INSTRUMENTATION AMPLIFIER

When an application calls for both high gain and a CMRR of 80dB or greater, experienced experimenters often turn to special devices called instrumentation amplifiers. These remarkable devices were once bulky and expensive, but today, they can be purchased for just a few dollars as an integrated circuit. To make the construction as simple as possible, I designed this ECG around the Rolls-Royce of instrumentation amplifiers, the AD624AD from Analog Devices, which you can buy from Digi-Key ( digikey.com) for under $25. You select a gain of 1,000 with the AD624AD by simply shorting certain pins together, and at this setting the amp’s CMRR exceeds 110dB.

The AD624AD is easy to use, but experienced gadgeteers should also feel free to experiment with less expensive options, such as the Analog Devices AD620AN. And if you’re a real Daniel Boone-type maker, you can construct your own instrumentation

Make: 157