Instrumentation amplifier

+ IN

+ 9 3_{+ 7}

6 741

2

– ₄ – 9

GAIN = 1 + ^200,000/_Rg = 1000 IF R_g = 200Ω 100K 100K

100K

R_g

+ 9 2_– 7

6 741

3

+ 4 – 9

Shielded cable output

+ 9 27

–

6 ₃ 741

+ ₄

– 9

100K

100K

100K

– IN

All resistors are 1% metal oxide ¼ watt

Once you’ve connected body electrodes to our circuit, plug the Go Link into your laptop’s USB port, plug the voltage probe into the Go Link, and wire the probe to our circuit’s ground and low-pass filter output. For safety, the laptop should be unplugged.

SAFETY

Unless you’re doing something exotic, battery-powered instruments are generally safe to connect to people. The Go Link is powered through its USB port, so if the laptop you’re using is unplugged, you’ve got no worries.

Unlike the editors of MAKE, I personally think it’s OK to use our ECG device with a plugged-in computer, provided you take some additional precautions; see more discussion on my website at sas.org/make.html. The 10M (that’s 10 million ohms!) resistor between the subject and ground will choke any current in the unlikely event that an AC adapter fails and the laptop fries. The 47K resistors also limit the current. Even with a freak power surge, the subject’s arms lie outside of any path between the electrodes, so if she experiences any pain, she’ll be able to grasp and pull off the wires. Keep someone else in the room, or if that’s not possible, make the electrode leads short and run ECGs while standing. That way, if lightning does strike, you’ll break the connection by falling down.

In any case, never attach electrodes to the arms, and don’t connect the device to anyone in weak health. No matter how much of a do-it-yourselfer you are, don’t fire up your homemade ECG if you

think you might be having a heart attack. Resist your curiosity and dial 911.

In hospitals, some monitoring equipment adds yet another layer of safety: opto-isolation. In this scheme, battery-powered devices connected to the patient transmit readings via LEDs to matching optical sensors inside a wall-powered display device. Since the only link between the two devices is a stream of photons, large currents can’t reach the patient through any wires. If you care to, you can add opto-isolation to your ECG to protect against a meteor-falling-from-the-sky-probability-level chain of events. Opto-isolators come in DIP-style ICs that you can plug into your system between the filter and the data logger. You’d ground the LED transmitter side to the amplifier circuit’s ground, and the photodiode receiver to the data logger or to ADC’s ground.

INTERPRETING THE RESULTS Once you’ve mastered the art of tracking your ticker, you’ll need to decipher what it all means. There’s just one must-have reference, Rapid Interpretation of EKG’s by Dale Dubin, MD. You’ll find this easy-to-master mainstay of medicine on nearly every doctor’s bookshelf in America. Dr. Dubin also happens to be a good friend of the Society for Amateur Scientists, so we’re able to offer new copies of this classic to MAKE readers at a 30% discount; see sas.org/make.html.

Dr. Shawn (Shawn Carlson, Ph.D.) is the founder and executive director of the Society for Amateur Scientists ( sas.org) and a MacArthur Fellowship winner.

Make: 159