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Surely you or a friend of you owns a smartphone. You probably have a one with you at that moment.
Not without reason a smartphone is smart
. It does tasks that previously required multiple devices.
For example, you can use the smartphone to make phone calls, send and receive text and picture messages, take pictures, shoot videos or surf the Internet.
The smartphone reacts (apparently) intelligently to changes in its environment. You are certainly familiar with some examples:
By tipping on the screen with your finger, the smartphone recognizes where you typed and, for example, an app is launched or a letter is written.
It detects if you hold it vertical or horizontal and adjusts the screen to it.
If you pull it closer to your ear to make calls, the brightness of the screen will be reduced, which saves energy and increases the battery life.
The reason for the extremely diverse applications of smartphones lies in their equipment with a sophisticated miniature electronics and a number of technical sensors.
From the picture you gain an impression of the complex inner life
of a smartphone.
On the front you will find the following elements:
- Battery charging socket
- Speakers
- Headphone jack
- Camera sensor
- Processor
- SD card reader
- Motor for the vibration alarm
And on the back you will find the following elements:
- Rear of the touch-sensitive display
- Proximity sensor
Home-Button
- Microphone
But even apart from the sensors in the smartphone: Technical sensors have become indispensable in our modern world. Among other things, they are used for measurements and monitoring. Here are two examples
Speed camera and radar control
For the safety of the road users, but also for the protection of noise speed controls are carried out. This is done with optical sensors that can analyze the transit time of radar waves.
Air quality measurement
Dirty air makes you sick. To check the air quality, the concentration of various air pollutants is determined at many (but far too few) measuring stations.
In particular, when it comes to monitoring compliance with legal limit or benchmark values, it is not sufficient for sensors to only roughly record physical or chemical data of the environment.
During speed controls, sensors are interested in the exact speed of passing vehicles.
In air quality monitoring, the concentration of a substance, e.g. of nitric oxide in the air, is measured.
But even in your normal school day you know the problem of bad air. You enter a classroom in which a class had previously been taught and was not aired. It stinks. The students who come out did not notice that.
In order to objectify the subjective perception of odors, one can use gas sensors.
In this workshop we will deal with the applications and the functionality of these small technical sensory organs
and smell with them.
Get acquainted the test setup and sensor.
Can the sensor distinguish water from apple juice and non-alcoholic beer?
Introduction to a simple model of the sensor.
Understand the basic operation of the sensor and investigate the temperature dependence of the sensor resistance.
Fingerprint
of water, apple juice and alcohol-free beer.
Investigation of the temperature dependence of the sensor.
Temperature dependence of the sensor explained with a simple model.
The fingerprint
of different gases as basis for the calibration process.
The core of the experiments in this workshop is a circuit board. The left and right photo shows their top and bottom.
First of all, let's take a closer look at the bottom side.
On the bottom of the circuit board you can see the gassensor.
The gas sensor has a membrane on the surface (black marked), which protects the sensitive interior from liquids. However the gas mixture can penetrate this protective layer.
At the edge of the sensor you see four solder joints. Two of the four connections are for power supply (red marked).
With the other two connections (blue marked) you can tap the voltage of the gas sensor.This will be discussed later in detail.
The following photos show a close-up of the gas sensor without membrane and protective cap (left) and a picture of the sensor under a microscope (right).
The sensor has a sensitive layer (the circular yellow-green film in the photos). This layer interacts with a gas mixture entering through the membrane. What exactly happens there, you will learn in the theory phase.
Now we will have a look at the top of the board. Here you can see an USB port that is needed for the power supply. If the sensor operates, the red LED lights up.
The gas sensor uses two adjustable resistors, so-called potentiometers. These can be adjusted with a coarse and fine controller. What effect the potentiometers have on the gas sensor, we will work out in the experimental phase.
The two sockets on the left (red and black) are connected to the two connections on the gas sensor, which are used to measure the voltage. The voltage at the gas sensor is also called heating voltage (see labeling between the connections on the board).
The heating voltage is measured on the left via a multimeter.
We measure the sensor resistance via the two connections on the right (see labeling between the connections). To do this, connect a second multimeter on the right. Incidentally, the sensor resistance is the most important parameter in this workshop.
We will start with the first setup.
Connect the board to a power source (PC) with a micro USB cable. If the red LED lights up, proceed by clicking on Next step
.
Take one of the two digital multimeters and set the measuring range to 20 Volt DC as in the following picture:
Now connect the Digital Multimeter to the connectors on the left side of the board using two banana plugs. The structure should look like the following picture:
With the adjustable resistors (potentiometers), you can regulate the heating voltage U , which is applied to two of the four connections. Fill in the range in which the heating voltage can be regulated at the sensor in the lower two fields.
Check again the minimum heating voltage.
Check again the maximum heating voltage.
You determined the minimum and maximum of the heating voltage well. You can edit the values by clicking on Change
or continue by clicking on Continue
.
Now set the heating voltage to U = 1,2 V!
Before we clarify what it should be good for, to be able to vary the electrical voltage in this area, let's look at the other two connections. The two sockets on the right side of the board are connected directly to the other two terminals of the sensor. Here you can measure the electrical resistance of the sensor, which will be the most important measure in this workshop.
Now connect the other multimeter with two banana plugs and set the range for the resistance measurement to 200 kΩ.
In this first experiment, we will investigate the behaviour of the sensor in an atmosphere, saturated with water and apple juice. Take a jar labeled water
, open it and carefully screw it under the board. THE SENSOR MAY
NOT BE WET BECAUSE IT WILL BE DAMAGED OTHERWISE.
Reminder: Heating voltage U = 1,2 V.
Wait at least 60 s (make use of the timer) and enter the sensor resistance of water in the table. Repeat the experiment with apple juice!
Liquid
Water
Apple juice
Resistance (U = 1,2 V) in kΩ
Check your measurement of the sensor resistance of water.
Check your measurement of the sensor resistance of apple juice.
Your measurement values are in the expexted range. You can still change the values by clicking on Change
or go on by clicking on Continue
.
Now set the heating voltage U = 2.6 V and repeat the measurements for water and apple juice. Wait at least 60 seconds again until you fill in the sensor resistance.
Liquid
Water
Apple juice
Resistance (U = 1,2 V) in kΩ
Resistance (U = 2,6 V) in kΩ
Try to explain how it is possible to use the sensor to distinguish water from apple juice!
Check your measurement of the sensor resistance of water.
Check your measurement of the sensor resistance of apple juice.
Enter an explanation!
Thank you for the explanation. With Change
you can revise your explanation at any time.
Carry out the experiment again with alcohol-free beer and the heating voltages U = 1.2 V and U = 2.6 V. Wait at least 60 seconds again until you read the resistance.
Liquid
Resistance (U = 1,2 V) in kΩ
Relative value in relation to water
Resistance (U = 2,6 V) in kΩ
>Relative value in relation to water
Water
100%
100%
Apple juice
%
%
Non-alcoholic beer
%
%
Check your resistance measurement for non-alcoholic beer at a heating voltage of U = 1.2 V.
Check your resistance measurement for non-alcoholic beer at a heating voltage of U = 2.6 V.
Your measurement values are in the expected range. You can always adjust the values by clicking on Change
.
Which heating voltage is better suited to differentiate between water, apple juice and non-alcoholic beer, based on the sensor reaction? Click on show help
to get additional information.
Please enter an explanation!
Thank you for your explanation!
In the following, we will take a closer look at the way the gas sensor works. Here we use a simplified model that we will encounter more often during the workshop.
First, we explain the main components of the model.
The sensor is made of conductive material.
The surface of the sensor (red marked) is in contact with the outer atmosphere, which we will ignore for now.
The sensor is connected to an ammeter, which measures the current in the sensor.
Currently no current flows through the sensor.
In the conductive sensor material is a certain number of electrons, which are freely movable and thus can transport charge. One speaks therefore of free charge carriers.
Currently the free charge carriers are at rest.
If a voltage is applied to the sensor, the free charge carriers start moving: A current flows through the sensor.
The arrows (
) symbolize the velocity of the free charge carriers. The longer the arrow, the faster the speed.
If there is air around, oxygen atoms can bound onto the surface (
) by connecting chemically with free electrons.
The bound electrons are no longer free and therefore no longer contribute to charge transport.
Select what happens when the surface of the sensor is covered with oxygen particles of the gas mixture air.
The number of free charge carriers .
The velocity of the free charge carriers .
The current, going through the sensor, .
The sensor resistance .
- The velocity of the free charge carriers is represented by the arrow length.
- The pointer of the ammeter indicates the current.
- According to Ohm's law, voltage U, current I and resistor R have the relation R = U / I.
At least one answer is incorrect.
All answers are correct!
This time, the air is additionally admixed with another gaseous substance, the so-called target gas (
).
The target gas can react with the oxygen particles on the sensor surface. The oxygen particles bound with the target gas release the still bound electrons. As a result, more free charge carriers are available for charge transport again.
The current at the sensor depends crucially on the reaction rate of the target gas with the oxygen on the sensor surface.
The greater the reaction rate (the more likely reactions to take place), the lower the oxygen coverage and the more free electrons there are
Choose what happens when there is target gas around.
The number of free charge carriers .
The velocity of the free charge carriers .
The current, going through the sensor, .
The sensor resistance .
- The velocity of the free charge carriers is represented by the arrow length.
- The pointer of the ammeter indicates the current.
- According to Ohm's law, voltage U, current I and resistor R have the relation R = U / I.
At least one answer is incorrect.
All answers are correct!
As already mentioned, the current strength of the sensor and thus also the sensor resistance depends on the reaction rate of the target gas.
Compare the reaction rates of the target gases with water, apple juice and non-alcoholic beer. Refer to your measurements for the sensor resistors at a heating voltage of 1.2 volts.
The order of reaction rates is incorrect.
The order of reaction rates is correct.
The reaction rate of a target gas depends on four factors:
1. Concentration of the target gas: The more target gas contained in a volume (unit e.g. g/cm³), the more likely reactions will be with the oxygen (higher reaction rate).
2. Oxygen coverage on the sensor surface:: The more the surface is covered with oxygen, the more likely reactions will be with the target gas.
3. Type of target gas: At the same concentration and oxygen coverage, more reactive target gases have a higher reaction rate (with the surface oxygen).
4. Temperature of the sensor material: The influence of the temperature of the sensor material is very complex. Not only does it have a direct impact on the reaction rate (reactions are more likely to occur at higher temperatures). In addition, the temperature also affects the oxygen coverage and the number of free charge carriers. Let's get closer to the influence of temperature experimentally ...
The last experiment showed that the heating voltage has an influence on the sensor resistance. But what exactly causes the heating voltage at the sensor?
Under the sensor is a long, thin platinum wire, which is wound very tightly. At the two ends of this wire, the heating voltage is applied. The higher the heating voltage, the greater the current flowing through the platinum wire.
Electricity has the property of heating the conductor through which it flows. This is called the heat effect of the electric current. You can change the heating voltage with the regulator.
The stronger the current, the more the platinum wire is heated, i. the higher the heating temperature of the sensor.
The diagram shows the approximate relationship between the applied heating voltage U and the heating temperature θ of the platinum wire. You can use the mouse to read value pairs from the diagram. Answer the following questions:
a) At what temperatures were the first two experiments performed?
b) Determine the minimum and maximum heating temperature that you can set with the board.
Check again the specified heating temperatures.
The specified heating temperatures are correct.
In the next experiment we want to investigate the influence of the sensor temperature θ on the sensor resistance R for different substances.
In any case, you will measure the relationship R (θ) for water. In addition, select another substance for which the context should also be measured:
Choose a substance!
Now set the heating voltage U = 2.8 V and screw your glass with water to the holder under the sensor.
On the next slide you have to follow the following steps:
In the diagram you can compare the values of water and . In addition, you see the values for .
As you can see, the measurement series for the different substances differ.
Usually, at school, you're entering metrics with a linear axis scale. However, it is often helpful to apply one or both of the logarithmic axes instead of linear axis scaling.
In the diagram, in addition to the linear representation, you can select a representation in which the measured values (on the y-axis) are plotted logarithmically. Try it out!
Give at least one advantage and one disadvantage of the linear and logarithmic representation:
Advantages of the linear representation:
Disdvantages linear representation:
Advantages logarithmic representation:
Disadvantages logarithmic representation:
Please enter advantages and disadvantages
By clicking on change
you can revise the advantages and disadvantages.
Describe how it is possible for you to distinguish between water, apple juice and non-alcoholic beer. Enter your answer in the text field.
Your explanation is too short!
The resistance curve is like a fingerprint for each target gas.
We now want to understand why the sensor resistance is related to the heating temperature. For this we use the already introduced model of the sensor.
Short repetition: The sensor is connected to an ammeter, which measures the current through the sensor. The arrows represent the velocities of the free charge carriers: the longer an arrow, the faster the charge carrier. Since an operating voltage is applied to the sensor, a current flows.
We do not consider the outer atmosphere for now.
The color of the sensor represents the temperature of the sensor caused by the heating element:
At the moment the sensor has a low heating temperature.
Increasing the heating voltage increases the heating temperature.
Try it with the buttons below the picture!
By the way: The sensor material is a so-called thermistor . That the higher the heating temperature, the better the sensor material conducts: the free charge carriers become faster.
Choose what happens if the heating voltage is increased.
The temperature of the sensor .
The number of free charge carriers .
The velocity of the free charge carriers .
The current through the sensor .
The sensor resistance .
- The velocity of the free charge carriers is represented by the arrow length.
- The pointer of the ammeter indicates the current.
- According to Ohm's law, voltage U, current I and resistor R have the relation R = U / I.
At least one answer is incorrect.
All answers are correct!
Describe the relationship between the heating voltage, the current through the sensor and the sensor resistance in your own words. Use the pictures above as an aid. You can also use back to navigate to the previous slides.
Your explanation is too short!
Thanks for the explanation. Use change
to revise your statement at any time.
What happens now, if the sensor is in contact with air without target gas and, at the same time, the heating voltage is regulated?
Short reminder: Due to the presence of air in the environment, free charge carriers are bound to oxygen particles () at the sensor surface. For the current then less free
charge carriers are available.
The heating voltage is currently low and therefore also the heating temperature of the sensor.
Increasing the heating voltage increases the heating temperature and thus the speed of the free charge carriers.
Try it with the buttons below the picture!
However, the higher heating temperature results in a second effect:
Both effects (higher velocity of the free charge carriers, more bound charge carriers) are now to be considered.
Choose what causes the increase from a low to a medium heating voltage, taking into account the influence of the air.
The temperature of the sensor .
The number of free charge carriers .
The speed of the free charge carriers .
The current through the sensor .
The sensor resistance .
- The velocity of the free charge carriers is represented by the arrow length.
- The pointer of the ammeter indicates the current.
- According to Ohm's law, voltage U, current I and resistor R have the relation R = U / I.
At least one answer is incorrect.
All answers is correct!
The heating voltage is now changed from a medium voltage to a higher voltage. Choose what causes the heating voltage to increase again.
The temperature of the sensor .
The number of free charge carriers .
The velocity of free charge carriers .
The current in the sensor .
The sensor resistance .
- The velocity of the free charge carriers is represented by the arrow length.
- The pointer of the ammeter indicates the current.
- According to Ohm's law, voltage U, current I and resistor R have the relation R = U / I.
At least one answer is incorrect.
All answers are correct!
Increasing the heating voltage (1) leads to a higher the velocity of the free carriers, but due to the higher temperature of the sensor (2), more and more air particles also bond on the sensor surface. The result: fewer and fewer free charge carriers contribute to the electricity. In their impact on the current, both effects are in opposite directions.
If the temperature of the sensor is low, many free charge carriers are present. Their speed is low. The current is relatively small / the resistance is relatively high.
If the temperature of the sensor is medium (hot, but not extremely hot), the current reaches a maximum, so the sensor resistance is a minimum. In this case one speaks of the highest sensitivity of the sensor.
If the temperature of the sensor is high, the velocity of the free charge carriers is high, but only a few are present. Again, the current is relatively small / the resistance is relatively high.
The four diagrams show different dependencies of the sensor resistance on its temperature. Indicate which diagram is closest to sensor operation in air.
Confirmagain.
Select the diagram in which the marked temperature points to the temperature of highest sensitivity.
Confirmagain.
In the diagram on the right you can see your measurements for water and again. What is the temperature and sensor resistance of the highest sensitivity.
Please check your answers!
Thank you!
Finally, let us consider the case of variable sensor temperature for a measurement in air, which additionally contains a target gas ().
Decide on the following statements whether they are true or false.
continueto get to the next slide.
continue-button.