Data logging is the automatic collection and
saving of information. Having the ability to
automatically log a string of measurements
and save them for later study and analysis can
help transform you from an experimenter into
a scientist.
Trees are natural data loggers, and I
described how to analyze their annual growth
rings in MAKE Volume 19. So are layers of
snow (Volume 21). And you can log sunlight
intensity for entire days using blueprint paper
(Volume 26).
Electronic data logging provides an
enormous range of opportunities for amateur scientists. Back in the 1980s, I would
connect a sensor to the joystick port of a PC
like RadioShack’s Color Computer or IBM’s
PCjr, and write simple data logging programs
in BASIC. When analog-to-digital interface
boards became affordable, my logging
became more sophisticated. Still, these
methods required a computer dedicated
solely to the logging operation.
Data logging changed dramatically when
miniature, standalone loggers were introduced. These devices provide real-time
logging without the need for a dedicated
computer. They’re activated and downloaded
by a computer, but in between, they operate
independently. Data loggers are available
from Onset Computer, Jameco, Omega
Engineering, SparkFun, and others. Maker
Shed ( makershed.com) also sells a data
logging shield for Arduino microcontrollers.
Interfacing Sensors to Loggers
You can buy tiny data loggers that store
temperature readings over time. Other loggers
record light, carbon dioxide, pressure, and
other parameters. But what if you want to log
a parameter for which there is no logger?
You can build your own logging system from
scratch or from published plans. Or you can
do as I’ve done and design DIY sensor circuits
that can be connected to the voltage input(s)
of commercial loggers. This approach frees up
considerable time, since software is already
available for these loggers.
If a sensor produces an output voltage that
doesn’t exceed the allowable input voltage for
the logger, no circuitry is needed (unless the
signal is so small that it requires amplification). A typical logger has an allowable input
range of 0 to 2. 5 volts. This means you can
safely and directly log the voltage of many
kinds of disposable and rechargeable power
cells and batteries.
COUNTRY SCIENTIST
By Forrest M. Mims III,
Amateur Scientist
USING SENSORS
WITH DATA
LOGGERS
Voltage Divider Sensor Interface
If your sensor is resistive it won’t produce a
voltage, so you’ll need to connect it as half of
a voltage divider. The simplest interface circuit
for resistive sensors is the single resistor or
potentiometer circuit shown in Figure A. It can
be used with light-sensitive photoresistors,
temperature-sensitive thermistors, and other
sensors having a resistance that changes with
pressure, touch, weight, acceleration, rotary
motion, and so forth.
R1 is a resistor or pot connected in series
with the resistive sensor to form a voltage
divider. Using a pot allows the sensitivity of the
circuit to be easily adjusted. The free end of
the sensor is connected to the logger’s positive supply voltage, and the free end of R1 is
connected to the logger’s ground at the input.
The junction of the sensor and R1 is connected to the logger’s positive input. Be sure to
check the polarity of the logger’s input before
making the connections.
It’s best to use the logger’s supply to power
this interface, since this will avoid applying
excessive voltage to the input. If the positive
supply isn’t available externally, you can open
146 Make: makezine.com/30
