diaphragm, and perhaps contraction of the stomach if
you’ve recently eaten.
Similarly, measurement of the perfusion of your biceps
is going to vary with a sneeze, a squeeze of your hands,
and other biological activity, not to mention factors in the
environment. Think a 5G pull in an F16.
Optical Heart Rate Monitoring
For this article, I’m going to focus on optical heart rate
monitoring — the same variety of monitoring used by the
Fitbit watch, as opposed to the EKG variety popularized
My first watch based heart rate monitor — a ‘70s
sports watch from Timex — was founded on optical
principles discovered well over 100 years ago. That is, with
each contraction of the left ventricle, a bolus of blood rich
in red blood cells (RBCs) is ejected into the aorta.
Within a few milliseconds, this pulse reaches the
fingertips, ear lobes, and other peripheral areas, opening
the small capillaries and temporarily filling them with a
rush of RBCs. This transitory mob of red blood cells
absorbs and reflects light differently from surrounding
If you measure the changes in amplitude of light
passing through capillaries in a thin layer of tissue (such as
an earlobe or fingertip), then heart rate can be easily
calculated in beats/minute. For an earlobe probe, this
entails fixing an LED on one side of an earlobe and a
receiver on the opposite side.
Depending on the design, desired noise immunity,
and need to measure other blood parameters (such as
blood oxygenation), the wavelength is usually red, IR, or a
combination of the two.
Now, let’s say we’re going to monitor heart rate
during your daily workout on a treadmill or spinning bike.
Assume that it’s ideal to exercise at between 50%-85% of
your maximum heart rate, and that this maximum is 220
minus your age.
For example, if you’re healthy, athletic, and 20 years
old, your maximum heart rate is about 200 beats/minute.
For maximum aerobic conditioning, you’d want to run on
the treadmill with a heart rate of between 100 and 170
beats/minute. Cruise at 70 beats/minute and you’re
wasting your time. Sprint at 175 or 180 beats/minute for
more than a minute or two, and you might end up on a
stretcher en route to a cardiac care unit.
With this physiology background behind us, let’s get
to the project.
The key hardware for this project is an inexpensive
Arduino-compatible ear-clip heart rate sensor by Grove
($13 at www.seeedstudio.com). If you own a solderless
Grove shield, then construction takes all of five seconds.
Simply attach one cable to the shield and the other to
the sensor, and you’re ready to go. Figure 1 shows the
Grove shield and leads going to and from the sensors and
LEDs. Figure 2 shows one of the output LEDs. Because
the limiting resistor is built into the unit, there’s no need
for additional components. Just plug and play.
The red cable provides USB connectivity during
development and adequate power for operation. No
external power is required. If you’re working without a
special shield, then you can quickly connect the signal,
By Bryan Bergeron
FIGURE 2. Grove green output LED. Note the built-in
series limit resistor.
To post comments on this article and find any associated files and/or downloads,
go to www.nutsvolts.com/magazine/issue/2017/12.
December 2017 27
FIGURE 3. Ear-clip sensor with LED visible on one side
of the clip.