This is the last consecutive video tutorial that I’ll be doing. I hope to continue doing more of these in the future, but this is the last one that I’ll be doing on a weekly basis. This week, I’m going to be talking about interrupts on the Arduino now there’s two types of interrupts that you can use at timer, interrupted or Hardware interrupt Arduino has good libraries built in to handle hardware interrupts how our timer interrupts were quite a bit more difficult. I’M. Not going to focus on them in this video, but if you’re interested in using timer interrupts for some reason go visit. My blog, I made a post a few months ago about timer interrupts, and I have a very long detailed code example about how you use them and it’s well commented, so that should help you get get. You started with that. If you want to use hardware, interrupts that you know you’re in the right place, that’s, where we’re going to start today, hardware interrupts are really great because they allow you to execute code on the Arduino while still looking from input from the outside world. So, for example, you could be running some code that turns a motor or something but still look for a button press that that changes, the speed of the motor or anything like that. Ordinarily, you’d have to accomplish this by pulling the button state continuously in the loop. But if you’re doing something in the loop that takes a while like running a for loop for example, then you might not be able to do that.
Interrupts handles it by letting you feed something into a pan. Their Arduino that’s constantly pulling it and checking it to see if it changes state and then, if it does it’ll execute a subroutine, then you can specify so let’s start talking about the technical details of interrupts. Now let’s talk about the different kinds of interrupts of the Arduino supports. There are four in total. The first time is low. This means any time an input to the Arduinos interrupts pin is low. It will keep executing that interrupts function over and over again. Usually we don’t want to do this. One I’ve never used it personally. The next kind is called change. This means it will execute a function any time the pin changes from high to low or from low to high. So if we have a pin input that looks like this it’ll change at these three points, the next kind is called rising, rising execute a certain subroutine any time it detects that an interrupt pin goes low to high, so wouldn’t execute here, but it would execute here And here falling as the exact opposite, falling interrupts execute when an input goes from high to low just like it does here and here, but it wouldn’t execute here. These are the two that we’ll use most commonly, especially when you’re using a button input let’s, not talk about debouncing. I first mentioned bouncing in episode 2 of the tutorial series when we first hook the button up to the Arduino to control something in the software.
We ran into this issue where, when you push a button, instead of just going from low to high, it actually jumps up and down, first a little bit before it settles on a value of 5 volts. This is going to be huge problem in interrupts, because one we can’t use the delay function inside, interrupts and delay was our primary means and software appealing with this, we would just read it and then delay until after this had passed and then read it again to Get the actual value, but if we can’t use delay, functions and interrupts. How are we going to deal with this if we use this straight up in an interrupt function? We’Re going to have a problem because there’s several rising edges that occur here and we only want one rising edge. So what will happen is we’ll actually execute the interrupt function several times, instead of just one which is going to cause errors in our program. The reason you can’t use the delay function in interrupts is because it depends on the same timer that’s used to handle the interrupt on the harder. We know let’s talk about how we can fix this. The answer is our hardware solution, so that we don’t have to use any functions in the Arduino software here’s. What our circuit looked like the first time we hooked the button up to the Arduino. We had a 10k pulldown resistor to ground a connection from that resistor to the pin and a button that connected from there to 5 volts.
When the button was undepressed. The pin was pulled down to ground through the 10k resistor. When the button was pressed, it was pulled up to 5 volts through the button now let’s figure out how we can use a similar setup, but do it with a deep bounce signal coming out. This is the setup we’ll use to accomplish a debouncing in hardware before it ever gets into the software of the Arduino first off we’re, going to change our pulldown resistor to a pull up. Resistor I’ll explain why that is in a minute. We’Re still going to use a 10k resistor but we’re going to add a capacitor across the button. I’Ll also explain that in just a moment this is an inverting Schmitt trigger the inverting Schmitt trigger will smooth out the signal that we’re going to create from this RC circuit or resistor capacitor circuit and then invert it back. The reason we use the pull up. Resistor is because we’re using an inverting Schmitt trigger which changes the signal from high to low and from low to high. Therefore, we wanted to change the default state from low default state here to high default state here, and then this will switch it back to what we want after we implement this. The signal going in will look like this exactly what we want for an interrupt awesome, but now let’s see why that’s happening the reason behind this circuits ability to debounce. Our signal in hardware is called an RC circuit.
Rc stands for resistor capacitor, and it just means that we have a resistor and a capacitor in series. So let’s take a simple example here. If we have five volts going through a 10k resistor and a 10 micro farad capacitor exactly is set up in the schematic. I just showed you we can find a time constant, called tau time constant equals the resistance times the capacitance value. The time constants is the amount of time it takes for the signal to decay from five volts down to ground there’s a decay time, because capacitors store a charge. That means it’s going to take a little while from to go to five volts down to ground, because this holds charging it. If we do this calculation out, we’ll see it the decay. Time is 0.1 seconds more than enough time to allow the button to be bounced. But less than ordinary human reaction time. So if we now look at this graph here, which shows voltage verse time, this explains the RC constant that I was talking about. If I push the button down right here, it’s going to take some time for a ZK from five volts down to ground you’ll, see that the capacitor helps smooth the bouncing that we saw before, but makes it no longer linear and no longer just drop straight down. Now it decays down to ground when we release the button right here, it will slowly curve back up from ground to five volts.
This is also determined by the RC time. Constant now, let’s, add our inverting Schmitt trigger the environment. Trigger does two things. The first thing is: is it flips all logic levels, one becomes zero and zero become one, but the more important thing is that it’s going to smooth out this curve. It has very certain voltage points after which it’s sets the signal to it default value. So at a certain point, when it hits a certain voltage decaying here, the Schmitt trigger is going to set it high, which is what happens right here and then the same thing going back up once it hits a certain voltage value, it’ll get set low. So there Schmitt trigger turns this inverted into a square wave, and that gives us exactly what we want to go into the interrupt on the Arduino awesome here’s. What the circuit looks like wired up we’re using the same push button as before here’s our 10 micro farad capacitor, and a 10 kilo ohm resistor. This is our inverting Schmitt trigger. I see it actually comes with six inverting Schmitt triggers in it we’re just using one of them. It gets power on the top right most pin and is connected to ground on the bottom left most pin. This is a very common configuration for 74 series, integrated circuits, of which this is one here’s. The code that we’re going to be running on this circuit to see if our hardware debouncing works, does it look familiar? It should it’s the exact same code that we ran into toriel 2 before we added software debouncing.
When we ran this in tutorial 2, we ran into all kinds of issues with the not LED not turning on when it should and not turning off when it should now we’re. Using that exact same code that was prone to error in tutorial 2, and it should work thanks to our hardware debouncing in tutorial 10, so let’s load it onto the Arduino and see how well it works. Ok, now that the software is downloaded onto the Arduino, keep an eye on this LED which is connected to pin 13 and we’ll control. With this button, as we set up in the program on off on off works perfectly, and we didn’t have to do any software. Debouncing remember what a pain that was last time and all we had to do was add two components to the circuit awesome. Now that we know how to do a button correctly in hardware, we can use it as an interrupt into the circuit without have to worry about any bouncing issues. So let’s use a switch input into the circuit to do something interesting and make a kind of mood. Color LED chooser that you control with your hands here’s. What we’ll need to accomplish this first off we’ll have three different colored LEDs. You can choose a W color you want. I chose yellow red and green. These should be connected to three PWM enabled pins in the Arduino, so you can change their brightness. Each one will have a current limiting resistor of 150 ohms, as we usually do next up I’m going to attach an infrared distance sensor.
This is one that we’ve used in several tutorials before it’s, a sharp infrared distance sensor. It has voltage connection a ground connection and outputs, an analog value which will attach to analog input 0 on the Arduino. Lastly, we have our D bounce button input so just like I showed you before. We have our 10k resistor, our 10 micro farad capacitor, if you’re using electrolytic capacitor make sure the negative end goes to ground because those are polarized and we have our switch here and our inverting Schmitt trigger here, which then feeds into pin 2 I’ve chosen pin 2 Because pin 2 is one of the two interrupts enabled pins on the Arduino it’s interrupt zero, so we refer to it in the program will actually be referring to zero, not okay, great, and what this switch will do is when we push a button, it will change Between the LEDs and the distance sensor will allow us to the brightness of each LED, so you can place your hand above the distance. The distance sensor to change the brightness of the LED that’s currently selected, get it to brightness. You like push the button and let go the next LED, and then you can set the brightness for that LED and so you’ll be able to set the brightness for all three LEDs and then leave them set at that value. Just by moving your hand up and down over the circuit, it should be pretty cool, so let’s write the code for it.
I’Ve already set up a framework for our code. Let’S start by going through all the comments that I made and filling in the appropriate information. The first thing we have to do is choose which interrupt we’re connected to so instead of saying we’re connected to pin 2 we’re going to say, we’re connected to interrupt 0 because that’s really what we care about so we’ll say the button interrupt is to have the Button pin is equal to 0, which is pin 2 on the arduino. I will just comment that so we know next, when you can figure all the pins the LEDs are connected to int, yellow LED is connected to 11 int red LED is connected 10 and green. Led is connected to 9 now I’m gon na. Do something interesting here, which is create a fake, LED that I’m going to call null LED the reason I’m doing this is that we can lock the LEDs to a certain brightness level, because we’ll be scrolling through each of the LEDs to set their brightness. We want to choose the opportunity to scroll tooling. It doesn’t exist so that these stay, where they are without currently flux, always fluctuating with the infrared sensor. You’Ll see what I mean in a minute when I implement this and then the last thing is to set the pin for the distance sensor. Int just pin equals 0 analog input. 0. Okay, now let’s set the pin modes. When you set these, all these outputs pin mode red LED is an output pin mode green LED is also an output, yellow, LED also an output, and we don’t really need set this well just do it anyway, pin mode null LED and output.
I just chose 6 because there’s another pwm pin, but doesn’t really even have to be a pwm pin to be honest, ok and now let’s attach our interrupt so to do that we use a command called attach interrupt and we feed it. Three things: what interrupt or connected to which we defined earlier is button and so interrupt 0, a function that we want to run when the interrupt is detected and when we want to run the interrupt. So remember I talked about this earlier in this episode rising. We wanted to tap on. The button goes from low to high, which should be perfect, based on the setup of our button, where are connecting to interrupts zero, which is sent by button int and we’re, going to run. This function called swap whenever that button is detected and will write swap in just a moment. Okay, before we write swap let’s write the the loop here, so the loop is just going to be doing what we want the program to do whatever led it happens. To have selected at the moment, we’ll have it change the brightness based on the infrared distance value. So we’ve done this before we’ll do list equals analog. Read the distance pin well change that to your brightness by mapping it distance zero, 1023, 0, 255, okay, good and then we’ll just have it write that to the LED but gee? How do we know which LED we’re currently on well? Okay, let’s put it in and then we’ll make a variable to control it analog right, we’ll call it selected LED and brightness.
So now you can see exactly we need to do here. We need to use this swap function to change. What selected LED is so let’s define selected LED up at the top here there’s one very important thing, though, which is that you need to define anything that gets modified by an interrupt function as a volatile, so we put volatile and selected LED equals let’s start it Off on the green LED to begin so what volatile means is that this integer can potentially change in the middle of execution here if you don’t define it as volatile you’re going to run into all kinds of weird things happening: ok, perfect, that’s! To do what we want and we’ll start it off as green LED now. All I have to do is write swap so what’s swap should do is every time the button is pressed. It should increment, through the LEDs and change selected, LED from green to red and then red, to yellow and then yellow to null, so that it locks the LEDs on whatever brightness setting. We have them and then from the all black tea greens that we can do it again. Such is going to be a couple of statements: that’s free, straightforward right, so, if selected led is currently set to green LED, remember, we use our double equal sign and if statements then we’ll change it to red LED and if selected or we can do else. If actually to save us some processing time else, if selected LED equals red, LED we’ll change it to yellow else, if again oops, I have a spelling error up here, huh else, if selected LED, I also forgot my double equal sign.
Hopefully you guys caught that right and I have a capital S I’m. Making yards all over the place here selected LED equals, yellow LED. Then we want to change it to null and last but not least otherwise. We’Ll change it back to green. All right, let’s just check it over for mistakes. That looks good, good, good, good, good, good. Okay, that looks good and remember you. Don’T actually need brackets, and if you only have one line of code after an if statement or a while loop or a for loop, you don’t need brackets, always okay, so this code looks done now. Let’S run it on the Arduino and see how our neat little visualizer works. We have the code on here now so let’s see how it works. Let’S choose a brightness for green. First, we can see the brightness changes with distance pick something right there. Okay, now it moves to red, will make read a little bit dimmer. I think about there and now we can set yellow let’s, make yellow nice and bright okay cool, and then we press it again and it goes to that null LED and they’re all set at those levels. And then we can of course go through and change them again. If we want awesome thanks for watching my tenth Arduino tutorial, I hope you guys enjoyed learning that interrupts, they’re really really useful, and maybe you guys got some good ideas from the project we did today.
I think you can make a really cool musical theremin out of it. Maybe you guys want to experiment with that I’d love to see any suggestions that you guys have or projects. You could work on using that code that I showed you. This is going to be the last consecutive Arduino video that I’m making, as I mentioned at the beginning, I hope to make more of them in the future, but I can’t continue making them every week during the semester. It’S too crazy, hopefully I’ll be able to do some written tutorials though, and I’ll be able to answer you guys questions. If you have any questions, always remember, you can post them on YouTube on my blog or on element14.com, and I will check them out. So thanks again for watching, I really appreciate anyone who watched through the whole series and all the people who gave me feedback. It was really spectacular and I really enjoyed making these because of all the great feedback I got from you guys. So thanks again and I’ll see you soon, thanks to element14 for helping me to sponsor this video series. They were kind enough to provide a lot of the materials that I’ll be using to create. These tutorials feel free to go visit their website and element or team comm check out their community, which is a great place.
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