WEBVTT 00:00.050 --> 00:05.930 This is the solution for the activity number six, where you have to switch the LED that's turned on 00:05.930 --> 00:08.720 whenever you press on the push button. 00:08.720 --> 00:11.570 So we will start with all LEDs turned off. 00:11.570 --> 00:17.690 And then we will need to create some kind of algorithm inside this switch LED function, so that we 00:17.690 --> 00:23.030 can switch a different LED every time that we actually call the function. 00:23.030 --> 00:25.790 So every time the button is pressed. 00:25.790 --> 00:26.990 So let's get started. 00:26.990 --> 00:28.070 We will start with. 00:28.070 --> 00:31.460 So we will just ignore this code for now. 00:31.460 --> 00:35.390 And we will start with the imports and initializing the hardware. 00:35.390 --> 00:38.840 So we are going to import the Gpio zero library. 00:38.840 --> 00:41.030 So we will do from Gpio zero. 00:41.030 --> 00:42.650 And what do we need. 00:42.680 --> 00:46.850 Well we need an LED and then comma button. 00:47.090 --> 00:53.390 Then I'm also going to import from signal import pause okay. 00:53.420 --> 00:56.000 So that's the same thing we saw in activity five. 00:56.030 --> 01:00.940 You see from signal import pause so that we can actually Right. 01:00.940 --> 01:04.330 I'm just going to write it now pose here. 01:04.360 --> 01:04.750 Okay. 01:04.750 --> 01:10.150 So we are going to register this function that will be called when the button is pressed. 01:10.150 --> 01:14.440 But then we need to pose the program so that the program stays alive. 01:14.440 --> 01:17.410 And so we can actually see this function being called. 01:17.410 --> 01:21.400 And then well let's see if we need the time functionality or not. 01:21.400 --> 01:22.300 Probably not. 01:22.330 --> 01:27.160 But as we don't know if we need it right now, we probably don't need to import it. 01:27.160 --> 01:29.110 So I'm just going to keep those two imports. 01:29.110 --> 01:32.530 Then I will start to initialize my LEDs. 01:32.530 --> 01:35.860 So we have LED well here I can just call it LED. 01:35.890 --> 01:42.220 One for example is equal to LED and I will put 17. 01:42.220 --> 01:43.750 So that's going to be our first LED. 01:43.930 --> 01:51.430 And then led two is equal to LED with a Gpio number 27. 01:51.430 --> 01:58.150 And the third one LED three with so LED and Gpio number 22 okay. 01:58.180 --> 02:04.320 So just as you see on the image Those are the three gpios that we use for the LEDs. 02:04.320 --> 02:06.360 Then I can initialize the button. 02:06.360 --> 02:15.000 So I'm going to write it button like this is equal to button with uppercase and 26. 02:15.030 --> 02:15.480 All right. 02:15.480 --> 02:20.040 So we have our four components with our four gpios. 02:20.040 --> 02:22.320 And they are all configured here. 02:22.350 --> 02:28.140 Now the first thing I will do is I will actually power off all the LEDs. 02:28.140 --> 02:30.120 So it should be done by default. 02:30.120 --> 02:34.050 But just in case I want the LEDs to be powered off. 02:34.050 --> 02:45.780 So I'm just going to do led1 dot off and then LED two dot off and then led three dot off. 02:45.780 --> 02:50.760 And at this point you might think, well, maybe it would be nice to use a list, okay. 02:50.760 --> 02:56.670 And maybe some for loops and stuff like that to optimize the code instead of writing the same code three 02:56.670 --> 02:57.300 times. 02:57.300 --> 02:58.320 And you are right. 02:58.320 --> 03:01.460 And that's actually what we will do in the next activity. 03:01.460 --> 03:05.750 But for now, let's keep working with this without any lists. 03:05.750 --> 03:09.410 So we initialize the states for all the LEDs. 03:09.410 --> 03:14.450 We have our LED index, so we will start at zero. 03:14.480 --> 03:17.480 Then there is this function that I need to write the code in. 03:17.480 --> 03:23.120 And then we finish the program with button dot when pressed is equal to switch LED. 03:23.540 --> 03:26.690 So whenever the button is pressed this function is going to be called. 03:26.690 --> 03:34.610 And then we pause the program so that the program can keep monitoring the button state and call this 03:34.610 --> 03:35.360 function. 03:35.360 --> 03:39.950 And when we stop the program, well that's going to exit from the pose. 03:39.950 --> 03:43.820 So now let's write the code inside this function okay. 03:43.850 --> 03:47.810 So for now everything we've done is quite easy. 03:47.840 --> 03:50.510 That's things that we've done already before. 03:50.510 --> 03:54.170 But now what do we write inside this function. 03:54.170 --> 03:59.690 So we have three LEDs I started the LED index at zero. 03:59.690 --> 04:06.010 So let's say that the first LED will be zero, the second will be one and the third will be two. 04:06.040 --> 04:07.480 Okay, so that's an index. 04:07.480 --> 04:08.710 You start to count from zero. 04:08.740 --> 04:10.870 You go from zero, one and two. 04:10.900 --> 04:12.370 Because we have three LEDs. 04:12.370 --> 04:13.480 What I can do. 04:13.480 --> 04:19.660 So the algorithm I'm going to write is to say well we're going to check which index we are right now. 04:19.660 --> 04:26.380 And depending on that index we are going to turn on one LED only and turn off the other ones. 04:26.380 --> 04:33.190 And then we are going to increment the LED index by one so that the next time that we call the function 04:33.190 --> 04:33.430 again. 04:33.430 --> 04:39.970 So the next time that the user presses on the button, we can well, we can use another index and thus 04:39.970 --> 04:41.770 we can turn on another LED. 04:41.770 --> 04:46.000 So let's write this after the global LED index. 04:46.000 --> 04:53.230 Here I will need the user if because I want to check for example, if the LED index is equal with two 04:53.860 --> 04:56.830 equal signs very important two zero. 04:56.890 --> 04:58.420 So I will start with zero. 04:58.450 --> 05:00.690 If the LED index is zero. 05:00.720 --> 05:01.620 What do I do? 05:01.650 --> 05:05.160 Well, I will do led1 dot on. 05:05.520 --> 05:12.240 Then I do led two dot off and led three dot off. 05:12.270 --> 05:15.270 Okay, I turn on only one led. 05:15.300 --> 05:19.590 So the first one and I turn off the two other LEDs. 05:19.590 --> 05:21.090 And then what do I do? 05:21.120 --> 05:24.780 What I will do is I will do LED index. 05:24.780 --> 05:28.590 So I'm still in the if plus one. 05:29.070 --> 05:34.950 Because if you remember plus equal one is going to add this value to that variable. 05:34.950 --> 05:37.290 So let's say I press on the button. 05:37.320 --> 05:40.770 The first time switch LED is going to be called. 05:40.770 --> 05:43.650 We check is the LED index zero. 05:43.680 --> 05:45.840 Yes because that's the current value. 05:45.840 --> 05:47.700 So we turn on the first LED. 05:48.090 --> 05:54.450 And then we say that LED index is so the same value zero plus one. 05:54.450 --> 05:58.830 And note that I was talking about how to modify a variable. 05:58.830 --> 06:01.580 You need to use the global keyword here. 06:01.580 --> 06:03.410 And this is because of this line. 06:03.410 --> 06:09.560 If you try to run this code and you don't use the global, you will get an error here because you are 06:09.560 --> 06:12.770 trying to modify a global variable. 06:12.800 --> 06:14.870 So for that you need to use the global keyword. 06:14.900 --> 06:15.110 Okay. 06:15.200 --> 06:16.340 Just a tiny detail. 06:16.370 --> 06:16.820 Great. 06:16.820 --> 06:20.660 So that's going to work for the first iteration. 06:20.690 --> 06:20.900 Okay. 06:20.900 --> 06:23.630 So if we just run the program like this it's probably going to work. 06:23.630 --> 06:25.460 We're going to turn on the first LED. 06:25.730 --> 06:29.000 But then if we press again where nothing is going to happen. 06:29.000 --> 06:35.570 So I'm going to add an Elif here, Elif, and I'm going to do Elif. 06:35.600 --> 06:40.370 LED index is equal with two equal sign to one. 06:41.210 --> 06:48.140 So the first time LED index is zero, we do this and then we do plus one. 06:48.140 --> 06:51.770 We're going to exit from this if and exit from this function. 06:51.770 --> 06:56.090 But then the function is going to be called again the next time the button is pressed. 06:56.090 --> 07:00.140 And the next time the button is pressed, the LED index will not be zero. 07:00.140 --> 07:01.290 It will be one. 07:01.290 --> 07:02.700 So what do we do? 07:02.730 --> 07:04.920 LED one off. 07:05.250 --> 07:06.870 So we turn off the first LED. 07:08.040 --> 07:09.150 We turn on. 07:09.150 --> 07:11.970 So LED two dot on. 07:11.970 --> 07:13.740 We turn on the second LED. 07:13.740 --> 07:15.840 And we don't really need to do that. 07:15.840 --> 07:19.320 But I'm still going to be as explicit as possible. 07:19.320 --> 07:23.010 We also turn off the third LED because it's already off. 07:23.010 --> 07:25.140 But here it's more explicit. 07:25.170 --> 07:25.530 Great. 07:25.530 --> 07:34.800 And after I do this I will do the same thing LED index plus one, which means that after this LED index 07:34.800 --> 07:36.210 will be equal to two. 07:36.240 --> 07:42.390 And the next time we come to the switch led, then I'm simply going to add a Elif. 07:42.390 --> 07:45.480 And actually do I need to do an Elif? 07:45.690 --> 07:49.380 LED index is equal to two. 07:49.410 --> 07:54.840 We don't really need to do that, because that's going to be the last one, and we're not going to go 07:54.840 --> 07:55.650 after two. 07:55.680 --> 08:02.060 So I'm just going to put else that's going to work the same LED one dot off. 08:02.630 --> 08:05.510 LED two dot off and. 08:05.600 --> 08:07.850 LED three dot on. 08:07.880 --> 08:09.680 Okay, we turn on the third LED. 08:10.460 --> 08:13.610 So I put LS because I know that if it's not zero. 08:13.610 --> 08:18.290 If it's not one, it's going to be two because there is no other value after that. 08:18.320 --> 08:20.840 We don't have any fourth LED. 08:21.080 --> 08:21.830 Okay. 08:21.830 --> 08:27.740 And one important thing to do is when we reach the last index that we want, if I don't do anything 08:27.740 --> 08:29.600 else, well that's going to work the first time. 08:29.600 --> 08:34.250 We're going to turn on the first, second and third LED, but then nothing else is going to happen. 08:34.250 --> 08:38.720 So what we need to do is we need to go back to the first index. 08:38.720 --> 08:43.100 So in this case I will do LED index is equal. 08:43.100 --> 08:44.750 So just one equal okay. 08:44.780 --> 08:50.720 Because we assign a value is equal to zero and the function is complete. 08:50.720 --> 08:55.520 So the first time this function is called you're going to go to the first part of the. 08:55.550 --> 09:01.990 If the second time you're going to go to the second part of the if the third time you're going to go 09:01.990 --> 09:06.970 to the third part of the if and then the fourth time, you actually go back to the first one. 09:07.000 --> 09:08.860 ET cetera, et cetera. 09:08.890 --> 09:12.100 So looks like it's complete. 09:12.130 --> 09:18.250 Make sure that you have written the button when pressed is equal to switch LED without the parentheses 09:18.250 --> 09:18.880 here. 09:18.910 --> 09:19.420 Okay. 09:19.450 --> 09:20.830 Make sure it's the same name. 09:20.830 --> 09:24.760 And then make sure you have the pause at the end with the parenthesis. 09:24.760 --> 09:26.500 Because here you call the function. 09:26.530 --> 09:30.250 I'm going to run that and let's see what happens. 09:30.250 --> 09:33.490 So you can see now all LEDs are turned off. 09:33.520 --> 09:38.770 Now if I press and I release you see the first LED is turned on. 09:38.800 --> 09:42.760 I press again, the second LED and I press again. 09:42.940 --> 09:44.290 We have the third LED. 09:44.470 --> 09:47.050 And now if I press again we go back to the first one. 09:47.080 --> 09:47.890 ET cetera. 09:47.920 --> 09:48.820 ET cetera. 09:48.850 --> 09:49.630 Okay. 09:49.630 --> 09:57.040 And there's just one more thing I want to show you is that, um, let's see if that works at some point. 09:58.150 --> 10:01.860 So, of course, it never works when you want to show something to people. 10:03.060 --> 10:04.170 Maybe. 10:09.990 --> 10:12.090 Okay, you can see here what happened. 10:12.090 --> 10:13.110 So it was very quick. 10:13.110 --> 10:16.620 But what happened is that the blue LED was turned on. 10:16.620 --> 10:22.920 And then I pressed on the button and very fast it went to the green one and the red one again. 10:22.920 --> 10:24.720 But I just pressed once. 10:24.750 --> 10:25.290 Okay. 10:25.290 --> 10:26.850 So what happened here? 10:26.850 --> 10:32.040 Well, this is an issue that's actually a mechanical issue with the push button. 10:32.040 --> 10:32.880 It's not really an issue. 10:32.910 --> 10:35.730 It's just a behavior you can expect from a push button. 10:35.730 --> 10:41.700 So when you press on the button, what can happen is that the button is actually bouncing. 10:41.760 --> 10:41.940 Okay. 10:41.970 --> 10:44.430 So the mechanical part is bouncing a bit. 10:44.430 --> 10:49.950 And in some occasions it might look like you have actually pressed on the button twice. 10:49.950 --> 10:53.730 So it's not a software bug, it's just how the hardware is. 10:53.760 --> 10:54.270 Okay. 10:54.300 --> 10:59.070 And so depending on how fast you read from the push button from the software, maybe you're going to 10:59.070 --> 11:04.430 see that you press twice on the button, but actually you just press once, but the button is still 11:04.430 --> 11:06.230 physically bouncing. 11:06.260 --> 11:07.820 And so how to solve that? 11:07.850 --> 11:10.340 Well, here it's very easy with this library. 11:10.370 --> 11:12.860 You can go back to the button here. 11:12.860 --> 11:15.620 When we create the button we need to give the Gpio. 11:15.620 --> 11:18.050 But then we can give another argument to that. 11:18.050 --> 11:19.370 So I'm going to put a comma. 11:19.370 --> 11:23.450 And this argument is called bounce time. 11:23.450 --> 11:27.380 So you can just write bounce time like this is equal to. 11:27.410 --> 11:34.280 And let's give a value for example 0.05 which is 50 milliseconds. 11:34.280 --> 11:35.750 And what is this doing. 11:35.750 --> 11:41.390 Well it's going to say to the software that whenever we read a value from the push button, for example, 11:41.390 --> 11:46.910 if we see that the button is pressed, we're going to ignore the next values, the next few values over 11:46.910 --> 11:50.210 the next here 50 milliseconds. 11:50.210 --> 11:55.160 And this is very convenient because over the next 50 milliseconds that's a quite common value to use. 11:55.160 --> 11:57.530 You might have some physical bounces. 11:57.530 --> 12:03.370 So what you do is you just ignore those bounces, and it's only going to start to monitor the button 12:03.370 --> 12:05.800 again after 50 milliseconds. 12:05.830 --> 12:06.190 All right. 12:06.220 --> 12:09.550 So for the software it's gonna prevent the bounce to happen. 12:09.580 --> 12:12.940 I mean the bounce is still going to happen, but it's not going to read the bounce. 12:12.940 --> 12:14.170 And then for you as the user. 12:14.200 --> 12:17.650 Well 50 milliseconds is so fast you won't even see it. 12:17.650 --> 12:24.370 That basically means that you could press the button 20 times per second and it would still work correctly. 12:24.400 --> 12:27.880 Okay, but now I challenge you to press the button 20 times per second. 12:27.910 --> 12:28.120 Okay. 12:28.150 --> 12:30.400 As a human, it's very difficult to do that. 12:30.400 --> 12:32.950 So well, that's a small parenthesis. 12:32.950 --> 12:38.680 You can add this bounce time, which is going to solve the bounce issue that we have just seen previously. 12:38.680 --> 12:43.480 And if you want to learn more about this bounce well you can find many resources online. 12:43.510 --> 12:43.720 Okay. 12:43.750 --> 12:45.880 You just type push button bounce. 12:45.880 --> 12:49.480 You will find a lot of things related to Arduino as well. 12:49.510 --> 12:49.960 All right. 12:50.080 --> 12:54.340 But for this course here I'm going to keep this explanation here. 12:54.370 --> 12:54.970 Great. 12:54.970 --> 12:57.610 And this activity is now complete. 12:57.610 --> 13:00.820 And I have another one for you just in the next video.