Laboratory
Exercise - Sensors
Measuring
Some Sensor Voltages
In this lab exercise you will use some devices that produce a voltage that
changes with temperature and light intensity. They are:
-
A thermocouple,
-
An integrated circuit
temperature sensor - the LM35.
-
A photocell.
All of these devices are called sensors because they sense some sort of
physical quantity (temperature and light for these sensors) and produce
a voltage that changes when the physical quantity changes. Note,
not all sensors change voltage. Some sensors - like thermistors and
strain gages - change resistance as their controlling physical quantity
changes value.
Temperature Sensors
- For The Record
Two of the senosrs are temperature sensors. Temperature sensors come
in several varieties including the following.
-
Thermocouples, which are
inexpensive and widely used. However, thermocouples produce a voltage
out that is in the millivolt range. Here is a photo of a thermocouple.
-
Thermistors are temperature
sensitive resistors. Here is a photo of a thermistor.
-
Integrated circuit temperature
sensors, like the LM35, are designed to produce a voltage that is directly
proportional to temperature. Here is a photo of an LM35 integrated
circuit temperature sensor.
Two of the temperature
sensors produce a voltage output. They are:
-
The thermocouple,
-
The LM35.
We will look at those
sensors first. Here's what you have to do.
-
Measure the output voltage
of the thermocouple, and do that at two temperatures. Remember, you
have room temperature, and your body temperature!
-
You will need to connect
the LM35. Click
here to get the circuit for the LM35. Wire it up and measure
the same two temperatures.
Here are some questions
for you to answer.
-
Which sensor produces
the larger voltage?
-
Does either sensor have
a voltage so small that it might be difficult to measure?
-
Which sensor is most sensitive,
in terms of having a larger voltage for a given temperature.
-
Which sensor responds
quicker?
Now, there is one other
thing to do with the thermocouple.
-
Using a Data Acquisition
Unit (DAU) measure the temperature using the temperature capabilities of
the DAU. (You need a DAU to do this.)
Finally, do the following.
-
Measure the voltage across
the thermistor at room temperature and at body temperature.
-
Measure the resistance
of the thermistor at room temperature and at body temperature.
-
You will need to use the
resistance measurement capabilities of your instrument.
For your report answer
the following questions.
-
Which sensor produces
the largest voltage as temperature changes between room temperature and
body temperature? In other words, which sensor is the most sensitive?
-
Does any sensor have a
voltage so small that it might be difficult to measure?
-
Which sensor woud require
some circuitry in order to make it a usable sensor (one that produces a
voltage output)?
-
Order the sensors from
slowest to fastest as best you can.
-
Which sensor do you find
easiest to use? Why?
