Environmental Monitoring with Sensors: Hands-on Exercise
Now that you’ve seen a few types of sensors, along with some circuits that can be developed to
condition their responses, let’s spend a bit of time experimenting with these concepts.
Thermistor Investigation
Here we have a representative low-cost negative temperature coefficient (NTC) thermistor that is
specified to be 10 kΩ at 25 C. Our measurement of the component’s resistance is ~11.8 kΩ,
and not ~10 kΩ, due to our room temperature not being 25 C (77 F) and to any component
variations.
We can demonstrate that this is a NTC device by warming the component with our fingertips and
noting that the resistance of the device has dropped to less than 10 kΩ.
Now let’s design a basic voltage divider circuit consisting of a 10 kΩ resistor, R1, placed in series
with the thermistor, RT. If we have the top node set to 5 Volts and the bottom node of the series
circuit at ground, ‘GND’, then we can calculate the voltage at the center node, Vout, using the
following:
Now let’s build a circuit replicating the above diagram and measure the output voltage first across
the top and bottom of the circuit and we see it’s 5 Volts as expected. The red clip lead
corresponds to the positive terminal of the voltmeter and the black clip to the negative.
Now let’s measure the output at Vout, the node connecting the resistor and thermistor. Here we
see the output voltage is ~2.7 V at our room’s temperature. Again, our room is colder than 25 C
and thus the thermistor resistance, RT, is greater than the fixed resistor, R1; i.e., 10 kΩ. As such,
more voltage is dropped across the thermistor resulting in Vout being greater than 2.5 V.
Now lets increase the temperature of the sensor by again squeezing it between your fingers. The
result is that the thermistor’s temperature is now greater than 25 C and thus the thermistor
resistance, RT, is less than the fixed resistor, R1 = 10 kΩ. Now more voltage is dropped across
R1 than the thermistor resulting in Vout being less than 2.5 V.
At this point, you can ‘ruggedize’ your circuit so you can move it to a variety of locations of known
temperature and collect data to calibrate your circuit. As noted in the instructional clip, the
thermistor’s response of resistance vs. temperature is non-linear. That being said, one can come
up with an approximate ΔR/ΔT linear coefficient if the temperature range of interest is small.
No.
1
2
3
Condition
Room temp
Warmer
Cooler
Temperature (C)
Voltage (Vout)
From your data, develop a relationship between measured voltage, Vout, and temperature, C.
As expected, Vout goes down as the temperature increases. If we wanted Vout to go up with
increasing temperature you could redesign the circuit and swap the two components.
Strain Gauge Investigation
In the images below we see a strain gauge bonded to a thin steel cantilever beam. As noted in
the module, a strain gauge can be considered a long wire that is laid out in a zig-zag fashion. As
the strain gauge elongates or compresses, the resistance between the two leads (black wire and
red wire) increases or decreases, respectively.
With no strain on the beam, we have measured the resistance of the gauge, Ro, to be about 1 kΩ.
Bending the beam down introduces tensile force and stretches the strain gauge thereby
increasing its resistance. However, as seen in this measurement, the change is very small; only
3.6 ohms or 0.35%. Not only is this change very small but also as illustrated by the image the
amount of strain (i.e., bend) on the cantilever is significant. In practice one would expect
significantly less strain on a system and thus even much smaller changes in resistance. Such
very small changes in resistance are difficult to directly measure with accuracy.
Lifting up on the beam introduces compression forces on the strain gauge thus decreasing its
resistance as seen in this measurement. Again, the change is small, only 4.3 ohms or -0.43%.
Let’s now develop a Wheatstone Bridge using the strain gauge and three 1 kΩ resistors (R1, R2,
and R3) in a quarter-bridge configuration.
However, these components are not ideal and thus with no strain, the bridge is not balanced and
thus the voltage across the bridge, Vab, is not zero (i.e., Vab = 25.9 mV).
To balance the bridge, we replace the resistor in series with the strain gauge, R2, with a
potentiometer, which allows us to nearly balance the bridge under no load conditions (Vab = 0.231
mV).
Now under tensile forces, the voltage at node b increases as the strain gauge has higher
resistance than the potentiometer. The voltage at node a remains the same as there is no
change in the fixed resistors R1 and R3. Therefore we measure a negative change in Vab (i.e.,
Vab = -2.78 mV). We note that as we are measuring on a millivolt scale, this is a significant
change from the no strain condition (over one order of magnitude). The Wheatstone Bridge
therefore enables us to relate very small changes in voltages to small changes in strain gauge
resistance and by using the Gauge Factor enables one to determine the strain in the material to
which the gauge is bonded.
Under compressive forces, there is still no change in the fixed resistors R1 and R3 and thus the
voltage at node a remains the same. The voltage at node b now decreases as the strain gauge
has lower resistance than the potentiometer. The end result is we now see a positive change in
Vab (i.e., Vab = 2.73 mV).
Take Aways
Resistance-based sensors are just one type of sensor but are commonly used for a wide variety
of applications. Here we have demonstrated that sensors in which nominal resistance sees a
large amount of change with the phenomenon of interest (e.g., the thermistor) can be conditioned
with a simple voltage divider. However, even sensors that see little change with the phenomenon
(e.g., strain gauges) can be conditioned using a still relatively simple bridge circuit.
While data sheets and equations give us a sense of how the circuit will perform, one should
consider a calibration test to ensure the measured results are as expected as there can always
be component variations that may introduce bias and proportional errors.