the 16 digital I/Os.
• Senses various physical properties including
temperature, humidity, voltages, currents, light levels,
sound levels, etc., using the 16 analog inputs (ADC).
In this article, we will experiment with analog
temperature sensors. Temperature sensors include
thermocouples, thermistors, and solid-state devices.
Temperature sensors are associated with the sense of touch
We use temperature sensors every day to monitor the
indoor/outdoor temperature; for weather applications;
commercial appliances, industrial applications, and scientific
applications. In HVAC, thermostats control the room
temperature by measuring the ambient room levels and
comparing those to the set temperature. This way, the
furnace activates when the temperature is below the set
point. In automation, factory process control and robotics
are used everywhere. Even your laptop or PC uses a sensor
to monitor the CPU temperature. You can see then how
important this sensor is to us.
Types of Analog Temperature Sensors
There are various types of analog temperature sensors,
including lab glass thermometers; these include biological
sensors such as our own nerve cells near the surface of the
skin and electronic temperature sensors including analog
and digital kinds. One of the early analog temperature
sensors was developed in 1821, by the German-Estonian
physicist Thomas Johann Seebeck who discovered that
when any conductor is subjected to a thermal gradient, it
will generate a voltage. This is known as the thermoelectric
effect or Seebeck effect used in thermocouples.
Thermocouples have many commercial and research
uses including process control and factory automation.
These kinds of sensors can also be used for consumer and
commercial high temperature applications such as ovens,
furnaces, and stoves. For more information on the
thermoelectric effect, see the article relating to it on
Wikipedia ( www.wikipedia.com).
Type K or type J thermocouples (shown in Figure 2A)
are made by welding two wires of dissimilar metals
together; they measure high temperatures and are accurate
to within ±1 degree Celsius. They do require signal
conditioning using op-amps and a Wheatstone bridge, since
they generate a very weak voltage. This requires an op-amp
to amplify the signal to a range that our microcontroller can
read using the 10-bit ADC. Thermocouples measure the
temperature difference between two points, not the
absolute temperature. In order to measure a single
temperature, one of the junctions — normally the cold one
— is maintained at a known reference temperature; the
other junction is at the temperature to be sensed.
Thermistors shown in Figure 2B are another type of
analog temperature sensor similar to a resistor but whose
FIGURE 2A. A type K thermocouple used for scientific consumer
and factory automation applications.
FIGURE 2B. This is what thermistors look like. These are low-cost
sensors but a bit more difficult to use due to their non-linear nature.
The low-cost LM34
is a solid-state
device with three
wire leads that
readings in millivolts
proportional to the
resistance varies significantly with temperature. They are
low-cost sensors that can be used for high temperature
measurements. Although I was able to use them with the
VEX microcontroller, I found them to not be as accurate as
the LM34 temperature sensor shown in Figure 2C . They
are not recommended for these experiments since they are
not as accurate, and are much harder to use.
The low-cost LM34 Fahrenheit temperature sensor is a
solid-state device with three wire leads that return the
temperature readings in millivolts proportional to the
current ambient temperature surrounding the sensor. For
example, a temperature reading of 650 indicates a
temperature of 65.0 degrees Fahrenheit. These analog
readings are digitized using the microcontroller’s 10-bit
analog-to-digital converter (ADC). LM34s are linear in
nature and do not require calibration. Although they read
temperature in degrees Fahrenheit to ±1 degree precision,
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