It's a Switch

Educator intro

Length of the chapter: 1,5 - 3h

Adjust the length of the chapter according to the time you have at hand.

• 1,5 hours: allow 15-20 minutes to accomplish one of the final exercises.
• 1,5-3 hours: let the students make both final exercises, while allowing the more time to fine-tuning their mini projects.

Learning Objectives

Students learn…

• … to connect different switches to digital pins
• … to make switches work by programming: to set the pin mode to INPUT_PULLUP, to read the status of the switch with digitalRead(pin);
• … to program conditions with if statements, also using multiple conditions
• … about variables: how to create a variable, how to give it a value, how to use a variable to store the status of the program
• … to make sounds using a piezo speaker and the tone function

Depending on the final exercise, students also practice…

• …combining inventing and crafting skills that work with Arduino Exercise 1: Your Own Switch Check out the exercise instructions and get some crafting materials (cardboard, tin foil, paper glue, scissors, tape) ready if your students choose this exercise.
• …using multiple buttons with Arduino, utilizing ready-made code to make a more advanced project and to learn different possibilities Exercise 2: Reaction Game

Slides and Lesson Plans

Feel free to use these slides to introduce the second chapter to the students!

Plan 1 - Structured Lesson (90-125 min)

Plan 2 - Free-form lesson (90-140 min)

When there are several components in a circuit, you can share the ground (GND) pin with all of them. Often you need to share the 5V pin, too. This pin provides constant 5 volt voltage, which may be needed by many components at the same time (eg. potentiometers, sensors, motors) The long rows on the sides of the breadboard are meant for sharing the ground and voltage pins. 

In this chapter it is possible to connect every component to a separate GND pin, because there are three of these pins on an Arduino board. However, it is a good idea to practice using the ground and voltage rows of the breadboard - it will make the circuits easier to understand and debug.

Not all breadboards have color coded side rows. But when they do, students sometimes connect GND to a red row and 5V to a blue row. It's not dangerous. Paying attention to wire and row colors will make debugging easier later on as circuits become more complex.

It’s generally not a good idea to use pins 0 and 1 in circuits: they are reserved for serial communication between the board and computer, which means they will not work as reliable input or output pins in many cases.

If at this point the students upload the program , nothing will happen. That is because the loop is repeating super fast, over and over again. So fast in fact that the LED won’t have time to react to the digitalWrite commands!

void setup() {
  pinMode(10, OUTPUT);
  pinMode(2, INPUT_PULLUP);
}

void loop() {
  digitalWrite(10, HIGH);
  digitalWrite(10, LOW);
}

Another way to write the code could be:

void loop() {
  if (digitalRead(2) == LOW) {
    digitalWrite(10, HIGH);
  }
  else {
    digitalWrite(10, LOW);
  }
}

However, we’ve instructed the students to use two if statements, instead, in order to keep from teaching too many new things in one go.

void loop() {
  if (digitalRead(2) == LOW) {
    digitalWrite(10, HIGH);
  }
  if (digitalRead(2) == HIGH) {
    digitalWrite(10, LOW);
  }
}

There is a difference between using the if-else structure and only if statements.

• if-else: makes sure only one of the actions defined in the structure gets executed during a loop run
• consecutive if statements: when both if statements are true, the actions defined in both get executed during one loop run.

A button can either be pressed or not pressed - so there’s no way both if statements could be true at the same time in our code example. However, in some other program this could happen. It’s good to understand the difference: if you want only one thing out of several options to take place, choose the if-else structure. If you want to allow several options to take place, use if statements.

This will be extensively practiced, later on!

The description on using pullup resistors is kept compact on purpose. If the students are new to electronics, the concept and logic of pullup resistors can be a lot to absorb. 

If you would like to teach this topic at, for example, a physics lesson, here's some extra information!

Button circuit with an external resistor: pullup explained

When you connect buttons to an Arduino, you may get the following problem:

When the button is pressed down, the pin of the button is connected to ground (GND) and the value received from the button is LOW for certain. 

When the button is released up, the value of the port floats: the value is not connected to voltage, so it can be HIGH or LOW at random. 

With Arduino, you can use an extra resistor to fix this problem. Remember, that pressing the button always connects the legs of the right and the left side of the button to each other!

When the button is not pressed, the button's input pin 2 (green wire) is connected through the resistor to voltage of 5 V (red wire). Voltage is applied to the port, and the measured value is HIGH. The value is "pulled up", hence the name pull-up resistor. 

When the button is pressed down, the button connects pin 2 and ground, and the value is LOW. In principle, pin 2 is now connected to a 5 V voltage. However, electric current always travels "the easiest way", which in this case is to ground, because that route has the least resistance!

Connecting a circuit like this to Arduino every time you use a button would be pretty time-consuming. Luckily, Arduino has a similar built-in mechanism, which you can use by defining the button pin as INPUT_PULLUP in the setup:

pinMode(2, INPUT_PULLUP);

After that the button pin is connected to the internal pullup resistor on the Arduino board. You can define any pin (0-13) as an INPUT_PULLUP pin!

Remember, that when using a button, you should always use the pullup resistor! A good memory trick for how the button behaves with the pull-up resistor is this: when the button is up, its state is up (HIGH) and when the button is down, its state is also down (LOW).

Check out this great tutorial by Sparkfun:

https://learn.sparkfun.com/tutorials/pull-up-resistors