stamp sized heart rate receiver is so thin that I
almost overlooked it.
As shown in Figure 1, the basic kit also contains
a smattering of miscellaneous components including
a handful of capacitors and resistors, an LM358 op-amp, a pair of magnetic reed switches, a pair of rare
earth super magnets, signal diodes, four-direction tilt
sensor, and a variety of connectors. There is also a
modest supply of old fashioned pH paper.
Unless you're in the medical field, you may not
have encountered an SpO2 sensor. The fingertip
sensor (see Figure 2) is a disposable adhesive unit
that contains a pair of LEDs (one IR and one red)
and a detector. The ratio of red/IR absorbed as light
from the two LEDs travels through the tissues of a
fingertip, and then indicates the degree of oxygen
saturation; 98% is a typical level in a healthy person.
At a full sprint, my SpO2 drops to about 96% — the
same value that an asthmatic might have at rest.
Another atypical component is the nebulizer
which is a set of plastic tubes and containers designed to
create a mist or aerosol that can be inhaled. Like the
blood pressure cuff, the nebulizer is a stand-alone device.
You'll have to think of clever ways to add sensors to
automatically collect physiological data.
I run with a Polar T34 chest strap heart rate
transmitter, so it wasn't much of a surprise. However,
what did catch my eye was the matching receiver (see
Figure 3). My previous experience with a Polar receiver
was with a cumbersome 3" x 2" unit. The small,
unobtrusive receiver is definitely a plus.
Overall, there's a lot in the microMedic kit to keep
anyone busy for weeks, if not months. The non-medical
sensors are standard high quality components, and the
quality of the medical-specific components seems
appropriate for the price of the kit. I would have preferred
a non-disposable SpO2 sensor, but given that my clip-on
SpO2 sensor sells for $300, I can see the
need to go with an affordable option.
Depending on what you decide to build,
you'll likely require additional supplies. For
example, if you plan to use the blood
pressure cuff, then you'll need an inexpensive
nursing stethoscope to insure correct
placement of the cuff. At a minimum,
you'll probably want a project enclosure
of some type.
Figure 2. Working side
of the SpO2 sensor.
LEDs are in the upper
left cutout; sensor is in
the lower right cutout.
trouble consistently mapping colors to the color guide. For
example, I can guesstimate the mid-point between the pH
values of 3 and 4, but not between the values of 1 and 2.
What, for example, if you have two people reading test
results, and one is, say, red-green colorblind? We need a
Recall that pH paper is an indication of the hydrogen
ion concentration of a solution. Acids have a low pH, as
low as 1 or red. Bases have a high pH, as great as 11 or
purple. Neutral pH is 7 or green. Regular table vinegar has
a pH of about 2. 5 (orange-red).
The normal pH for blood is between 7. 35 and 7. 45,
or slightly alkaline. The pH of tears is limited to a narrow
range from about 6. 5 to 7. 5, or light green to dark green.
Urine, on the other hand, is more exciting — at least in
terms of reading pH with paper strips. The pH of urine
varies from about 4. 6 to 8, or orange-green to dark green.
Figure 3. Polar heart rate
transmitter strap and
Let's Build Something
Continuing with the idea of an objective
quantitative pH paper reader, let's use the
ColorPAL color sensor to quantify the color of
a test strip, using the color guide from the bag
containing the pH strips as a standard (see
Figure 4). I don't know about you, but I have
February 2013 47