Arduino senses the environment by receiving inputs from add-on devices such as sensors, and can control the world around it by adjusting lights, motors, and other actuators. In this class you will learn how and when to use the different types of sensors and how to connect them to the Arduino. Since the external world uses continuous or analog signals and the hardware is digital you will learn how these signals are converted back-and-forth and how this must be considered as you program your device. You'll also learn about the use of Arduino-specific shields and the shields software libraries to interface with the real world. Please note that this course does not include discussion forums.
Lecture 1.2 - Resistive Sensors
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Module 2
This module introduces sensors and actuators and discusses how to interface with them. We’ll examine different classes of sensors and actuators. For each type of sensor/actuator, we’ll examine the circuitry needed to interface with it. Additionally, we’ll take a look at the Arduino code needed to communicate with the sensors and actuators.
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Ian HarrisProfessor
Department of Computer Science
[MUSIC]
There are several different types of sensors.
Right now we'll talk about some simple sensors.
And a lot of simple sensors we'll call Resistive Sensors.
What they do is they change resistance, so they act like resistors.
Okay, but
they are resistors who resist changes according to some environmental effect.
So, I got, you can see on the right side of the slide, I got pictures of
three different resistors, or rather sensors which act as resistors.
And actually, you should have some of these in your kit.
That first one, the one all the way on the left, that's a Photoresistor.
So it just,
its resistance changes according to the brightness, okay, in the room.
The next one is the Thermistor, so
its resistance changes according to the temperature.
And the next one is Flex Resistor and it bends and
its resistance changes according to how much it's bent.
If it's bent a lot then it has a different resistance than if it's straight.
So these are all three sensors.
They sense things about the environment, and the way sense,
the way they transmit information is by changing a resistance.
So remember that our microcontroller, it doesn't sense resistance directly,
it senses voltage.
So we've got to make this, put a circuit around this thing.
Put this thing inside a circuit.
And in that circuit there has to be a voltage which changes when the resistance
of this sensor changes.
So that circuit it is what I'm showing there.
You can see the circuit up there in the top middle.
And the circuit has got, it's basically a voltage divider.
So at the bottom its got a resistor going to ground.
At the top it has this blue box.
Right? This blue box is gonna be a sensor,
one of these resistive sensors.
So it could be a photoresistor, it could be a thermistor,
it could be a flex resistor, it could be any of those.
As long as they are, they basically act as resistors up there.
And the resistances changes according to the environmental effect that you wanna
observe.
Now, [COUGH] you'll remember several lectures ago,
I talked about a voltage divider, actually when I was talking about potentiometers,
I showed a voltage divider.
And that circuit is exactly a voltage divider.
Okay, it's a voltage divider, if you remember a potentiometer.
Right?
It's got two terminals on the top and the bottom,
then it's got this middle terminal between two resistors.
And these resistors, their ratio, the ratio of their resistance,
can change in a potentiometer.
So similar things happening here.
The bottom resistor is just gonna be constant.
Okay, but the bottom resistor is this sensor.
And that changes according to the brightness, lets say of the photoresistor.
Or the temperature of the thermistor.
And so you see between the two resistors you got that point there
that we're observing, that's connected to, say, and Arduino pin.
We're gonna observe the voltage at that point.
And that voltage will go up and
down according to the resistance on that resistor.
So as the resistance changes on that resistor, the voltage will go up and
down and we can detect that.
And we'll have to take that pin, take that observation voltage and
wire that into an analogue pin, one of the analogue pins, A0 through A5 lets say.
And then we will have to call analogue read to read the voltage.
And based on the voltage value that we get, we can tell how bright it is or
how hot it is or something like that.
So that's the idea.
So that's the circuit that we are gonna try and
build around this resistive sensor to be able to read it properly.
So your connecting sensor in a voltage divider, which is that circuit there.
And the sensor itself,
since it's a resistive sensor it is the top resistor in this voltage,
or you can make it the bottom resistor too, it doesn't actually matter.
As the resistance changes the voltage changes according to Ohm's Law.
So, [COUGH] here's an example, you might have a photoresistor, so you can see,
that's the voltage divider.
The bottom resistor, I've made 10 kilo ohms, and the top resistor,
I have made a photoresistor.
That's the symbol for photoresistor.
It's basically a resistor with a circle around it, got some arrows coming in,
cuz it's detecting light.
And so, it's a voltage divider, and
let's say that this voltage divider, this photoresistor,
its resistance changes within a range depending on the brightness.
So, as brightness increases, the resistance of the photoresistor decreases.
That's generally how they work.
So, I'm making up some numbers here, but
let's say its resistance is 10 K Ohms at a certain level of brightness.
Okay, so at 10 K Ohms then that means if we look at our volts divider we've got
a 10 K Ohms resistance on the top and a 10 K Ohms resistance on the bottom,
which means that the voltage will be divided in half.
So since you've got five volts going from top to bottom, five volts on the top,
zero on the bottom that middle point that we're measuring will be 2.5 volts,
because the resistance are even.
So if it's at ten 10 K Ohms, if the photoresistor has 10 K Ohms of resistance,
then you'll get 2.5 volts.
But let's say the brightness changes.
Right? Let's say it gets more bright,
then the resistance goes down.
So now the resistance of the photoresistor is only 5 K Ohms.
Now when that happens, you've got a smaller resistor on top and
still got the big 10 K Ohm resistor on the bottom.
Right?
So now, what happens is since there's less resistance on top,
there's gonna be less voltage, what's called voltage drop,
voltage difference across the top resistor.
So now, the resistance that we measure, the voltage that we measure in between
those two resistors is gonna be higher, 3.3 volts.
Right?
Because the voltage is disproportionately across the bottom resistor,
since it's bigger, relative to the top.
So the point there is, that as the outside environmental, environment changes.
So, in this case, if it's a photoresistor, the brightness changes.
The resistance changes in this circuit, and
as the resistance changes the voltage changes on that pin that we're measuring.
And that pin that's going into one of the analog inputs of the arduino.
And we can measure that with an analog read.
So by calling analog read we can see the voltage value, and
we can detect the change.
And we can make something, some code that does something activates some device when
the light level goes above a certain threshold.
If the voltage that we measure is higher than X then we can do something.
So we can respond.
We can write code that reacts to the environment.
Okay, so those are resistive sensors, the ones I just described.
But then there are some other sensors that are a little bit more complicated, but
actually easier to use from our perspective.
Voltage-Controlling Sensors.
So the first sensors we looked at they're resistive.
When the environment changes, the resistance of the sensor changes.
In these devices, they don't output resistance.
They don't have a resistance.
They do have resistance, but that's not what you're measuring.
Instead they have a pin which outputs a certain voltage directly.
And this is good for us because Arduino, it directly senses voltage.
Right? So these type of sensors,
these voltage controlling sensors, they output voltage directly.
And so that's even easier for us.
We can just take their voltage out, put it right straight into one of the analog
inputs of the Arduino and measure its voltage.
Now to give you an example of one of this sensor,
of one of these sensors, a PIR motion sensor.
PIR standing for Passive Infrared motion sensor.
So that's what I have over there in the left.
You see that passive infrared motion sensor.
And yeah, actually, you've probably seen these things like this.
This is what you would have, like we have in this room actually, and
in a room in my office.
This is a motion sensor so
when you come in the room the lights go on, something like that.
So, they're using PIR motion sensors.
So, PIR motion sensor has three leads, three wires, three terminals,
whatever you call them.
One is connected to power, one is connected to ground, and
one is the signal that you're interested in, the voltage that you're interested in.
So, if we look at our motion sensor block over there, it's got one pin on the top,
one pin on the bottom and one pin on the right side.
So the pin on the bottom that's wired straight to ground.
The pin on the right that's wired straight to power.
Okay, the pin on the top is the one that you can see.
If you follow the pin on the top that goes to the left,
which is presumably going to our Arduino.
Right? Some analog input into our Arduino.
So the pin on the top is the one of interest, the signal of interest.
All right, and that pin is gonna have a voltage assigned by the motion sensor.
And the way this motion sensor works, is if, actually this particular
motion sensor, what it does is, it pulls the signal low when motion is detected.
So when it senses motion, it sets that voltage on top to zero volts.
Okay, so that's what's gonna happen.
Now, when it doesn't sense motion, it doesn't do anything to that wire on top.
Right?
So that wire, the voltage on that wire would be floating.
So that's why we need a 10 K Ohm resistor to power,
because if there's no motion then we don't want,
one thing that we'd never want to input to our server is a floating voltage.
A voltage that we can't tell if it's high or low.
Right?
We don't want a floating voltage, so we wanna make sure that, if there's no
motion, even though the motion sensor isn't assigning a value to this pin,
this wire gets some value.
Right?
And that value that it gets in this case should be a one, should be a high voltage.
Right?
Because the low is what happens when there's motion, so
we want high to be associated with no motion.
So, when there's no motion, the motion sensor doesn't do anything but
that pin is connected to power, to plus five through the 10 K Ohm resistor.
So that's why we need that 10 K Ohm resistor connecting it to power,
it's called a Pull Up Path.
We need that pull up path, so that even though the motion sensor isn't forcing
that pin to have any value, if there's no motion it'll just go high,
because it's connected to power through the resistor.
So this is a common type of sensor.
This is one type of sensor, it's a voltage-controlling sensor.
They're useful.
They're nice to use.
They're pretty easy to make the circuit to use them.
This is just one. There are several,
it's called Open-Collector.
So when you have a pin like that you pull low and it floats when there's no motion.
They call that open-collector,
so we have to pull it high it needs it's pull up resistor.
So that when there's no motion it goes to high it has a known value.
Okay, so there are lots of different type of sensors that are voltage controlling,
and here's a couple.
The first one,
the one over there on the left is the Triple Access Analog Accelerometer.
So, an accelerometer basically senses acceleration in three dimensions, so X, Y,
Z, it'll tell you the acceleration.
So this is useful for telling your orientation.
Right? Because there's always
the acceleration of gravity.
So you can tell the orientation of your device based on what this,
this thing reports back.
If it reports basically, and you know what gravitational pull is supposed to be,
what it's acceleration is.
So you can measure your acceleration in each axis and
tell what it's orientation is.
So that is a Voltage-Controlling Sensor, so
it has those holes on the side of that little board.
You generally solder pins onto those, it was called a Breakaway board.
And you solder those pins on there and then you can
wire those pins into the Arduino and sense the voltage on those pins.
And there are several of them, but one is X, one is Y, one is Z.
Those are the signals of interest.
And you can wire those into the analog input of the Arduino and
tell what the acceleration is in each axis.
[COUGH] The next one is the Dual Axis Gyroscope.
So a gyroscope actually reports angular velocity.
In only two dimensions this time.
But again, there are two signals in there that you can read and
see what the angular velocity is.
Now both of these type of things,
these are used in any kind of device that needs to sense orientation.
The two of them are used together, actually.
So, for instance, my quadcopters.
They all have these built into them.
Accelerometers and Gyroscopes are essential,
because it needs to know its orientation so it can stay level.
So, these type of devices are commonly used for that type of application.
Thank you.
[MUSIC]
