In this lab, you will learn the basics of how to control the real world with your code. We’re going to use a button and two LEDs.

The button is connected to pin 3, and the LEDs we’re going to use are connected to pins 10 and 12.

For this lab, we’re going to start out by just using LED 1 (pin 10) and the button. After we get LED 1 working, we’ll add LED 2 (pin 12) into the mix to make it a little more interesting.

Whenever the button is being pushed, we want LED 1 to turn on, but any time that the button is not being pushed, we want LED 1 to be turned off. On the Arduino, we can use a command (a function) called digitalRead. This will look at a pin and tell us whether it sees 5 volts or not. If the button is being pushed, pin 3 will have 5 volts flowing to it from the power supply. This is equivalent to 1 in binary.

There is also another function, called digitalWrite. digitalWrite will take a 1 or a 0 and apply it on an output pin. So, if you digitalWrite a 1 to pin 4, then that pin will be connected to 5 volts internally. Since there’s an LED connected to pin 10, it will now have power flowing through it and it will turn on.

However, before we can use either digitalWrite or digitalRead, we need to tell the Arduino which pins we’ll be reading from and which ones we’ll be writing to! If we don’t tell it what our intentions are, it’s very hard to predict what it will do when we read or write the values of the pins. There is a function called pinMode that allows us to set the direction of the pin. Saying pinMode(3, INPUT) will set pin 3 to be an input, which is a pin you can read values from. If we say pinMode(10, OUTPUT), it sets pin 10 to be an output, which is a pin you can write values to. If you write a 1 to a pin, the pin will turn on and go to 5 volts. If you write a 0 to a pin, the pin will turn off and go to 0 volts. Make sure to set the pinModes in the setup function. Everything else we’re doing in this lab goes in the loop function.

If we combine the two, we can connect a button to the LED using software! First, we want to read the value of the button. So,

bool buttonPushed = digitalRead(3);

and then we can “write” that value to the output pin controlling the LED.

digitalWrite(10, buttonPushed);

Challenge (for a grade): Rewrite that code to just use one line to control the LED, rather than two.

Now, LED 2 is on pin 12. So, let’s make it so that LED 2 is on when the user is not pushing the button. In C++, if you have a value that is either true or false, you can invert the logical value with an exclamation mark (!). So, !true is exactly the same as false. If I say bool good = true; and then bool notgood = !good;, notgood is equal to false.

Attempt to make LED 2 do the opposite of whatever you’re telling LED 1 to do. If you have trouble, ask a TA, but make sure to try it yourself, don’t just give up immediately. If you get it working with 4 lines of code, try to do it in 3 lines of code in the loop function, and then try to do it again with only 2 lines of code in the loop function.

There are a lot of different ways to make this work, but there are three ways that most people are likely to try, and if you can figure all of them out, you’re doing great! If you need help, there’s absolutely nothing wrong with that.

Troubleshooting Tips!

1. For now, make sure you end your lines with semicolons. Some lines don’t need semicolons, like blank lines and lines that just have a curly brace ({ or }) on it. You also don’t need semicolons on lines that declare a function, such as void loop().
2. If your code is uploading to the Arduino, but it isn’t working right, make sure that you wrote your pinMode statements in the setup function. Writing them in the loop function would work too, but it’s bad practice, because then you’re setting the directions over and over, which is wasteful.
3. If you’re still having problems, make sure that your circuit looks the same as the one above and then ask a TA for help.

Questions (answer each with one to two solid sentences of explanation.)

1. Which way of writing this code do you think is the best way?
2. Why are we using bool for buttonPushed instead of int or char or another type?
3. If you encountered any issues, write them here and discuss how you solved them.